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 North America. They are for supplying power to lighter-duty, general-purpose electrical devices. Twist-locking devices are used for heavy industrial and commercial equipment, where increased resistance to accidental disconnection is required. NEMA device numbers prefixed by an “ L” are curved-blade, twist-locking connectors; all 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. 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. An example of a locking connector is the L5–15R, while is also a 15 amp rated connector, but is a locking design which is not physically compatible with the straight-blade 5–15 design.

For a listing of the NEMA device numbers and a visual reference to the blade arrangement, please see the Marinco Industrial Catalog.