Yak99

Existing page NEMA connector is in need of a re-write. I've been putting this off for some time. I intend to edit for style first and add citations later.Yak99 (talk) 14:33, 2 July 2011 (UTC)

ANSI/NEMA WD-6 is the standard for alternating current power connectors rated between 125 and 600 volts in North America. Previous editions of the standard were known as ANSI C73, or ASA C73. The current NEMA standard has a different plug configuration for each voltage and phase configuration commonly used in North America, and for each ampere rating. Voltage ratings range from 125 to 600 volts, and ampere ratings range from 15 to 60 amperes. Each configuration is designed to be incompatible with devices of a different rating, although there are a few instances of deliberate overlap.

The current iteration of the ANSI/NEMA standard was mostly developed in the 1960s. Before this decade, the number of non-interchangeable devices available was much smaller. The resulting overlap in usage on different types of circuits caused accidents.

NEMA Nomenclature

NEMA devices are classified using the alpha-numeric formula A-##-##-A. The first numeral designates the circuit type; e.g. two-pole two-wire non-grounding, two-pole three-wire grounding, or three-pole three-wire grounding. The second numeral is the ampere rating of the device, which may be 15, 20, 30, 50, or 60. The "L" prefix, if present, denotes a locking device. The "R" suffix indicates a receptacle, and the "P" suffix indicates a plug.

Straight-blade vs. locking

Most NEMA devices are of the straight-blade type, having flat blades for the current-carrying conductors, and round or U-shaped pins for the equipment grounding conductor. An alternative is the locking type, which has two or more blades tracing the same radius. One blade of a locking connector (generally either the neutral or the ground) will have an L-shaped crook in it which keys the connector and locks it when given a clockwise twist. Locking devices are often referred to as "Twist-Locks". This term is a registered trademark of Hubbell, Inc. The alpha-numeric nomenclature came into use sometime in the 1960s. Although anachronistic, this article will refer to the alpha-numeric numbers for earlier devices where possible.

Grounding vs. non-grounding

Most NEMA configurations are of the grounding type, having one contact that is not intended to carry current during normal operation. The first grounding configurations to be introduced were the 5-15 and 6-15, which were standardized in 1951. Installation of 5-15R or 5-20R receptacles on general-use 125V circuits was made mandatory in 1962, and other standards followed.

Equipment requiring a grounding plug

• Refrigerators, freezers, and air conditioners
• Clothes washers, dryers, kitchen waste disposers, sump pumps and aquarium equipment
• Handheld and fixed power tools, unless double-insulated
• Computers and peripherals, unless double-insulated
• Equipment operating on systems exceeding 150V to ground
• Equipment operating in hazardous (flammable or explosive) atmospheres

In addition, any appliance with a metal frame is likely to require a grounding connection.

Equipment not requiring a grounding plug

• Portable table and floor lamps, unless used in healthcare facilities
• Double-insulated equipment
• Small residential kitchen appliances with exposed heating elements, such as toasters

Toasters are exempted because a grounding connection would actually increase the hazard if a metal object were inserted into an energized appliance.

Non-grounding configurations

Most of the non-grounding configurations are the legacy of the pre-1951 standard, and are seldom used today. Exceptions are the 1-15, 10-30, and 10-50 series, which remain in widespread use.

General-use plugs

The vast majority of electrical devices in North America are equipped with one of two straight-blade NEMA plugs, depending on grounding requirements.

1-15P

This plug was chosen as the standard by an industry committee in 1916, yet it took until about 1921 for it to beat out all competitors. There are two main variations. The non-polarized version has two parallel blades of the same width, while the the polarized version has one wider blade. Although polarized receptacles have been widely deployed since the mid-1920s, polarized plugs were uncommon before 1978. The wider blade is intended to be connected to neutral, and is required on lamps with screw-shell sockets, as well as devices with integrated receptacles or single-pole line switches. In recent years, appliances with none of these characteristics have been sold with polarized plugs. The 1-15P is known to international travelers as the Type A.

5-15P

This grounding plug was introduced about 1949, and was adopted as standard in 1951. It did not enter widespread use until after 1959, when a change in safety standards required certain appliances and handheld power tools to be so equipped. The list of appliances requiring grounding was significantly expanded in 1969.

Like all grounding plugs, the ground pin is slightly longer than the current-carrying blades, so that it makes contact first, and breaks contact last. The ground pin is either round or U-shaped, so that it cannot be accidentally inserted into an energized receptacle slot. As the 5-15P was the first grounding plug to be defined in the standard, all subsequent straight-blade grounding plugs have incorporated these design characteristics.

The National Electrical Code began requiring grounding receptacles in all buildings beginning in 1962, and even today there are many receptacles in older buildings which will not accept a 5-15P grounding plug. In these locations, cheater plugs continue to be widely employed, despite the hazard. The 5-15P is known to international travelers as the Type B.

General-use receptacles

In the United States, all buildings built after local adoption of the 1962 National Electrical Code are equipped with 5-15R, 5-20R, or both. Older buildings, particularly homes, may still be equipped with the older non-grounding types. All of these receptacles are rated 125V. The standard voltage in the US and Canada is 120V, with a tolerance of ±5%.

5-15R

This receptacle is overwhelmingly the most common type in Canada and the United States. It is the only type that may be used on 15A circuits in new construction. In the US, it may also be used on 20A circuits, where there is more than one receptacle on the circuit. This receptacle will accept any 1-15P, polarized or not, as well as the 5-15P. Appliances sold on the North American market are commonly designed to the large installed base of 5-15R receptacles. For this reason, continuous loads, such as air conditioners and space heaters, are usually designed to draw 12 amperes or less, or 80% of the circuit rating. Non-continuous loads, such as hair dryers, are allowed to draw up to 15 amperes.

The 5-15R and 5-20R must have the grounding contact connected to the grounding electrode via a separate conductor which is not part of the circuit, and installed in the same cable or conduit with the circuit conductors. An exception is made for retrofits in pre-1962 circuits where the circuit is protected by a ground-fault circuit interrupter. Unfortunately, there are many illegal and dangerous retrofits that lack either of these safety features.

In Mexico, widespread adoption of the 5-15R is comparatively much more recent, and many receptacles may not have the grounding contact connected.

5-20R

This receptacle may only be used on a 20A branch circuit. In addition to the 1-15P and the 5-15P, the 5-20R can also accept the 5-20P. Where a 20A branch circuit has only one outlet, the receptacle must have a 20A rating; thus, this receptacle is often installed for large appliances such as refrigerators and dishwashers, even though the appliance is usually equipped with a 1-15P. Also, some jurisdictions require commercial buildings to be equipped with 20A receptacles, possibly to deter contractors from installing the cheapest grade of devices.

The 5-20P is quite rare, although it is sometimes found on heavy-duty contractors' tools, such as core-drilling machines and electricians' hot boxes. It is also found on some commercial kitchen appliances. Rental equipment fitted with this type of plug is frequently mutilated in an attempt to get the plug to fit a 5-15R.

Canadian 5-20R

For a time, the Canadian Electrical Code did not recognize the US practice of allowing 15A equipment on a 20A circuit. Thus, the Canadian version of the 5-20R lacks parallel slots, and will not accept a 1-15P or a 5-15P. In recent years, however, the US practice has found acceptance in Canada. The Canadian equivalent of the US 5-20R is called a 5-20RA.

1-15R

This receptacle remains quite common in homes and some commercial buildings built before 1962. The vast majority installed since the 1930s will accept polarized plugs, and all will accept non-polarized plugs. Although prohibited in new construction, the 1-15R still does appear as part of installed equipment, for example, in the housings of bathroom exhaust fans. The 1-15R is still sold for replacement use in older buildings.

Non-NEMA double T-slot

This receptacle dates back to 1915, and was dropped from the NEMA standard in the 1950s or 1960s. Its appearance, and usually its voltage rating, cause much confusion, since it will accept both the 1-15P and the 250V 2-15P. The reason for the confusion is that the two plugs were commonly used on 125V systems until the 2-15P fell from favor in the 1920s. By the time the 2-15P was reclassified as a 250V device in the 1950s, it was likely already obsolete.

There are millions of double T-slot receptacles in use today. They are probably about as common as the 1-15R, and the vast majority will accept a polarized plug.

Non-grounding NEMA 10 devices

The NEMA 10 series is a legacy of the pre-1950 standards. There are 20A, 30A, and 50A versions. All are rated 125/250V, single-phase, and are intended for split-phase applications. NEMA 10 devices are recognizable by the angle of their blades which creates a "crow's foot" shape, although some other configurations share this characteristic. The neutral blade is slightly longer, for a first-make, last-break connection. There is no grounding contact.

Ranges and clothes dryers

The most common use of NEMA 10 is for connection of household electric kitchen ranges and clothes dryers. The 10-30R is commonly found for dryers, and the 10-50R for ranges. The National Electrical Code began allowing this type of connection in 1942, but since 1996, it is no longer legal for new construction, and NEMA 14 devices are used instead.

Most North American ranges and dryers have heating elements that operate at 240V, and other components, such as lights, timers, and motors, operating at 120V. These appliances require three conductors to operate; hence, when connected using a three-wire plug, the frame of the appliance is grounded through the neutral. This is considered a TN-C grounding system. In these installations, the neutral does carry current, and a broken neutral can give the user a shock, although this is fairly rare. This method of grounding was unique to these appliances, and continues to cause much confusion.

Cord installation

New ranges and dryers are commonly shipped without a cord, requiring the user to select the correct cord for their installation. To conform to the NEMA 10 grounding standard, a conductive jumper must be installed between the neutral terminal and the appliance frame. Appliances formerly shipped with the jumper connected, but in recent years they are shipped disconnected. To convert a NEMA 14 appliance to NEMA 10, the jumper must be added, and to convert from NEMA 10 to NEMA 14, it must be removed. This process is not standardized among manufacturers, and many DIY books and websites omit this advice. Consumers who do not fully understand this grounding issue should leave this conversion to a qualified professional.

Cords should always be selected to match the existing receptacle, rather than the other way around, unless an upgrade to NEMA 14 is considered.

Obsolete use

Other uses of the NEMA 10 series are effectively obsolete, although all of the NEMA 10 devices are still in production. When they were used, there was no definitive standard for wiring. A 10-20R, for example, may have been wired 125/250V without ground, 125V with ground, or 250V with ground, or even three-phase without ground. This lack of standardization was the impetus behind the improved NEMA standards.

The 10-20R is occasionally found in living rooms of homes that once had a large window air conditioner, if the home has been upgraded to central air conditioning.

This 15A device is dimensionally identical to the AS 3112. It is occasionally found in laundry rooms of homes that were wired to comply with the 1947 National Electrical Code, before the standardization of the 5-15R. This receptacle may also be found wired for 250V.

Grounding NEMA 14 devices

The NEMA 14 series, introduced in the mid-1960s, is the modern replacement for NEMA 10. It is rated 125/250V and intended for split-phase applications. Unlike NEMA 10 devices, however, the NEMA 14 devices have a fourth contact for grounding. The straight-blade 14-30R and 14-50R are commonly used for dryers and ranges, respectively. Where a NEMA 14 device is used, the appliance must have no internal connection between neutral and the appliance frame.

A NEMA 10 installation may be upgraded to NEMA 14 only if the existing branch circuit wiring has an insulated neutral conductor and an adequately-sized equipment grounding (bonding) conductor. This judgment is best left to a qualified electrician.

The 14-50R is also found in recreational vehicle parks, as larger RVs are designed to plug in to this type of receptacle. This connection was formerly common for mobile homes as well.