· For the most part, you are going to see 120 volts, 15 amp, and 20 amp. And 220 2 wire, 3 wire, and four wire types. Please remember not all 220volt outlets use a neutral (white) wire. To give you a general idea, let's say you just bought an air compressor from Home Depot. It requires 220 volts but the plug only has three prongs.