NEMA connector

Terminology

The terms used to describe electrical connectors and their parts are normally those included in IEC 60050, the International Electrotechnical Vocabulary, published by the International Electrotechnical Commission (of which the US is a full member). IEC 60050 is also available (no charge) as IEV Online. However, NEMA, the NFPA National Electrical Code, the National Electrical Contractors Association's NECA 130 Standard for Installing and Maintaining Wiring Devices and this article use some terminology which is not covered in IEC 60050 and may not be familiar to non-residents of North America.

The term plug is in general and technical use in all forms of English, however the term receptacle does not appear in IEC 60050 and the internationally accepted term is socket-outlet. Similarly, blade meaning the contact of a plug is not normally used outside North America, although blade contact is defined in IEC 60050, however the normal English term used in connection with mains plugs is pin.

Nomenclature

Most NEMA connectors are named following a simple alphanumeric code consisting of: a numeral preceding a hyphen, a numeral following the hyphen, and letters at the beginning and end of the code to indicate whether the connector is a locking type and whether it is a plug (male connector) or the corresponding receptacle (female connector).

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; others without a leading 'L' are straight blade and non-locking. Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required.

The numeral preceding the hyphen indicates the type's particular configuration; that is to say the number of poles (current-carrying terminals) and wires connected to it, the voltage meant to be used with, and whether for use with single- or three-phase power. A connector with ground terminal is described as having more wires than poles, e.g. two-pole, three-wire; or four-pole, five-wire; etc. A non-grounding device may be two-pole, two-wire; three-pole, three-wire; etc.

The numeral following the hyphen is the current rating of the device in amperes. This number is followed by the letter 'R' to indicate a receptacle or 'P' to indicate a plug.

As an example, the 5-15R is the common 125 V two-pole, three-wire receptacle rated for 15 A. 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 125 V rating, but is rated for 30 A.

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. 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.

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'.

Non-locking connectors

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 120 V 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 don't require grounding (earthing).

All NEMA 1 devices are two-wire non-grounding devices (hot-neutral) rated for 125 V maximum. 1-15P plugs have two parallel flat blades, ¹⁄₄ inch (6.35 mm) wide, 0.06 inches (1.524 mm) thick, ⁵⁄₈–²³⁄₃₂ inch (15.875–18.256 mm) long, and spaced ¹⁄₂ inch (12.7 mm) 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. 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 ⁵⁄₁₆ in or 7.938 mm 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.

The Japanese plug and socket with narrow faces appear physically identical to NEMA 1-15. The Japanese system incorporates stricter dimensional requirements for the plug housing, different marking requirements, and mandatory testing and approval by METI or JIS.

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.

NEMA 2

All NEMA 2 devices are two-wire non-grounding devices (hot-hot) rated for 250 V 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).

NEMA 5

All NEMA 5 devices are three-wire grounding devices (hot-neutral-ground) rated for 125 V 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 (4.763 mm) diameter round or U-shaped ground pin, ¹⁄₈ in (3.175 mm) longer than the power blades (so the device is grounded before the power is connected) and located from them by ¹⁄₄ in (6.35 mm) edge-to-edge or ¹⁵⁄₃₂ in (11.91 mm) 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 ¹⁄₂ in (12.7 mm) 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 1 in (25.4 mm) between power pins; 5-30 also has an L-shaped neutral blade. 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. If the ground pin is removed to make it fit a 1-15R receptacle or extension cord, the line/neutral polarity is lost.

The 5-15R is by far the most common electrical receptacle in North America in buildings built since the mid-twentieth century. It is usually installed in a twin duplex configuration (which may be on a common circuit, or alternatively wired with each receptacle on a separate circuit, sometimes switched).

In 46 of the 50 United States and all of Canada, tamper-resistant receptacles are required in new residential construction as of November 2013. These prevent contact by objects like keys or paper clips inserted into the socket. This is accomplished by an interlocking mechanism that needs to have both 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. (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) is called a "stinger". Generally, lighting technicians use these extension cords to deliver power to lights rated at 2000 W or less.

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.

NEMA 6

All NEMA 6 devices are three-wire grounding devices (hot-hot-ground) used for 208 and 240 V circuits and rated for 250 V 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 ²³⁄₃₂ in (18.256 mm) center-to-center. The 20 A plug has a blade rotated 90°, and the 6-20R receptacle may have 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).

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.

The 30 A plug and receptacle look similar to the 15 A one but larger. The higher-current versions are rare, with twist-locking plugs such as L6-30 or direct wiring more common. Generally 6-series non-locking plugs are used for such appliances as large room air conditioners, commercial kitchen equipment, and the occasional home arc welder. Single-phase 6-50 is commonly used on farms for silo unloaders, and is used with a 6-gauge flexible power cord up to 200 ft (61 m) long. Some manufacturers of electric vehicle charging stations equip their 30-40 A Level 2 EVSEs with a 6-50 plug on a short cord, though it is becoming less common, with manufacturers now favoring the more common 14-50 plug.

NEMA 6 devices, while specified as 250 V, may be used for either 208 or 240 V 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 rooms is typically supplied with three-phase 208 V.

NEMA 7

NEMA 7 devices are 2-pole and ground connectors rated at 277 V. The 15 A 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 neutral 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 10

NEMA 10 connectors are a now deprecated type that had formerly been popular in the United States for use with high-wattage electric clothes dryers and kitchen ranges. They are classified as 125/250 V non-grounding (hot-hot-neutral), and were designed to be used in a manner that indirectly grounds the appliance frame, though not in a manner consistent with most modern practice. The older practice 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 1996 editions. Since North American dryers and ranges have certain components (timers, lights, fans, etc.) that run on 120 V, 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.

NEMA 14

The NEMA 14 devices are four-wire grounding devices (hot-hot-neutral-ground) available in ratings from 15 to 60 A. The voltage rating is 250 V. Of the straight-blade NEMA 14 devices, only the 14-30 and 14-50 are in common use. The 14-30 is used for electric clothes dryers, the 14-50 is used for electric cooking ranges, and either may also be used for home charging of electric vehicles. 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 240 V when supplied by split-phase power, or 120 and 208 V if the supply is three-phase. The 14-30 has a rating of 30 A, and an L-shaped neutral blade. The 14-50 has a rating of 50 A, 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 Tesla Motors's Mobile Connector for vehicle charging, which recommends the installation of a 14-50 receptacle for home use.

NEMA 15

NEMA 15 connectors are similar in size and appearance to the 14-series, however they are meant for use with three-phase power up to 250 V, with no neutral conductor (hot-hot-hot-ground).

NEMA 18

NEMA 18 connectors are similar to 14 and 15-series devices, but for 120Y208 V three-phase with no ground conductor (hot-hot-hot-neutral).

NEMA 24

NEMA 24 connectors are two-pole and ground connectors for 347 V applications. These connectors are almost entirely confined to use in Canada, where 347Y600 V three-phase power is commonly available in commercial buildings and 347 V (single-phase) is used in lighting circuits.

NEMA TT-30

The NEMA TT-30 (TT stands for Travel Trailer) connector is a 120 V 30 A 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.

The appearance of this plug is sometimes confused with a NEMA 10 connector, rated for 240 V, but the NEMA TT-30 is a 120 V device. The hot and neutral blades are angled at 45° from vertical and 90° to each other, similar to NEMA 10 devices, but 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.

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, 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. Many specific types exist; only a few are listed below. Other types include special purpose connectors for boats, 400 Hz 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 15 A applications where a larger locking connector would not fit. ML1 connectors are two-pole, no ground, rated for 125 V. ML2 connectors are two-pole and ground, rated for 125 V. ML3 connectors are for three-pole no ground (hot-neutral-hot) rated for 125/250 V.

NEMA L5

NEMA L5 connectors are a series of two-pole and ground locking connectors rated for 125 V. L5-30 receptacles are common at marinas where power is provided for their guests.

NEMA L6

NEMA L6 connectors are rated for 250 V. They are intended for two-pole, three wire hot-hot-ground circuits with a nominal supply voltage of 240 or 208 V, depending on phase configuration. The L6 connector does not provide a neutral connection.

L6-20 and L6-30 connectors are commonly found on in-rack power distribution units in countries where the mains supply voltage is greater than 120 V. They are also found in the US for heavy-duty 240 V equipment such as welders, where the higher supply voltage allows a lower current draw. These connectors are thus found where industrial equipment or large power tools are commonplace.

NEMA L7

NEMA L7 are two-pole and ground connectors rated for 277 V. 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 480 V. Intended for three-wire hot-hot-ground circuits.

NEMA L9

NEMA L9 are two-pole and ground connectors rated for 600 V. Intended for three-wire hot-hot-ground circuits.

NEMA L14

NEMA L14 are three-pole and ground connectors rated for 125/250 V. Intended for three-pole, four-wire hot-hot-neutral-ground circuits with a nominal supply voltages of 240 or 208 V hot-to-hot and 120 V 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 250 V. Intended for three-phase circuits.

NEMA L16

NEMA L16 are three-pole and ground connectors rated for 480 V. Intended for three-phase circuits.

NEMA L17

NEMA L17 NEMA L16 are three-pole and ground connectors rated for 600 V. Intended for three-phase circuits.

NEMA L18

NEMA L18 are four-pole no ground connectors rated for 120/208 V. Intended for wye three-phase circuits.

NEMA L21

NEMA L21 are four-pole and ground connectors rated for 120/208 V. 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 L22

NEMA L22 are four-pole and ground connectors rated for 277/480 V. 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 347/600 V. 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

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.

Ground fault circuit interrupter (GFCI) receptacles

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). In the US and Canada they are required in many potentially wet locations, including outside outlets, bathrooms, some places in kitchens, basements, and crawl spaces. They work by comparing the current going out on the hot conductor to the current returning on the neutral conductor, and disconnect the circuit if the difference exceeds 4-6 milliamperes. They are cheaper than GFCI circuit breakers and can be wired to feed additional "downstream" receptacles so that putting one GFCI receptacle in a circuit protects all the plugs, lights, and switches downstream from it. They are also recommended for power tool outlets and locations where children might insert conductive objects into the receptacles.

Tamper-resistant receptacles

Starting with the 2008 National Electrical Code and the 2009 Canadian Electrical Code, 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, yet prevents shock hazards to a child that sticks a single, metal object into the receptacle. The new receptacles are expected to reduce the number of electric shocks to children because inserting a normal, two-blade electrical plug applies pressure on both sides of the receptacle to open an internal, spring-loaded shutter, but a foreign object fails to do so and therefore does not make contact with the live electrical contacts. 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.

AFCI receptacles

The National Electrical Code has been updated for 2014 and it addresses the use of Outlet Branch Circuit (OBC) Arc Fault Circuit Interrupter (AFCI) Receptacles as an alternative to breakers when used for modifications/ extensions, as replacement receptacles or in new construction. AFCI Receptacles work to address the dangers associated with potentially hazardous arcing conditions (parallel arcs and series arcs) by interrupting power to help prevent dangerous arc-faults that may lead to an electrical fire. 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. They are 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. When installed as the first receptacle on a branch circuit, AFCI receptacles provide series arc protection for the entire branch circuit. They also provide parallel arc protection for the branch circuit starting at the AFCI receptacle.

Surge protective receptacles

Surge protective devices are designed to reduce the random energy surges of voltage transients and electrical noise on the power supply line that can damage sensitive electronics such as TVs, computers, and smart appliances. They are available for 120 V, 15/20 A 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 of UV stabilized thermoplastic with high cold impact resistance to withstand the elements. Required by the 2008 National Electrical Code in outdoor damp or wet locations, WR receptacles should be installed in patio, deck, and pool areas. They are available in a variety of models including GFCI and tamper-resistant. For added protection, WR receptacles should be protected by Extra-Duty While In-Use or Weather-Resistant Covers. These covers are ruggedly constructed to seal out moisture (dripping or condensing), dust, debris, and insects, while providing easy access to receptacles to allow homeowners to use power tools, trimmers, sprinkler systems, and pumps with confidence and improved safety.

Leak-current detection and interruption (LCDI) cordsets

Damaged power cords of portable air conditioners cause many electrical fires and about 350 deaths per year. To combat this, each portable air conditioner sold in the United States is now required to have a leakage current detector interrupter (LCDI) or 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. 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, typically molded-on plugs. The protection device is equipped with "Test" and "Reset" buttons on the housing.

Color code

The color of a device neither identifies its voltage class nor power system. Because the colors are not identified by national standards, the purpose of color-coding a receptacle is set by the building owner. In this case, building owners may select brown, ivory, white, almond, grey, and 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:

Break-away tabs

Most duplex receptacles have metal tabs connecting the top and bottom receptacles. These tabs are pre-scored so that they can be broken off to allow the top and bottom receptacles to be wired onto separate circuits. 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", or "half-split".

Neglecting to break off the tabs when replacing a split receptacle can disable a switched outlet, or cause a short circuit, or accidentally parallel two circuits, depending on how the receptacle had been wired.

Original Hubbell plug and receptacle

An early American electrical plug and receptacle was invented by Harvey Hubbell and patented in 1904. Hubbell's first design was a receptacle which screwed into a lampholder (like the early lampholder plugs), but with a separable plug with pins or blades (U.S. Patent 774,250). The 1906 Hubbell catalog shows this plug with a flush mounting receptacle for use in wall or floor. Other manufacturers adopted the Hubbell pattern, and by 1915 they were widespread. In 1913, Hubbell patented a parallel blade plug (U.S. Patent 1,064,833). In 1916, Hubbell received a further patent for a polarized version of the parallel blade plug where one blade was both longer and wider than the other (U.S. Patent 1,180,648). In the modern polarized version of NEMA 1-15, both blades are the same length, and only the width varies.

American 125 V, 15 A / 250 V, 10 A ("Australian" style)

A patent for this obsolete plug and receptacle was filed in 1915 under U.S. Patent 1,179,728. It predated the NEMA receptacle and plugs, but was similar in function to NEMA 10-20. The plugs and receptacles used in countries such as Argentina, Australia, and China are based on this type, and are physically compatible.

NEMA 1-15 style quintuplex receptacle

This is a five-way receptacle from c. 1928, and is able to accept modern ungrounded polarized NEMA 1-15P plugs. The receptacle is obsolete, as the current NEMA standard only allows a maximum of three receptacles in a single-gang receptacle box.

NEMA 1-15 "lightswitch" style receptacle

This is a rare lightswitch style 1-15 receptacle. It has one vertical receptacle, and it can accept modern ungrounded NEMA 1-15 plugs. It is obsolete as it has little insulation, thus is a minor fire hazard when used with high-wattage devices.

Related standards

The dimensions and configurations for NEMA connectors are given in ANSI/NEMA standard WD-6. Underwriters Laboratories maintains UL Standard 498, 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 and its associated specification sheets. This specification references WD-6 and UL 498, and provides additional durability and electrical safety performance criteria for connectors intended for military use.