Museum of Plugs and Sockets logo, small North America
Heavy duty plugs and sockets
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North America is a mine of interesting plugs and sockets. Different devices for 125V, 208V, 250V, 277V and higher, rated at 15A, 20A, 30A, 50A or 60A; single phase, split-phase or three-phase systems. The four plugs at right illustrate the variation in pin configuration starting with a basic domestic 125V-15A (NEMA 5-15P, bottom left). Moreover, many plugs are available in a straight blade and a locking design (see nos. 17-21).

There are over 50 types in the category heavy duty,
but even a museum can't have all of them. The images below show a small, but reasonably representative selection.

For each plug and socket the official US code
is stated, given by the National Electrical Manufacturers Association. The NEMA code provides the following information: first number: class of device (voltage dependent); second number: maximum tolerated current: P: plug; R: receptacle (= socket).
4 US plugs


NEMA 5-30R NEMA 5-30P NEMA 7-15R NEMA 7-15P

1, 2 125V, 30A socket and plug (NEMA 5-30R,P). The plug measures 11 x 6.5 x 4.8 cm (3.2 to 3.6 cm long pins excluded). It can handle a 2.5 cm thick cable. Huge compare to a 125V - 15A plug and therefore a typical example of 'heavy duty'. It is more likely to expect such devices at farms and workshops than residences.
3, 4 277V, 15A socket and plug (NEMA 7-15R,P). 277V is the result of a neutral to ground connection in a 480V three-phase circuit (see page on three-phase transformers). In a workshop where 480V is nessesary, it might be more economic to use 277V single phase for lighting. Various 277V types of lamp are available, as incandescent lamps, screw base compact fluorescent light bulbs, fluorescent tube lights and LED lighting.


NEMA 6-15R NEMA 2-20R NEMA 2-20P NEMA 6-30/50P

NEMA 6-30R NEMA 6-30P NEMA 6-50R NEMA 6-50P

5 250V, 15A socket (NEMA 6-15R). The corresponding plug is one of the four plugs shown in the image top right (NEMA 6-15P).
6, 7 250V, 20A socket and plug, without earth pin (NEMA 2-20R,P). The socket is an older design by Hubbell; the plug has been bought in 2000.
8 Components of an 'assemble-your-own' 250V, 30A or 50A plug (NEMA 6-30P or 6-50P). Images no. 10 and 12 show the results. The kit also allows the construction of new, non-existing plug types; an interesting feature, but unfortunately an A-Y-O socket does not exist.
9, 10 250V, 30A socket and plug (NEMA 6-30R,P).
11, 12 250V, 50A socket and plug (NEMA 6-50R,P).


NEMA 14-30R NEMA 14-30P NEMA 14-50R NEMA 14-50P

13, 14 125/250V, 30A split-phase socket and plug (NEMA 14-30R,P). GND: earth connection (= equipment ground); W: ground, comparable to neutral. Y-W = X-W = 125V; X-Y = 250V. Plug no. 14 is a part of a (replacement) power supply cord recommended for dryers.
The next page, on Japanese heavy duty plugs and sockets, has a scheme that explains the principle of split-phase electrical power.
15, 16 125/250V, 50A split-phase socket and plug (NEMA 14-50R,P). See no. 13 for explanation of abbreviations. Plug no. 16 is recommended for electric stoves.


NEMA L6-30R Voltage color coding NEMA L6-30P NEMA L17-30R,P

US Variload 30A 600V plug US Variload 30A 600V plug

17, 18 250V, 30A locking socket and plug (NEMA L6-30R,P). Hubbell Twist-Lock sockets may have a colour code indicating 125, 250, 277, 480, 600 Volt AC and 125/250V split-phase.
19 600V, 30A  3-phase locking connector and plug (NEMA L17-30R and P). The angled pin (right) = equipment ground. Any combination of two of the three other pins gives 600V. This four wire device does not have a system ground (neutral in a Y-shaped 3-phase transformer). In a high voltage distribution system it is common not to have a neutral wire because loads are connected phase to phase.
20, 21 Hubbell 30A, 250V AC/DC-600V AC Variload locking plug. The center pin is the equipment ground. N = 3-phase system ground (neutral in an Y-shaped 3-phase transformer). X, Y and Z are phase connectors. Voltage depends on combination of connectors that is active. Standard 3-phase, 4 pole, 5 wire plugs have a round center pin. Variload have differently shaped center pins.

The Hubbell catalog gives the following information about Variload Twist-Lock.
In many large plants and laboratories the number of circuits of different ratings and frequency is greater than the number of blade configurations available in conventional wiring devices.
Prior to NEMA configuration standardization the solution was “Variload Twist-Lock” devices with four non-interchangeable center pin and slot configurations that can be designated to different circuits.
Plug no. has a triangular center pin. The arrow shaped pin of no. 21 is unusual and not mentioned in the catalog.


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