The National Electrical Manufacturers Association (NEMA) has developed standards for the configuration of locking and nonlocking plugs and receptacles. The differences in the plugs and receptacles are based on the ampacity and voltage rating of the device. The configuration also ensures receptacles of a lower voltage or ampere rating are not interchangeable with receptacles having a higher rating. For example, the two most commonly used receptacles are the NEMA 5-15R, , and the NEMA 5-20R, The NEMA 5-15R receptacle has a 15-ampere, 125-volt rating and has two parallel slots and a ground pinhole. This receptacle will accept the NEMA 5-15P plug only, The NEMA 5-20R receptacle
