This document describes which voltages can be derived at the electrical outlet receptacle from the existing wiring found in common electrical boxes for both residential and commercial applications. This information is applicable to the United States only, although Canadian applications will be similar.
Generally a 120 / 240 volt, three-wire system is used in residential applications.
120 Volt Applications:
For 120 volt applications, there should be three wires in the box. The expected color of the wires should be green (the green wire is ground, but it could also be a bare wire rather than green), white (neutral), and black (phase also known as hot). Voltage between the phase and neutral conductors will be 120 volts nominal. When connecting a receptacle (usually a NEMA X5-XX), the ground wire bonds to the green terminal, neutral bonds to the silver terminal, and hot bonds to the copper-colored terminal. A qualified person can change a 120 volt dedicated box to a 240 volt box by connecting the neutral wire to a suitable dual-pole circuit breaker on the opposite phase leg. This conductor shall be wrapped with black tape. A suitable 240 volt receptacle will be installed in the box.
240 Volt Applications:
If the box is being used for a 240 volt application, there should be three wires in the box. The expected colors of the wires should be green (the green wire is ground, but it could also be a bare wire rather than green) and two black wires (phase conductors). Voltage between the two phase conductors should be 240 Volts. When connecting a receptacle (usually a NEMA X6-XX), the green (or bare) wire bonds to the ground lug and the phase conductors bond to the X and Y lugs (either phase conductor can go to either lug). A qualified person can change a 240 volt box to a 120 volt box by taking one of the phase conductors and connecting it to the neutral bus in the circuit breaker panel. This conductor shall be wrapped with white tape. A suitable 120 volt receptacle will be installed in the box.
Generally a 120 / 208 volt, five wire system derived from a transformer with a wye secondary is used in commercial applications. There are three hot phase conductors (colored red, blue, and yellow), a neutral conductor (colored white), and a ground wire (colored green). Some or all of these wires may show up in the box.
120 Volt Applications:
For 120 volt applications, any one of the hot phase conductors will be connected to the copper-colored terminal on a suitable NEMA X5-XX receptacle. The neutral conductor will be connected to the silver terminal. The ground wire shall connect to the green terminal.
208 Volt Applications:
For 208 volt applications, any two of the hot phase conductors will be connected to the X and Y terminals of a suitable NEMA X6-XX receptacle. The ground wire will be connected to the ground lug (colored green).