If you don’t do electrical work very often, it can be confusing and can lead to questions. Is the black wire hot? Which wire is hot, black or white?
The first rule is there are no rules. So if you see lots of different colors in an electrical box, you probably need to call an electrician. But if you see three or four colors, you can probably figure it out.
Always turn off the power at the breaker box when you’re working on electrical outlets. Sometimes for testing you have to turn the power back on. Have a helper turn the power off and on for you so you don’t leave bare wires hanging out of the wall that may be live.
Which wire is hot, black or white?
In the most basic 110-volt electrical setup, you’ll find two wires: black and white. The black wire is hot. The white wire goes by many names that all mean the same thing: neutral, common, or return. Power flows from the box to the outlet through the black wire and back to the box through the white one. The neutral wire can also be gray.
Sometimes you’ll find a white wire with a piece of black electrical tape wrapped around it. By convention, that means the wire is hot.
If you have a ground wire, it’s green or bare. The ground wire is for safety.
When you connect an outlet or light switch, the black wire goes to the brass screws. The ground wire goes to the green screw. The white wire goes to the white-metal screws on an electrical outlet.
In a 110-volt setup, an additional wire, usually red or blue, normally indicates a hot wire controlled by a light switch. Under those conditions, a black wire should be always hot and always on. The additional wire is hot but switched off at the switch. I’ve covered red wires in light fixtures and light switches before.
What about 220 volts?
In a 220-volt setup, you have two 110-volt leads. In that case, it can be different. If you have a three-wire 220-volt setup, the white wire can also be hot. A white wire being used this way should be marked, but it might not be. So be careful with 220.
In a 4-wire 220-volt setup, white should be common. The hot wires can be black and any other color besides white, gray, or green.
Green or bare wire is still ground.
Using a multimeter to map out mystery wires
Mapping out mystery wires is different depending on whether you’re dealing with 110 or 220. You have to do this with the power on, so be very careful. Spread the wires far enough apart that they won’t accidentally touch each other or anything else and short out. If you’re uncomfortable working with live wires, call an electrician.
You’ll also need a multimeter. Set it to AC, which is usually indicated by either V AC, or V~ on the dial.
Once you figure out what the mystery wires are, writing a note on the back of the faceplate with a permanent marker is always a good idea. That way, if you ever have to work on the circuit again, you know what’s what.
Mapping mystery wires in 110 volts
If you find an odd-colored wire, you can use a multimeter to figure out what it does. With the power back on, touch one lead from the multimeter to the white wire and the other lead to the mystery wire. If it reads 110 volts, it’s a hot wire.
If you find a mystery wire that reads a very low voltage (less than one volt) with a multimeter, there’s a very good chance it’s a hot wire controlled by a light switch somewhere. If a mystery wire reads exactly zero, you still would do well to check to make sure there isn’t a light switch somewhere controlling it.
If you have more hot wires than you need for your application, cap the unused wire off with a blue wire nut to prevent short circuits. Tape a note to the wire indicating that it’s hot, if you wish. That may help you avoid confusion in the future.
Mapping mystery wires in 220 volts
To map out 220, first you need to find your neutral. Touch any two non-green, non-bare wires. If they read 220 volts, both wires are hot. If they read 110, one of them is neutral. Find two wire pairs that both read 110 volts, and then the wire that’s in common with both 110 volt readings is your neutral.