Ungrounded (left) and grounded (right) power plugsCommon North American 125-volt receptacles. All accept a 1-15P plug; the two on the left also accept grounded 5-15P plugs; the second from the left also accepts 5-20P plugs. The NEMA 5-15R device on the far left is most common; the two rightmost designs are typically seen in older buildings.
NEMA connectors are power plugs and sockets used for ACmains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. NEMA wiring devices are made in current ratings from 15 to 60 amperes (A), with voltage ratings from 125 to 600 volts (V). Different combinations of contact blade widths, shapes, orientations, and dimensions create non-interchangeable connectors that are unique for each combination of voltage, electric current carrying capacity, and grounding system.
NEMA 1-15P (two-pole, no ground) and NEMA 5-15P (two-pole with ground pin) plugs are used on common domestic electrical equipment, and NEMA 5-15R is the standard 15-ampere electric receptacle (outlet) found in the United States, and under relevant national standards, in Canada (CSA C22.2 No. 42[1]), Mexico (NMX-J-163-ANCE) and Japan (JIS C 8303).
Other plug and receptacle types are for special purposes or for heavy-duty applications.
The dimensional standard for electrical connectors is ANSI/NEMA WD-6 and is available from the NEMA website.[2]
In the early days of electrification, residential use was almost exclusively for illumination, with rooms normally having just a single electrical outlet spot in the center. Along with his lightbulb, Thomas Edison developed the Edison screw in the early 1880s, patented 1881.[3] The Edison screw was very successful, and quickly became the first de facto standard for electric connection.[4] In the early 1900s, table and floor lamps became more popular, and sockets were mounted on walls for secondary connections. One big disadvantage of screw connectors was that the cord inevitably got twisted after being connected to the receptacle.
Harvey Hubbell's inventions
In 1903, Harvey Hubbell filled the U.S. patent 774,250, for a lightbulb socket adaptor and plug, also a standalone receptacle. The adaptor was screwed into the lightbulb socket, leaving a flat face with two holes to conveniently attach the plug. The same patent had a second design, with a wall attachable receptacle, capable of receiving the same plug, thus being the first socket and plug design patented in the US. Later in 1904, he changed the design to flat blades (a design later incorporated in the NEMA 2 series), filled under the U.S. patent 774,251 Both these patents were granted in November 1904.
In 1910, Hubbel worked on improving his popular flat blade design, filled U.S. patent 1,064,833 in 1912. The new design had parallel blades, a more compact design and was easier to manufacture. This design was improved once again in 1915, introducing a polarized plug under the U.S. patent 1,180,648. The flat blade plug which in subsequent years evolved to become NEMA 1-15 was born.
Some of Hubbel's patents:
U.S. patent 774,250, fig. 1, illustrating a lightbulb socket adaptor and an attachable flat blade socket for it.
U.S. patent 1,064,833, fig. 1 and 3. First patent showing the parallel blades which became NEMA 1-15.
U.S. patent 1,180,648 fig. 4 and 5, showing a concept for polarized receptacle and plug.
Other American manufacturers
It is worth noting that in addition to Hubbell's system, circulated a large variety of different plugs and receptacles, some of them compatible with Hubbell's, some not. In 1919, Hubbell unsuccessfully tried to prevent other manufacturers from making receptacles and plugs to the dimensions used by Hubbell. The report of the court proceedings[5] includes a comprehensive review of the development of the art in the US prior to 1919, based on evidence presented to the Court. Separable plugs had been available for more than a decade prior to Hubbell's 1904 design.
NEMA
In 1926, the National Electrical Manufacturers Association (NEMA) was founded by the merger of the Electric Power Club and the Associated Manufacturers of Electrical Supplies[6] and represented manufacturing companies of the electrical segment in a national level. NEMA provided a forum between companies for reaching standardization, but it was not until the 1940s that NEMA started to publish standards on receptacles and plugs, much later than UK,[7] France and Germany.[8] Since NEMA was a forum between manufacturers and not an governmental body, any decision had to be thoroughly discussed and agreed upon by its members, which substantially slowed the standardization process.
Nomenclature
NEMA receptacles, with their common US uses listed (in purple text)
NEMA connectors are named following an alphanumeric code consisting of:
(prefix L for locking) numerals – numerals (suffix R for receptacle, P for plug).
There are two basic classifications of NEMA connectors: straight-blade and locking. The metal conductive blades are often informally called prongs (as in 3-prong plug). Numbers prefixed by 'L' are curved-blade, twist-locking connectors. Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required.
The numerals preceding the hyphen encode the number of poles (current-carrying terminals) and wires connected to it, the voltage, and single- or three-phase power.[citation needed] A connector with ground terminal has more wires than poles: two-pole, three-wire; or four-pole, five-wire; etc. A non-grounded device may be two-pole, two-wire; three-pole, three-wire; etc.[citation needed]
The numerals following the hyphen is the current rating of the device in amperes. This number is followed by the letter 'R' to indicate a receptacle (socket) or 'P' to indicate a plug (prongs).
As an example, the 5-15R is the common 125V two-pole, three-wire receptacle rated for 15A. The L5-15R, while sharing the same electrical rating, is a locking design that is not physically compatible with the straight-blade 5-15 design. The 5-30R has the same two-pole, three-wire configuration and 125V rating, but is rated for 30A.
Although there are several non-grounding device types in the NEMA standards, only three of them are in widespread use today. These are the two-pole 1-15, still in use in millions of buildings built before the 1960s, and the three-pole 10-30 and 10-50.
Other types of NEMA connectors that do not follow this nomenclature include: the ML series (so-called "Midget Locking" connectors named for their diminutive size), TT (for connecting travel trailers and other recreational vehicles to external power sources), SS series ("ship-to-shore" connectors for connecting boats to shore power) and the FSL series (used in military and aircraft applications).
The small hole near the end of the power (non-ground) blades of some NEMA plugs is used for convenience in manufacturing; if present, it must be of specified diameter and position.[2] Small specialized padlocks are available to fit these holes, allowing "lockout" of hazardous equipment, by physically preventing insertion of locked plugs into a power receptacle. Since at least 1949, numerous receptacle devices have also been invented to use these holes to hold the prongs inside the receptacle slots, using a corresponding latch or locking mechanism. [9]
The blades of a NEMA connector are identified within the dimensional standard as follows: 'G' identifies the grounding conductor, 'W' identifies the (grounded) neutral conductor, and 'X', 'Y', and 'Z' are the "hot" line conductors. Single-phase connectors have only a single terminal identified as 'X' or two terminals, 'X' and 'Y'. Three-phase connectors will use 'X', 'Y' and 'Z'.[2]
NEMA non-locking connectors all use blades of various flat and folded shapes (except for the round pins used on grounding connectors). The plugs can be detached from the receptacles by pulling back on the plug body. These connector families have been designed so that connectors of differing types cannot be accidentally intermated.
NEMA wall receptacles can be found installed in any orientation. Neither NEMA nor the US National Electrical Code nor the Canadian Electrical Code specify a preferred orientation, but the National Electrical Contractors Association's National Electrical Installation Standards (NECA 130-2010) specify that the preferred location of the ground is on top. When the ground blade of a receptacle is on the bottom, the neutral blade is on the upper left and the hot blade is on the upper right. All descriptions below assume this orientation (i.e., clockwise order is ground, neutral, hot for 120V receptacle versions; and counter-clockwise for plug versions).
NEMA 1
This "2-prong" design, with two flat parallel non-coplanar blades and slots, is used in most of North America and on the east coast of South America on lamps; consumer electronics such as clocks, radios, and battery chargers; and other double-insulated small appliances that do not require grounding (earthing).
All NEMA 1 devices are two-wire non-grounding devices (hot-neutral) rated for 125V maximum. 1-15P plugs have two parallel flat blades, 1⁄4 inch (6.4mm) wide, 0.06 inches (1.5mm) thick, 5⁄8–23⁄32 inch (15.9–18.3mm) long, and spaced 1⁄2 inch (12.7mm) apart.
1-15R receptacles have been prohibited in new construction in the United States and Canada since 1962, but remain in many older buildings, and this obsolete design is still available for repair use only. Since January 1, 1974, all new power outlets are required to have a ground connection, using grounded receptacles (typically 5-15R or 5-20R) that accept both grounded and non-grounded plugs. [13] Replacement of obsolete NEMA 1 receptacles requires either rewiring with an additional ground conductor for a NEMA 5 receptacle, or a NEMA 5 receptacle complete with a ground fault circuit interrupter for two-wire non-grounded configurations (when a ground conductor is not available).
Ungrounded NEMA 1 plugs are still popularly used by manufacturers of small appliances and electronic devices because of the design's low cost and compact size, and they are upward compatible with modern grounded NEMA 5 receptacles. Standards permit ungrounded plugs where the appliance does not require grounding due to low risk of leakage current, such as on double-insulated devices.
In older plug designs both blades were the same width, so the plug could be inserted into the receptacle either way around. Many plugs manufactured since 1948 are polarized; the neutral blade is 5⁄16in or 7.9mm wide, 1⁄16in or 1.6mm wider than the line blade, so the plug can be inserted only one way. Polarized 1-15P plugs will not fit into unpolarized receptacles, which possess only narrow slots. Polarized 1-15P plugs will fit 5-15R grounded receptacles, which have the same wider slot for the neutral blade. Some devices that do not distinguish between neutral and line, such as internally isolated AC adapters, are still produced with unpolarized narrow blades. Cheater plug adapters allow a "3-prong" grounded 5-15P plug to be mated to a non-grounded 1-15R receptacle. The adapters include a spade lug to allow connecting to ground, often via the cover screw used to attach the outlet faceplate. These adapters are illegal in some jurisdictions, in particular throughout Canada.[14]
There are some obsolete 1-15R or 1-20R (120 V, 15 or 20 A) receptacles which are mechanically able to accept 1-15P (120 V), 1-20P (120 V), 2-15P (240 V), or 2-20P (240 V) plugs. These receptacles are typically found in older residential buildings and are not allowed to be installed under current NEC codes. In addition to the lack of grounding, these obsolete receptacles could allow a connected device to overheat and create a fire hazard if a device designed for the wrong voltage is connected. These problematic outlets are easily identifiable due to their lack of ground, along with both openings on the receptacle face being a sideways T-shaped opening that appear to be mirrored on the vertical center line of the face. Due to the potential danger of a voltage mismatch, whenever possible these receptacles should be replaced. Depending on local code, replacement with a "repair" approved non-grounded receptacle may be sufficient.
The Japanese plug and socket with narrow insulating faces appear and work physically identical to NEMA1-15, and such non-grounded receptacles are still common in Japan (though grounded 5-15R and 5-20R receptacles are slowly becoming more common). The Japanese system incorporates stricter dimensional requirements for the plug housing, different marking requirements, and mandatory testing and approval by METI or JIS.[15]
NEMA standards exist for 1-15P, 1-20P and 1-30P plugs, and the 1-15R receptacle. There are no 1-20R and 1-30R receptacles, because 1-20P and 1-30P can mate with a corresponding NEMA 5 receptacle.
A non-polarized 1-15P plug has 2 identical blades.
One blade is wider on a polarized 1-15P plug.
AC adapter with integrated plug pins matching an unpolarized NEMA 1-15P
Newer NEMA 1-15 plugs with wider protective faces (middle right) have a safety advantage; plugs for electric toys (right) as well as very early plugs (left) have noticeably wide faces to discourage touching the electrical contacts.
A NEMA 1 cord from 1932, with wide protective face to prevent touching the electrical contacts.
Japanese socket with grounding post, for a washing machine
Japanese socket having the narrowest socket face with grounding post, for an air conditioner
NEMA 2
All NEMA 2 devices are two-wire non-grounding devices (hot-hot) rated for 250V maximum. Although standards exist for 2-15, 2-20 and 2-30, this series is obsolete, and only Hubbell still manufactures 2-20 devices (for repair purposes).[16]
NEMA 3
This series of devices is specified for 277-volt, two-wire, non-grounding devices. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.[citation needed]
NEMA 4
This series of devices is specified for 600-volt, two-wire, non-grounding devices. Identically to the NEMA 3 series, this is "reserved for future configurations" and no designs for this series exist and no devices have been manufactured.[citation needed]
NEMA 5
NEMA 5-15P plug and NEMA 5-15R receptacle (different scales, blade spacing is 0.5 inches (12.7mm) for both.) Each receptacle also accepts an ungrounded plug, whether polarized or unpolarized.Typical 5-15R residential receptacle5-20R T-slot receptacle mounted with the hole for the ground pin at the top. The neutral connection is the wider T-shaped slot on the right.
All NEMA 5 devices are three-wire grounding devices (hot–neutral–ground) rated for 125V maximum, with the 5-15, 5-20 and 5-30 being grounded versions of the 1-15, 1-20 and 1-30, respectively. The addition is a 3⁄16-inch (4.8mm) diameter round or U-shaped ground pin, 1⁄8in (3.2mm) longer than the power blades (so the device is grounded before the power is connected) and located from them by 1⁄4in (6.4mm) edge-to-edge or 15⁄32in (11.9mm) center-to-center.
Compared to the 5-15P plug, the 5-20P plug has the neutral blade rotated 90° and shifted so its inner edge is approximately 1⁄2in (12.7mm) from the hot blade. The 5-20R receptacle has a T-shaped neutral hole, to accept both 5-15P and 5-20P plugs. An acceptable alternative version of the 5-20R receptacle has a rectangular slot that will only accept 5-20P plugs. The 5-30 and 5-50 are physically larger, with 1in (25.4mm) between power pins; 5-30 also has an L-shaped neutral blade.[17] These larger sizes are uncommon, as twist-locking plugs are generally used for high-current applications.
The neutral blade on 5-15P plugs is not always wider than the line blade, since the ground pin enforces polarity.
The Electrical Safety Foundation International has stated: "Never remove the ground pin (the third prong) to make a three-prong plug fit a two-prong outlet".[18] In addition to the dangers of breaking a ground connection, removing the ground pin to make it fit a 1-15R receptacle or extension cord, may result in the live–neutral polarity being lost.
The 5-15R and 5-20R are by far the most common electrical receptacle in North America in buildings built since the mid-twentieth century. It is usually installed in a duplex configuration; two receptacles may share a common circuit or may each be wired separately, sometimes to a switch.
In 46 of the 50 United States[19] and all of Canada, tamper-resistant receptacles are required in new residential construction as of November 2013[update]. These prevent contact by objects like keys or paper clips inserted into the socket.[20] This is accomplished by an interlocking mechanism that requires hot and neutral blades inserted simultaneously to release the small doors blocking the slots. The grounding slot is not blocked by a door.
In stage lighting for film and theater, this connector is sometimes informally known as PBG (Parallel Blade with Ground), U-ground, Edison or Hubbell, the name of a common manufacturer.[21] (The name "Hubbell" can be confusing as several different connectors share this name depending on the company, industry, and use.) In the motion picture and TV production industries, an extension cord that uses this type of connector (usually with 12 AWG or 10 AWG wire)[22] is called a "stinger".[citation needed] Generally, lighting technicians use these extension cords to deliver power to lights rated at 2,000watts or less.[23]
Internationally, the NEMA 5-15P plug and NEMA 5-15R receptacle are the basis for the International Electrotechnical Commission's IEC 60906-2 standard IEC system of plugs and sockets-outlets for household and similar purposes–Part 2: Plugs and socket-outlets 15 A 125 V a.c. and 20 A 125 V a.c.[24]
NEMA 6
All NEMA 6 devices are three-wire grounding devices (hot-hot-ground) used for 208 and 240V circuits and rated for 250V maximum, with the 6-15, 6-20 and 6-30 being grounding versions of the 2-15, 2-20 and 2-30, respectively. The 6-15 resembles the 5-15, but with collinear horizontal pins, spaced 23⁄32in (18.3mm) center-to-center. The 20A plug has a blade rotated 90° (opposite blade from what would be the "line" blade on a 2-15 or 5-15 plug. This prevents accidental insertion of plugs into outlets that use different voltages), and the 6-20R receptacle has a T-shaped hole to accept both 6-15P and 6-20P plugs (similar to the 5-20R receptacle accepting 5-15P and 5-20P plugs). The 50A (and 5-50) receptacles look like bigger versions of the 5-15A. 6-15R and 6-20R receptacles are usually manufactured on the same assembly line as "Industrial" or "Commercial" grade 5-15R and 5-20R receptacles, with all 4 receptacles sharing the same "triple wipe" T contacts behind the varying faceplates. The faceplate bonded onto the receptacle determines the final configuration of the receptacle.[citation needed]
NEMA 6 devices, while specified as 250V, may be used for either 208 or 240V circuits, generally depending on whether the building has a three-phase or split-phase power supply, respectively. The NEMA 6-20R or 6-30R found in many hotel and motel rooms is typically supplied with either split-phase or two phases of three-phase 208V.
The higher-current receptacles are rare, with twist-locking plugs such as L6-30 and direct wiring being more common. Generally, 6-series non-locking plugs are used for such appliances as large room air conditioners and commercial kitchen equipment; the occasional home arc welder also uses 6-50.[25] Single-phase 6-50 is commonly used on farms for silo unloaders, and is used with a 6-gauge flexible power cord up to 200ft (61m) long. The 6-50 receptacle charges electric vehicles at the same 32- or 40-ampere maximum from charging stations, on a short cord, as the more versatile 14-50 with its fourth prong neutral not used currently for any electric vehicle charging purposes.[26]
NEMA 6-15 (Green "U"-shaped contact is ground.)
NEMA 6-50 with dustproof flexible shroud, used on a farm silo unloader
NEMA 7
NEMA 7 devices are three wire/prong (hot-neutral-ground) connectors rated at 277V. The 15A 7-15 plug has the crowsfoot current carrying pins of the Type I plug, but with a U-shaped earth pin. The 7-20 version has an enlarged line/hot pin. 7-30 is a larger diameter connector, with an L-shaped neutral, while the 7-50 has an enlarged neutral pin, compared with the hot.
NEMA 8
NEMA 8 devices are specified for three wire, two-pole, grounding devices for 480 volts. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.
NEMA 9
NEMA 9 devices are specified for three wire, two-pole, grounding devices for 600 volts. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.
NEMA 10
The now-deprecated NEMA 10-30 has a neutral pin (at top of photo), but does not have a dedicated safety grounding pin.
NEMA 10 connectors are a now deprecated type that had formerly been popular in the United States for use with high-power electric clothes dryers, kitchen ranges, and other high-power equipment. NEMA 14-30R and -50R connectors have generally replaced NEMA 10 equipment for these applications. NEMA 10s are classified as 125/250V non-grounding (hot-hot-neutral), and were designed to be used in a manner that indirectly grounds the appliance frame to the neutral, which was common before the requirement of a separate safety ground was incorporated in the National Electrical Code.
As commonly used, 10-30 and 10-50 plugs required the frame of the appliance to be indirectly grounded via a strap connecting to the neutral blade. Safe operation relied on the neutral conductor in turn being connected to system ground at the circuit breaker or fuse box. If the neutral conductor were to break, disconnect, or develop high resistance, the appliance frame could become energized to dangerous voltages. Modern practice is to require a separate safety grounding conductor whose only purpose is to divert unsafe voltages, and which does not carry significant current during normal operation.
Relying on the neutral conductor was a legal grounding method for electric ranges and clothes dryers, under the National Electrical Code from the 1947 to the 1993 editions (banned in 1996 edition). Since North American dryers and ranges have certain components (timers, lights, fans, etc.) that run on 120V, this means that the neutral wire indirectly used for grounding would also carry current, even under non-fault conditions. Although this is contrary to modern grounding practice, such "grandfathered" installations remain common in older homes in the United States.[27]
NEMA 11
NEMA 11 series devices are three wire, three-pole, non-grounding devices for 3-phase 250-volt designs and equipment parts are specified by NEMA for 20-ampere (11-20), 30-ampere (11-30), and 50-ampere (11-50) devices.[citation needed]
NEMA 12
NEMA 12 series devices are three wire, three-pole, non-grounding devices for 3-phase, 480-volt equipment. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.
NEMA 13
NEMA 13 series devices are three wire, three-pole, non-grounding devices for 3-phase, 600-volt equipment. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.
NEMA 14
These sockets and plugs are four prong (see receptacle chart above) grounding devices (hot–hot–neutral–ground) available in ratings from 15 to 60A. The voltage rating is 250V. Of the straight-blade NEMA 14 devices, only 14-50 and 14-30 are in common use and either may be used for home charging of electric vehicles. The 14-30 is limited to 30 amperes (24 amperes continuous) and used mainly for some electric clothes dryers and some electric cooking stoves. The NEMA 14 connectors are essentially the replacements for the older NEMA 10 connectors described above, but with the addition of a dedicated grounding connection.
All NEMA 14 devices offer two hots, a neutral, and a ground, allowing for both 120 and 240V when supplied by split-phase power, or 120 and 208V if the supply is three-phase. The 14-30 has a rating of 30A, and an L-shaped neutral blade. The 14-50 has a rating of 50A, and a straight neutral blade sized so that it does not mate with 14-30 connectors.
NEMA 14-50 devices are frequently found in RV parks, since they are used for "shore power" connections of larger recreational vehicles. Also, it was formerly common to connect mobile homes to utility power via a 14-50 device. Newer applications include electric vehicle charging.[28] The 6-50 is a smaller size receptacle lacking a neutral fourth prong, exclusively providing 240 V, also used for electric vehicle charging station purposes.[29]
NEMA 14-30 and 14-50 receptacles
NEMA 14-30 clothes dryer receptacle and plug
A NEMA 14-50 supplying 12kW (9.6kW sustained) on a 240V circuit to recharge an EV
NEMA 15
NEMA 15 are three-pole and ground connectors (phase A, phase B, phase C, ground) rated for 208V. Intended for delta three-phase circuits with ground and no neutral. The straight blades all carry one of the three phases.[citation needed]
NEMA 20
NEMA 20 series devices are specified for 347/600Y three-pole, four-wire, non-grounding devices. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.[citation needed]
NEMA 21
NEMA 21 series devices are specified for three-pole plus neutral, five-wire grounding devices for 3-phase 120/208Y supplies. According to NEMA, NEMA 21 straight-blade devices are "reserved for future configurations", so no designs for this series exist and no devices have been manufactured. There are however NEMA L21 series locking devices for 20- and 30-ampere devices specified and available for these applications.[citation needed]
NEMA 22
NEMA 22 series devices are specified for three-pole plus neutral, five-wire grounding devices for 3-phase 277/480Y supplies. According to NEMA, NEMA 22 straight-blade devices are "reserved for future configurations", so no designs for this series exist and no devices have been manufactured. There are however NEMA L22 series locking devices for 20- and 30-ampere devices specified and available for these applications.[citation needed]
NEMA 23
NEMA 23 series devices are specified for three-pole plus neutral, five-wire grounding devices for 3-phase 347/600Y supplies. According to NEMA, NEMA 23 straight-blade devices are "reserved for future configurations", so no designs for this series exist and no devices have been manufactured. There are however NEMA L23 series locking devices for 20- and 30-ampere devices specified and available for these applications.[citation needed]
NEMA 24
This section needs expansionwith: "24" is hot-linked elsewhere in this article but is not fully described anywhere in the article. You can help by adding to it. (October 2023)
NEMA 24 Series devices are specified for two pole, three-wire grounding devices for 347V A/C power.[30]
NEMA TT-30
NEMA TT-30 plug and receptacle. (The center hole on the receptacle is not a contact.)
The NEMA TT-30 (TT stands for Travel Trailer) connector is a 120V 30A recreational vehicle standard (hot-neutral-ground), also known as RV 30. The TT-30R receptacle is commonly available in nearly all RV parks in the United States and Canada, and all but the largest RVs manufactured since the 1970s use this plug to connect to power feeds.[citation needed]
The appearance of this plug is sometimes confused with a NEMA 10 connector, rated for 240V, but the NEMA TT-30 is a 120V device. The hot and neutral blades are angled at 45° from vertical and 90° to each other, unlike NEMA 10 devices (where the angles are 30° and 60° respectively), also the plug is slightly smaller than a NEMA 10 and larger than ordinary 5-15P plugs. The ground pin is round, like those on straight-blade NEMA grounding devices. Referring to the picture, the orientation is the same as the NEMA 5 plug and receptacle, with the neutral blade on the lower right.
Adapters are available with the TT-30P plug on one side and a 5-15R or 5-20R receptacle on the other side. When a power feed cord is detachable from an RV, an L5-30P is usually used on the RV end of the cord.[citation needed]
Twist-locking connectors
Twist-locking connectors were first invented by Harvey Hubbell III in 1938 and "Twist-Lock" remains a registered trademark of Hubbell Incorporated,[31][32] although the term is used generically to refer to NEMA locking connectors manufactured by any company. Locking connectors use curved blades. Once pushed into the receptacle, the plug is twisted and its now-rotated blades latch into the receptacle. To unlatch the plug, the rotation is reversed. The locking coupling makes for a more reliable connection in commercial and industrial settings, where vibration or incidental impact could disconnect a non-locking connector.
Locking connectors come in a variety of standardized configurations that follow the same general naming scheme except that the designations include an "L" for "locking". Locking connectors are designed so that different voltages and current ratings can not be accidentally intermated.[33] Many specific types exist; only a few are listed below. Other types include special purpose connectors for boats, 400Hz circuits such as used for aircraft, and direct-current applications.
One apparent disadvantage of twist-lock connectors is that in the event that the cable is accidentally pulled too hard, rather than the plug falling out of the receptacle, exposed conductors may come out of the plug, causing dangerous shorts or shock hazards if the circuit is live. This is resolved in most cases by the connector having a robust integral strain relief.
ML
ML-series "Midget Locking" connectors are for 15A applications where a larger locking connector would not fit. [34]
ML-1 connectors are two-pole, no ground, rated for 125V.
ML-2 connectors are two-pole with ground, rated for 125V.
ML-3 connectors are for three-pole no ground (hot-neutral-hot) rated for 125/250V.
SS
SS-series "Ship-to-shore" connectors are for 50A marine shore-power applications.
SS1-50 connectors are two-pole with ground, rated for 125V.
SS2-50 connectors are three-pole with ground (hot-neutral-hot) rated for 125/250V.
NEMA L1
NEMA L1 series devices are single-pole plus neutral, two-wire, non-grounding devices for 125 volts single phase. Designs and devices for 15-ampere devices (L1-15) exist.
NEMA L2
NEMA L2 series devices are two-pole, two-wire, non-grounding devices for 250 volts single-phase. Designs and devices for 20-ampere devices (L2-20) exist.
NEMA L3 and L4
These devices would have been for 277- and 600-volt two-pole, two-wire non-grounding devices similar to the straight-blade NEMA 3 and 4 families, but were never specified by NEMA.
NEMA L5
NEMA L5 connectors are a series of two-pole and ground locking connectors rated for 125V. L5-30R receptacles are common at marinas that provide power to docked boats. They are also found on some RVs for connecting to shore power. RVs in the US are equipped for 120V 30A or 240V 50A service, and use a cord to connect to a receptacle at the campsite, usually on a power pedestal with one or more receptacles providing 120V 30A (TT30R), 240V 50A (14-50R), or 120V 15/20A (5-20R) service. Locking receptacles appropriate for the voltage and current are used on the RV end of the cord, along with non-locking plugs on the end connecting to the pedestal.
NEMA L6
NEMA L6 connectors are rated for a maximum of 250 volts. They are intended for two-pole, three wire, line-line-earth (or hot-hot-ground) circuits with a nominal supply voltage of 208 or 240 volts, depending on phase configuration. The L6 connector does not provide a neutral connection.
L6-20 connectors provide a maximum of 20 amperes and are commonly found in power distribution units (PDUs) used in the information technology sector. Most often, these connectors can be found in server rooms and data centers where the connectors are used to power equipment such as servers, backup systems and UPS units.
L6-30 connectors provide a maximum of 30 amperes and tend to be used in heavy-industry sectors. For example, welders and other manufacturing machinery where industrial equipment or large power tools are commonplace.
NEMA L7
NEMA L7 are two-pole and ground connectors rated for 277V. Typically, these connectors are found in commercial or industrial lighting circuits, especially where metal halide lamps are common.
NEMA L8
NEMA L8 are two-pole and ground connectors rated for 480V. Intended for three-wire hot-hot-ground circuits.
NEMA L9
NEMA L9 are two-pole and ground connectors rated for 600V. Intended for three-wire hot-hot-ground circuits.
NEMA L10
NEMA L10 series devices are two-pole plus neutral, three-wire, non-grounding devices for 125/250 volts single-phase. These are deprecated due to the lack of grounding but L10-20 and L10-30 devices are specified by NEMA and are commercially available.
NEMA L11
NEMA L11 series devices are three-pole, three-wire, non-grounding devices for three-phase 250-volt devices. Designs exist for 15-ampere (L11-15), 20-ampere (L11-20), and 30-ampere (L11-30) devices, and L11-20 and L11-30 devices were commercially available from at least one manufacturer (Bryant Electric).
NEMA L12
NEMA L12 series devices are three-pole, three-wire, non-grounding devices for three-phase 480-volt devices. Designs exist for 20-ampere (L12-20), and 30-ampere (L12-30) devices, and L12-20 and L12-30 devices were commercially available from at least one manufacturer (Bryant Electric).
NEMA L13
NEMA L13 series devices are three-pole, three-wire, non-grounding devices for three-phase 600-volt devices. Designs exist for 30-ampere (L13-30) devices and L13-30 devices were commercially available from at least one manufacturer (Bryant Electric).
NEMA L14
NEMA L14 are three-pole and ground connectors rated for 125/250V. Intended for three-pole, four-wire hot-hot-neutral-ground circuits with a nominal supply voltages of 240 or 208V hot-to-hot and 120V hot-to-neutral.
These connectors are common on household backup generators, and on racks of power amplifiers in large audio systems.
NEMA L15
NEMA L15 are three-pole and ground connectors rated for 250V. Intended for three-phase circuits.
NEMA L16
NEMA L16 are three-pole and ground connectors rated for 480V. Intended for three-phase circuits.
NEMA L17
NEMA L17 are three-pole and ground connectors rated for 600V. Intended for three-phase circuits.
NEMA L18
NEMA L18 are four-pole no ground connectors rated for 120/208V. Intended for wye three-phase circuits.
NEMA L19
NEMA L19 series devices are three-pole, four-wire, non-grounding devices for three-phase 277/480-volt devices. Designs exist for 20-ampere (L19-20), and 30-ampere (L19-30) devices, and L19-20 and L19-30 devices were commercially available from at least one manufacturer (Bryant Electric).
NEMA L20
NEMA L20 series devices are three-pole, four-wire, non-grounding devices for three-phase 347/600-volt devices. Designs exist for 20-ampere (L12-20), and 30-ampere (L20-30) devices, and L20-20 and L20-30 devices were commercially available from at least one manufacturer (Bryant Electric).
NEMA L21
L21-30 plug and receptacle
NEMA L21 are four-pole and ground connectors rated for 120/208V. Intended for wye three-phase circuits with both neutral and ground. The pin in the middle is ground, and the blade with a right angle on the tab is neutral. These connectors are common in live event power distribution. Many event production companies use power distributors with camlock connectors for feeder cable, and 12 or more L21-30 connectors which can each be broken out to three individual 120V circuits via the use of a stringer box.[citation needed]
NEMA L22
NEMA L22 are four-pole and ground connectors rated for 277/480V. Intended for wye three-phase circuits with both neutral and ground. The pin in the middle is ground, and the blade with a right angle on the tab is neutral.
NEMA L23
NEMA L23 are four-pole and ground connectors rated for 347/600V. Intended for wye three-phase circuits with both neutral and ground. The pin in the middle is ground, and the blade with a right angle on the tab is neutral.
Additional safety features
A North American NEMA-5 receptacle halfway through a corrective re-installation. The fluorescent green spacers, which firmly attach the receptacle to the metallic sub-structure, are made necessary by using 5⁄8in (16mm) drywall instead of 1⁄2in (13mm), or by slightly-misplaced device box housings.
Over time, electrical codes in the US and Canada began to require additional safety features in the basic NEMA 5-15R and 5-20R configurations to address specific electric shock hazard concerns. The safety features listed below are not mutually exclusive; for example, tamper-resistant GFCI receptacles are available.[35]
Tamper-resistant GFCI duplex receptacle type 5-20RA, which can take 5-15 and 5-20 grounding plugs and 1-15 non-grounding plugs
These versions of the 5-15R or 5-20R receptacle are residual-current devices, and have "Test" and "Reset" buttons (and sometimes an indicator light which may be normally on or normally off per the vendor's design). In the US and Canada, GFCI protection is required for receptacles in many potentially wet locations, including outside outlets, bathrooms, and some places in kitchens, basements, and crawl spaces. This is an expedient way to provide that protection at the receptacle itself.[36]
These safety devices work by comparing the currents flowing in the live and neutral conductors, and disconnect the circuit if their difference exceeds 4 to 6 milliamperes. Installing a single receptacle is often cheaper or more convenient than providing this GFCI protection at the circuit breaker. In addition, a tripped GFCI receptacle may be more easily noticed and reset, as compared to a tripped GFCI located in a remote circuit breaker panel far from the point of usage.
Like most current interrupting devices, a GFCI receptacle can optionally be wired to feed additional "downstream" outlets; correctly installing one GFCI receptacle in a circuit can protect all the plugs, lights, switches, and wiring which receive power from it. This allows for cost-effective retrofits on older installations where branch circuits were often daisy-chained between wet and dry locations. A GFCI receptacle may be installed indoors where it is sheltered from dampness and corrosion, while still protecting an outdoors receptacle wired downstream.
GFCIs are also recommended for power tool outlets and any locations where children might insert conductive objects into the receptacles.[36]
Tamper-resistant receptacles
Starting with the 2008 National Electrical Code[37] and the 2009 Canadian Electrical Code,[38]listed tamper-resistant receptacles that address electric shock hazards to children must now be installed in almost all areas of new or renovated dwellings. According to statistics cited by the NFPA, the code change adds only $40 to the cost of building an average, 75-receptacle home in the US.[citation needed]
This safety measure reduces shock hazards to a child that attempts to insert a single conductive object into the receptacle. Inserting a normal, two-blade electrical plug applies simultaneous pressure on both sides of the receptacle to open an internal, spring-loaded shutter, but a foreign object fails to do so and therefore cannot make contact with the live electrical contacts.[39] However, the device can still be defeated by inserting two objects simultaneously. Despite its weaknesses, the tamper-resistant receptacle is superior to protective plastic outlet caps which must be individually installed on each receptacle (and are a choking hazard when removed), and to sliding covers that children easily learn to defeat.[37]
AFCI receptacles
The National Electrical Code has been updated for 2014 to address the use of Outlet Branch Circuit (OBC) Arc Fault Circuit Interrupter (AFCI) receptacles as an alternative to traditional circuit breakers when used for modifications, extensions, replacement receptacles, or in new construction. AFCI receptacles reduce the dangers associated with potentially-hazardous arcing conditions (parallel arcs and series arcs), by interrupting power to arcing devices (e.g. a damaged appliance cord) that might otherwise not draw enough current to trip the primary circuit protection device.
AFCI protection is mandated by the 2014 Code in residential family rooms, dining rooms, living rooms, kitchens, parlors, libraries, dens, bedrooms, laundry rooms, sunrooms, recreation rooms, closets, hallways or similar rooms.[citation needed] It is also required in dormitory units. AFCI receptacles look similar to GFCI receptacles in that they have a "Test" and "Reset" button on the face of the device for localized testing. This saves a homeowner a trip to the breaker panel, should the device trip. Unlike AFCI breakers, AFCI receptacles can be used on any wiring system, regardless of the panel.[citation needed] When installed as the first receptacle on a branch circuit, AFCI receptacles can provide series arc protection for the entire branch circuit. They also provide parallel arc protection for the branch circuit downstream of the AFCI receptacle.
Surge protective devices are designed to reduce the random energy surges of voltage transients and electrical noise on the power supply line, which can damage sensitive electronics such as TVs, computers, and smart appliances. They are available for 120V, 15/20A applications, in different form factors such as surge protective receptacles in single, duplex, four-in-one, and six receptacle configurations, as well as surge-protective power strips. These devices provide point-of-use protection and are the last line of defense in a whole-house surge protection network.
Weather-resistant receptacles
Weather-resistant (WR) receptacles are made with ultraviolet-resistant insulating materials having excellent cold-temperature impact resistance to withstand longterm exposure to weathering and abuse. Metallic components are required to be resistant to corrosion. Mandated by the 2008 National Electrical Code in outdoor damp or wet locations, WR receptacles are required in patio, deck, and pool areas.[citation needed] They are available in a variety of variations, including GFCI and tamper-resistant.
For added protection, WR receptacles should be shielded by "Extra-Duty While In-Use" or "Weather-Resistant" covers.[citation needed] These covers are ruggedly constructed to keep out moisture (either dripping or condensing), dust, debris, and insects, while providing easy access to receptacles to allow their use with power tools, trimmers, sprinkler systems, and pumps.
Leak-current detection and interruption (LCDI) cordsets
Damaged power cords of portable air conditioners have caused many electrical fires, and about 350 deaths per year.[citation needed] To combat this, the 2017 National Electrical Code (NEC)[40] requires each portable air conditioner sold in the United States to have either a leakage current detector interrupter (LCDI) or a ground-fault circuit interrupter (GFCI) protective device built into its power cord. The device can be integral with the power plug, or a separate module within 12 inches of the plug. The protection device is equipped with "Test" and "Reset" buttons on the housing. An LCDI cord has a fine wire mesh around the conductors, and circuitry to detect current leaking from the conductors to the mesh, which would happen if the cord were damaged or frayed. The plugs are normal NEMA 5-15, 5-20, 6-15, 6-20, or 6-30 plugs, depending on the air conditioner design, and are typically molded-on designs.
Color code
The color of a device neither identifies its voltage class nor power system. Because the colors are not specified by NEMA standards, the purpose of color-coding a receptacle may be set by the building owner, who may select brown, ivory, white, almond, grey, or black receptacles in the 5-15 configuration to blend with the decor of a room.
However, although colors are not standardized by NEMA, some industries utilize colors for certain applications, following de facto standards:
A receptacle with a green dot is a so-called "hospital grade" device; The primary reason to use a hospital-grade receptacle at a patient bed location is to ensure that a receptacle with a greater contact tension is provided to minimize possibilities that an attachment plug supplying medical or life support equipment may be disconnected because the attachment plug slipped out of the receptacle. Also such devices are tested to survive harder use while limiting electrical leakage, compared to wiring devices intended for residential or commercial purposes.[citation needed][41]
Per UL Standard ANSI/UL 498, a receptacle (any color) with an orange triangle, is an isolated ground (IG) device, where the grounding pin of the receptacle is connected to ground independently of the frame of the receptacle and wiring outlet box. This is also a requirement of the National Electrical Code (NFPA 70), Article 406. While neither UL nor the CSA require the face of the receptacle to be a specific color, most IG receptacles are orange.[42]
A red receptacle may indicate a special-service outlet such as one connected to an emergency standby power source. The Canadian Electrical Code requires that "essential" receptacles in hospitals, connected to emergency power systems, must be red.[43]
Most duplex receptacles have metal tabs connecting the top and bottom receptacles. These tabs can be broken off to allow the top and bottom receptacles to be wired onto separate circuits.[44] This may allow for one switched receptacle for a lamp, or for two separate supply circuits when heavy loads are anticipated. Two branch circuits may optionally share a common neutral wire terminating on duplex receptacles, a condition sometimes referred to as "split-wiring", "split-receptacle",[45] or "half-split".
Related standards
The dimensions and configurations for NEMA connectors are given in ANSI/NEMA standard WD-6.[2]Underwriters Laboratories maintains UL Standard 498,[46] which specifies construction performance (e.g. durability, electrical safety, and fire-resistance) for NEMA connectors. These additional requirements allow connectors to be manufactured to be compliant with the National Electrical Code. The Defense Logistics Agency and General Services Administration maintain Federal Specification W-C-596[47] and its associated specification sheets. This specification references WD-6 and UL498, and provides additional durability and electrical safety performance criteria for connectors intended for military use.
IEC 60320 Appliance couplers for household and similar general purposes is a set of standards from the International Electrotechnical Commission (IEC) specifying non-locking connectors for connecting power supply cords to electrical appliances of voltage not exceeding 250 V (a.c.) and rated current not exceeding 16 A. Different types of connector are specified for different combinations of current, temperature and earthing requirements. Unlike IEC 60309 connectors, they are not coded for voltage; users must ensure that the voltage rating of the equipment is compatible with the mains supply. The standard uses the term coupler to encompass connectors on power cords and power inlets and outlets built into appliances.
A power cord, line cord, or mains cable is an electrical cable that temporarily connects an appliance to the mains electricity supply via a wall socket or extension cord. The terms are generally used for cables using a power plug to connect to a single-phase alternating current power source at the local line voltage. The terms power cable, mains lead, flex or kettle lead are also used. A lamp cord is a light-weight, ungrounded, single-insulated two-wire cord used for small loads such as a table or floor lamp.
Electrical wiring in North America follows the regulations and standards applicable at the installation location. It is also designed to provide proper function, and is also influenced by history and traditions of the location installation.
A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that interrupts an electrical circuit when the current passing through a conductor is not equal and opposite in both directions, therefore indicating leakage current to ground or current flowing to another powered conductor. The device's purpose is to reduce the severity of injury caused by an electric shock. This type of circuit interrupter cannot protect a person who touches both circuit conductors at the same time, since it then cannot distinguish normal current from that passing through a person.
The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring and equipment in the United States. It is part of the National Fire Code series published by the National Fire Protection Association (NFPA), a private trade association. Despite the use of the term "national," it is not a federal law. It is typically adopted by states and municipalities in an effort to standardize their enforcement of safe electrical practices. In some cases, the NEC is amended, altered and may even be rejected in lieu of regional regulations as voted on by local governing bodies.
A distribution board is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure. Normally, a main switch, and in recent boards, one or more residual-current devices (RCDs) or residual current breakers with overcurrent protection (RCBOs) are also incorporated.
AC power plugs and sockets connect devices to mains electricity to supply them with electrical power. A plug is the connector attached to an electrically-operated device, often via a cable. A socket is fixed in place, often on the internal walls of buildings, and is connected to an AC electrical circuit. Inserting the plug into the socket allows the device to draw power from this circuit.
IEC 60309 is a series of international standards from the International Electrotechnical Commission (IEC) for "plugs, socket-outlets and couplers for industrial purposes". They are also referred to as "pin & sleeve" connectors in North America or as "CeeForm" connectors in the entertainment industry. The maximum voltage allowed by the standard is 1000 V DC or AC; the maximum current, 800 A; and the maximum frequency, 500 Hz. The ambient temperature range is −25 °C to 40 °C.
Industrial and multiphase plugs and sockets provide a connection to the electrical mains rated at higher voltages and currents than household plugs and sockets. They are generally used in polyphase systems, with high currents, or when protection from environmental hazards is required. Industrial outlets may have weatherproof covers, waterproofing sleeves, or may be interlocked with a switch to prevent accidental disconnection of an energized plug. Some types of connectors are approved for hazardous areas such as coal mines or petrochemical plants, where flammable gas may be present.
In electrical engineering, ground and neutral are circuit conductors used in alternating current (AC) electrical systems. The neutral conductor receives and returns alternating current to the supply during normal operation of the circuit; to limit the effects of leakage current from higher-voltage systems, the neutral conductor is often connected to earth ground at the point of supply. By contrast, a ground conductor is not intended to carry current for normal operation, but instead connects exposed metallic components to earth ground. A ground conductor only carries significant current if there is a circuit fault that would otherwise energize exposed conductive parts and present a shock hazard. In that case, circuit protection devices may detect a fault to a grounded metal enclosure and automatically de-energize the circuit, or may provide a warning of a ground fault.
A camlock or cam-lock is an interchangeable electrical connector, often used in temporary electrical power production and distribution, predominantly in North America. Originally a trade name as Cam-Lok, it is now a generic term. Each camlock connector carries a single phase, pole, or conductor; multiple camlock connectors will be used to make a complete electrical supply or circuit.
AS/NZS 3112 is the harmonised/joint Australian and New Zealand Standard for AC power plugs (male) and sockets (female). The standard is used in Australia, New Zealand, Fiji, Tonga, Solomon Islands, Papua New Guinea and several other Pacific island countries. In Argentina and China an almost-identical design is also used, but these are separate standards, despite their appearance. The International Electrotechnical Commission (IEC) "world plugs" website defines this design as Plug Type I'.
A stage pin connector, also known as a grounded stage pin (GSP), grounded pin connector (GPC) or theater paddle (TP), is a standard cable type for theatrical lighting in North America and in many countries in the theatre world.
An automobile auxiliary power outlet in an automobile was initially designed to power an electrically heated cigarette lighter, but became a de facto standard DC connector to supply electrical power for portable accessories used in or near an automobile directly from the vehicle's electrical system. Such include mobile phone chargers, cooling fans, portable fridges, electric air pumps, and power inverters.
An electrical outlet tester, receptacle tester, or socket tester is a small device containing a 3-prong power plug and three indicator lights, used for quickly detecting some types of incorrectly-wired electrical wall outlets or campsite supplies.
Socapex is a brand of electrical connectors, known in the entertainment industry primarily for their 19-pin electrical connectors, commonly known as Socapex connectors, and used in film, television, and stage lighting to terminate the ends of a multicable. They are wired with six hot/live pins, six neutral pins, six ground/earth pins, and a final central pin used to aid alignment of the male end of the connector with a female receptacle. The Socapex was first created by a company called Socapex in 1961, which later on became Amphenol Socapex. "Socapex" became a brand name owned by Amphenol Socapex, the term "Soca" is now often applied to similar off-brand connectors as a genericized trademark.
A cheater plug, AC ground lifter or three-prong/two-prong adapter is an adapter that allows a NEMA 5-15P grounding-type plug to connect to a NEMA 1-15R non-grounding receptacle. They are needed to allow appliances with 3-wire power cords to plug into legacy ungrounded receptacles found in older buildings. The use of such an adapter avoids the need to replace receptacles, but is potentially hazardous if the grounding tab is not connected to electrical ground. These adapters are illegal in some jurisdictions, in particular throughout Canada. A safer and more reliable alternative identified in the US and Canadian electrical codes is to replace the outlet with a Ground Fault Circuit Interrupter (GFCI) breaker outlet.
In building wiring installed with separate neutral and protective ground bonding conductors, a bootleg ground is a connection between the neutral side of a receptacle or light fixture and the ground lug or enclosure of the wiring device.
Hubbell Incorporated, headquartered in Shelton, Connecticut, is an American company that designs, manufactures, and sells electrical and electronic products for non-residential and residential construction, industrial, and utility applications. Hubbell was founded by Harvey Hubbell as a proprietorship in 1888, and was incorporated in Connecticut in 1905.
There are approximately 20 types in common use around the world, such as AC power plugs and sockets, and many obsolete socket types which are still found in older buildings.
References
↑ CSA Standard C22.2 No. 42-10: General use receptacles, attachment plugs, and similar wiring devices. Mississauga, Ontario: Canadian Standards Association. 2010. p.1.
↑ U.S. patent 0,251,554 (applied 1881, awarded 1881) Two examples of screw system appear, including the kerosene-can lid, but their overall formations are incompatible with today's Edison sockets. No explicit claims for the screw system. Beware patents.google.com's preview mode lacks several pages, must download full PDF.
↑ Schroeder, Fred E. H. (1986-01-01). "More "Small Things Forgotten": Domestic Electrical Plugs and Receptacles, 1881-1931". Technology and Culture. 27 (3): 525–543. doi:10.2307/3105384. JSTOR3105384. S2CID112928824.
↑ [Harvey Hubbell Inc. V. General Electric Co. et.al. (262 Fed.Rep. 155) District Court of the United States, Southern District of New York, October 20, 1919. Reported in "The Trade Mark Reporter" Vol 10, 1920, Pages 164–175. The court observed that Hubbell's plugs were readily distinguishable from those of other makes by the provision of "nicks" in the pins of Hubbell's plug pins, designed to provide retention when mated with Hubbell's receptacles, so there was no confusion as to their origin. The report of the trial is available from Google Books here: https://books.google.co.uk/books?id=XOgsAAAAYAAJ&printsec=frontcover&dq=the+trademark+reporter+volume+10&hl=en&sa=X&ved=0ahUKEwjHgZr29YvXAhUCSRoKHWQaDOAQ6AEIJjAA#v=onepage&q&f=false
↑ C22.1-15—Canadian Electrical Code, Part I: Safety Standard for Electrical Installations (23rded.). Canadian Standards Association. 2015. Rule 24–106(6). ISBN978-1-77139-718-6.
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U.S. patent 774,250 , fig. 1, illustrating a lightbulb socket adaptor and an attachable flat blade socket for it.
U.S. patent 1,064,833 , fig. 1 and 3. First patent showing the parallel blades which became NEMA 1-15.
U.S. patent 1,180,648 fig. 4 and 5, showing a concept for polarized receptacle and plug.
