Black is "hot" because it carries voltage & current. White is nuetral or "common" because it does NOT carry voltage & current but provides a "common" tie point for the return path of current and is ultimately connected to ground. The bare wire (sometimes green) is a ground wire tie directly to earth ground.
Subsequently, one may also ask, what is the common wire when wiring a light switch?
Wiring a One Way Switch. A one way light switch has two terminals which is a common marked as COM or C. The common is for the live wire that supplies the input voltage to the switch. The other terminal is marked as L1 and is the output to the light fixture.
What color is the common wire?
The protective ground must be bare, green or green-yellow striped. Hot (active) wires may be any other colors except these. However, common practice (per local electrical inspectors) is for the first hot (live or active) wire to be black and the second hot to be red.
Is the common wire black or white?
White. The "common" is the "neutral" or "ground" wire, depending on the type of circuit. In normal US residential wiring, you'll have a black "hot" wire, a white "neutral" or "common" wire, and a green or bare "ground" wire.