Sometimes you’ll hear someone refer to the common wire when discussing electrical wiring. Not everyone is born knowing this stuff, but no one wants to sound dumb, either. So what is the common wire in electrical wiring, and what does it mean?
The common wire is normally the white wire, at least in the United States, and is often called the neutral wire. It’s also called common because all circuits in the house typically have the white wires tied together, which means every circuit has that wire in common.
Why a common wire?
The common wire is a trick that saves wiring and simplifies electrical panel design. If you recall your science classes in school, you probably built simple circuits using a battery and a light bulb. Your house wiring works on the same principle, just on a larger scale and at higher voltages. Electrical current travels from the source to the bulb, then back to the source. The wire that carries electricity to the source is the hot wire. In household wiring, this wire is usually black, though it can also be red.
The wire that carries electricity from the bulb back to the source is the neutral wire, or common wire. This wire isn’t always hot, though you also can’t make any assumptions about whether there’s power on the wire or not at any given point in time.
If you open an electrical panel, you’ll usually find that all of the white wires in the house are tied together on a bus bar, while individual black wires connect to each circuit breaker. That’s why people sometimes refer to it as common.
Why the light switches need to live on the black wire
In theory, if you break the circuit on either wire, it will stop the flow of electricity. In practice, you need to put light switches on the hot black wire rather than the white neutral wire. The reason is because all of the white wires connect. So if you put a switch on the white wire, the power may jump the switch and find a different path to the box, through another nearby white wire that you can’t see.
This is why traditional toggle-style light switches only need the black wire, and the white wires are all tied together in the back of the box, or in some cases, there may not even be any white wires in a box that just has light switches.
Is the common wire also ground?
When you try to use the common wire as ground, that’s called bootlegging. It’s not the same, it’s not legal, and it’s dangerous. Sometimes people use the two terms interchangeably because the concepts are extremely similar, but it’s not safe to use the two wires interchangeably.