NEMA connector
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NEMA connectors are connectors used for mains electricity (in North America and other countries) that conform to the standards set by the National Electrical Manufacturers Association. Some types are found in nearly all buildings in the United States. Similar and interchangeable connectors are used in Canada and other countries using the same type of receptacle, although there are some exceptions. NEMA wiring devices are made in current ratings from 15 to 60 A, and electrical potential (voltage) ratings from 125 to 600 V. Different combinations of contact blade widths, shapes, orientation, and dimensions give non-interchangeable connectors that are unique to a particular voltage, current capacity, and grounding system. The dimensional standard for electrical connectors is ANSI/NEMA WD–6 and is available from the NEMA Web site. [1]
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[edit] NEMA nomenclature
There are two basic classifications of NEMA device: straight-blade and locking. The straight-blade 5–15 and 5–20 are found nearly everywhere in countries using the NEMA standards, and are intended for supplying lighter-duty, general-purpose electrical devices. Twist-locking types are used for heavy industrial and commercial equipment, where increased resistance to accidental disconnection is required. Numbers prefixed by L are curved-blade, twist-locking connectors; others are straight blade and non-locking.
The numeral preceding the hyphen in NEMA nomenclature indicates the configuration, that is, the number of poles, number of wires, voltage, and whether single- or three-phase. A grounding type of device will be described as two-pole, three-wire; or four-pole, five-wire; etc. A non-grounding device will be two-pole, two wire; or three-pole, three-wire; etc.
The numeral following the hyphen is the rating of the device in amperes. The number is followed by the letter R to indicate a receptacle (female connector) or the letter P to indicate a plug (male connector).
As an example, the 5–15R is the common 125 V two-pole, three-wire receptacle. The L5–15R, while sharing the same rating, is a locking design which is not physically compatible with the straight-blade 5–15 design. The 5–30 has the same two-pole, three-wire configuration and 125 V rating, but is rated 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.
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]
[edit] 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) and the plugs can be detached from the receptacles by pulling back on the plug body. The connector families have been designed so that grounding connectors for 120 V and 208/240 V cannot be accidentally intermated.
NEMA wall receptacles can be found installed in any orientation. Neither NEMA, U.S. National Electrical Code nor Canadian Electrical Code specify a preferred orientation, and different orientations may be found in the same building. However, the orientation with the ground pin on the bottom is the most common, which places the neutral pin on the upper left and the hot pin on the upper right. All descriptions below assume this orientation.
[edit] NEMA 1
All NEMA 1 devices are two-wire non-grounding devices rated for 125 V maximum. NEMA 1-15P plugs have two parallel flat blades, 1/4 inch wide, 0.06 inch thick, 5/8–23/32 inch long, and spaced 1/2 inch apart, and are commonly found on household lamps and consumer electronics such as clocks and radios. 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; 1–20P and 1–30P mate with a corresponding NEMA 5 receptacle. Plugs and receptacles may be polarized to preserve the identity of the neutral conductor; the blade connected to neutral will be wider (5/16 inch) than the other, and polarized sockets will only accept a polarized plug in the correct orientation.
[edit] NEMA 2
All NEMA 2 devices are two-wire non-grounding devices rated for 250 V maximum. Although standards exist for 2–15, 2–20 and 2–30, this series is obsolete, and no such devices are currently manufactured. A 2–15P, if one could be found, would plug into a 6–15R or 6–20R (240 V), or a “T-slot” (120 V), with equal ease.
[edit] NEMA 5
All NEMA 5 devices are three-wire grounding devices 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 diameter round or U-shaped ground pin, 1/8 longer than the power blades and located 1/4 inch below them. (1/4 inch is edge-to-edge; 15/32 inch center-to-center.) The 20 A plug has the neutral blade rotated 90° and shifted so its inner edge is approximately 1/2 inch from the hot blade. The 5–20R receptacle may have a T-shaped neutral hole, to accept both 5–15P and 5–20P plugs. The 5–30 and 5–50 are uncommon, as twist-locking plugs are generally used for high-current applications. They are larger, with 1 inch between power pins.
The 5–15R is by far the most common electrical outlet in North America in buildings built since the mid-twentieth century. It is usually installed in a duplex configuration (which may be on a common circuit or with each receptacle on a separate circuit, sometimes switched).
The standard 5–15 (and its two prong counterpart) is sometimes called an “Edison plug” in the film and theatre industry, where it distinguishes it from more specialized theatrical connectors. This is perhaps a reference to the Consolidated Edison Company of New York, which supplies power to most New York theatres.[citation needed] In the photography industry, the 5-15 is known as a "Stinger."
[edit] NEMA 6
All NEMA 6 devices are three-wire grounding devices used for 208 V and 240 V circuits and rated for 250 V maximum, with the 6–15, 6–20 and 6–30 being grounded 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 inch center-to-center. The 20 A plug has the neutral blade rotated 90°, and the 6–20R receptacle may have a T-shaped neutral hole, to accept both 6–15P and 6–20P plugs. The 30 A plug and socket look similar to the 15 A one but larger. The higher-current versions are rare, with twist-locking plugs and 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.
NEMA 6 devices, while specified as 250 V, may be used for either 208 V or 240 V circuits. Visitors to North America should not regard the NEMA 6–20R or 6–30R found in many hotel rooms as a source of 240 V power, since in most cases, they are supplied with 208 V.
[edit] NEMA 10
NEMA 10 devices are a curious throwback to an earlier time. They are classified as 125/250 V non-grounding, yet they are usually used in a manner that effectively grounds the appliance, though not in a manner consistent with most modern practice.
As commonly used, 10–30 and 10–50 plugs have the frame of the appliance grounded through the neutral blade. This was a legal grounding method under the National Electrical Code for electric ranges and electric clothes dryers from the 1947 to the 1996 edition. Since North American dryers and ranges have certain parts (timers, lights, fans, etc.) that run on 120 V, this means that the wire used for grounding is also carrying current. Although this is contrary to modern grounding practice, such installations remain common in the United States and are relatively safe, because the larger conductors used are less likely to be broken than the smaller conductors used in ordinary appliance cords, and the current carried is small.
Persons moving their older appliances to newer NEMA-14-equipped buildings (or vice-versa) should have the cords replaced by a qualified electrician.
NEMA 10–20 devices are very rare nowadays. There is also a similar obsolete design, lacking a NEMA configuration number, rated 125 V, 15 A / 250 V, 10 A which is nearly identical to the AS/NZS 3112 standard used in Australia/New Zealand. These are also extremely rare.
[edit] NEMA 14
The NEMA 14 devices are four-wire grounding devices available in ratings from 15 A to 60 A. Of the straight-blade NEMA 14 devices, only the 14–30 and 14–50 are common. The 14–30 is used for electric clothes dryers and the 14–50 for electric cooking ranges. The voltage rating is 250 V. They are essentially the replacements for the NEMA 10 connectors above with the addition of a separate grounding connection.
All NEMA 14 devices offer two hots, a neutral and a ground, allowing for both 120 V and 240 V (or 120 V and 208 V if the supply system is three-phase rather than split 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 will not fit in the slot of a 14–30.
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 for mobile homes to be connected to utility power via a 14–50 device.
[edit] NEMA TT–30
NEMA TT–30 is a 30 A, 125 V recreational vehicle standard which may also be known as RV 30. It is frequently (and sometimes disastrously) confused for a NEMA 10–30. The RV receptacle is common in nearly all RV parks in the United States and Canada, and all but the largest RVs manufactured since the 1970s use this plug. The hot and neutral blades are angled at 45° from vertical and 90 degrees to each other, similar to NEMA 10 devices. The plug is slightly smaller than a NEMA 10 but larger than ordinary 5–15 plugs. The ground pin, however, is round, like those on straight-blade NEMA grounding devices. Referring to the diagram, the orientation is the same as the NEMA 5 plug and socket, with the receptacle neutral on the lower right. Due to the appearance of this plug, many people assume that it is to be wired for 240 V, but this is a 120 V device. Adapters exist with the TT–30 plug on one side and a 5–15 or 5–20 socket on the other side. When the cord is detachable from the RV an L5–30 is usually used on the RV end of the cord.
[edit] Twist-locking connectors
  L21–30 plug and receptacle |
Twist-locking connectors were first invented by Harvey Hubbell III in 1938 and “Twist-Lock” remains a registered trademark of Hubbell Incorporated[3], 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 very reliable connection in commercial and industrial settings.
Locking connectors come in a variety of standardized configurations that follow the same general naming scheme except that the designations all begin with an “L” for “locking”. Locking connectors are designed so the different voltages and current ratings can not be accidentally intermated[4]. Many specific types exist; only a few are listed below. Other types include miniature locking connectors, and special purpose connectors for boats, 400 Hz circuits such as used for aircraft, and direct-current use.
One argument against 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, the exposed conductors may come out of the plug, causing dangerous shorts if the circuit is live.
[edit] NEMA L5
NEMA L5 connectors are a series of locking connectors used for 120 V, single-pole circuits. The L5–30 is a very popular configuration even on circuits not requiring the 30 A current capacity, owing to its very rugged construction. The L5–30 is the standard configuration for 120 V boat shore power. And -20P are used for Server PDU units.
[edit] NEMA L6
NEMA L6 connectors are a similar series of twist-locking connectors used for 208/240 V circuits.
[edit] NEMA L7
NEMA L7 connectors are used for 277 V circuits (typically, commercial or industrial lighting circuits).
[edit] NEMA L21
The type L21–30–P and L21–20–P are 30 A and 20 A plugs for 208/120 V three-phase connections which include neutral and ground. These plugs connect to type L21–30–R and type L21–20–R receptacles, respectively. The pin in the middle is ground, and the blade with a right angle on the tab is neutral.
[edit] Ground fault circuit interrupter (GFCI) receptacles
These versions of the 5–15 receptacle are residual-current devices and have Test and Reset buttons (and sometimes an indicator light). In the U.S. 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 live connector to the current returning on the neutral, and will 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” outlets so that putting one GFCI receptacle in a circuit will protect 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.
[edit] Leak-current detection and interruption (LCDI) plugs
Because portable air conditioners have caused many electrical fires, those sold in the United States now must have leak-current detection and interruption (LCDI) plugs. The cords have a fine wire mesh around the conductors and circuitry to detect any current leaking from the conductors to the mesh, which would happen if the cord was damaged or frayed. These could be NEMA 5–15, 5–20, 6–15, 6–20 or 6–30 plugs, depending on the air conditioner. The plug is equipped with “Test” and “Reset” buttons on the housing.
[edit] Color code
The color of a NEMA device does not identify the voltage class or power system for the device. Since the colors are not regulated by national standards, the purpose of color-coding a receptacle is set by the building owner. Brown, ivory, white, almond, grey, and black receptacles in the 5–15 configuration are selected to blend with the decor of a room.
- Blue receptacles may indicate built-in surge suppressors.
- An orange receptacle may be an isolated ground device, where the grounding pin of the receptacle is connected to ground independently of the frame of the receptacle and wiring outlet box.
- A red receptacle may indicate a special-service outlet such as one connected to an emergency standby power source.
- At least one manufacturer makes a yellow receptacle which identifies it as corrosion-resistant.
- A receptacle with a green dot is a so-called “hospital grade” device; such devices are tested to survive harder use than wiring devices intended for residential or commercial purposes. The NEMA standard does not define green as a color for wiring devices.
[edit] Break-away tabs
Duplex receptacles usually have break-away tabs to separate the top and bottom halves so that the two outlets can be placed on separate circuits. This may allow for one switched outlet for a lamp, or for two circuits to be used when heavy loads are anticipated. Two branch circuits may share a neutral terminating on duplex receptacles.
[edit] See also
- IEC connector, an international electrical connector standard commonly used in computer equipment
- Domestic AC power plugs and sockets
[edit] References
- ^ ANSI/NEMA WD–6, the standard for US electric power connectors
- ^ NEMA standard WD 6 Wiring Devices Dimensional Specifications, page 17
- ^ Hubbell Corporate History
- ^ Hubbell Twist-Lock Wiring Devices and Safety Enclosures
[edit] External links
- Georgia State University - Physics Department - Household Wiring ~ Polarized Receptacles (diagram)
- NoOutage.com - NEMA Configurations (socket configurations)
- Power Cabling - NEMA Configurations (socket configurations)
- Stay Online - NEMA Plug & Receptacle Configurations Reference Chart (Straight Blade) (socket configurations)
- Stay Online - NEMA Plug & Receptacle Configurations Reference Chart (Locking) (socket configurations)
- System Connection - Power Cord NEMA Mold Chart (socket configurations)
