Diagram of different receptacles in common use in North America:
Courtesy of Wikipedia
- NEMA 6-series: Nominally 250V. The grounding conductor supplying this receptacle might be conduit or a derated, bare or green insulated conductor (such as an 8 AWG copper ground conductor with 6 AWG copper hot conductors). The grounding conductor is not intended to serve as a neutral, therefore it is not permissible to derive 125V power from this receptacle. 6-50 is typically used with welders.
- NEMA 10-series: Nominally 125/250V. This receptacle has a shared grounding/neutral conductor. The neutral wire must be insulated and the same gauge as the hot conductors. In practice, this is often not the case; there are many poor installations that simply use an uninsulated grounding wire for the neutral connection. Note that the chassis of the appliance is bonded (connected) to the neutral wire at a terminal block where the cordset enters the appliance; there should be no other connection. 10-50 is typically used with kitchen ranges (and 10-30 for clothes dryers) prior to 1999, when their use was deprecated by the National Electrical Code; the 10-series are now classed as "ungrounded" receptacles and plugs.
- NEMA 14-series: Nominally 125/250V. This receptacle has separate ground and neutral conductors. The neutral conductor should be the same gauge as the hot conductors; the grounding conductor might be conduit or might be a derated, bare or green insulated conductor (for example, 6 AWG hot and neutral conductors; 8 AWG grounding conductor). 14-50 is typically used with kitchen ranges, but may be used for any application requiring both 125V and 250V power up to 50A. The 14-series effectively replaces the 10-series.
If you ever see a receptacle with two T-shaped slots, be very cautious and always test it against a separately-derived ground (i.e., a 3-wire extension cord plugged into a known, good receptacle). This receptacle, designated NEMA 2-15, was originally designed to be usable for either 125V or 250V; it is a very, very bad design because a person could connect a 125V appliance into 250V causing unpredictable results.