Two basic classifications define NEMA devices: 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. 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 amperes number is followed by the letter R to indicate a receptacle (female connector), or the letter P to indicate a plug (male connector).