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I'm installing some lights in my basement, and will be replacing the single gang box in the hall (currently only for hall light) with a double gang box. The idea is to accommodate the hall light switch and the basement light switch in the same box, but I want to inform anybody who comes after that they will have to turn off two breakers to safely work in the box.

What's the best way to label the box, so people know there are two circuits in the box?

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up vote 7 down vote accepted

Put a divider between the two sides of the box:

Gang box divider

Then simply label each side of the gang box as belonging to the appropriate circuit.

Note: Make sure the divider you pick will fit your gang box. I've picked up dividers that don't before.

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I write the circuit number(s) on the back of the cover plate any time I open one up. Our house (built ~1979, CT) has numerous places where multiple circuits are in the same multi-gang box.

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I was just about to go through my house and map that out, but wasn't sure how to record it. I do not know why I didn't think of that. – Spike Aug 1 '11 at 0:28
When we mapped our house out, I made a document organized like the breaker panels (we have 3) and included as much detail as I could. I have a copy hanging on the wall next to the main panel, and use it often; the labels under the plates are faster when they are there. – TomG Aug 1 '11 at 0:37
Cover plate labels can be a problem, if say I remove them all to paint and don't remember which one went where. Could be loads of trouble down the line. – Tester101 Aug 1 '11 at 16:45
Maybe labeling the switch/receptacle itself is a better option. – Tester101 Aug 1 '11 at 16:45

You can wrap each hot wire in red tape leaving a 1 or 2 inch tail of tape stuck together and write the circuit breaker # number on it. IE: CB#4/ CB#9. Technically, you're not supposed to have two breakers represented in the same box, but I bet it is old wiring, so my "Common Sense" Code applies in this case.

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I couldn't find anything that said you can't have multiple circuits in a box, do you know of any specific codes that mention this? – Tester101 Jul 30 '11 at 0:18
I couldn't find anything that prohibited two branches in one box. In retrospect, I guess it is common where a lighting switch is co-located with a recpt and they are on separate circuits. What I was thinking was like devices in one box, where it is frowned upon as convention, not a code issue. When roughing in, we leave long loops and write complete descriptors on the jacket. My bad, sorry for the misleading info. I do like to use the tape label method however. – shirlock homes Jul 30 '11 at 10:04
This could be troublesome, since you may have to actually pull the switch/receptacle out of the box to be able to get a good look at the wire. – Tester101 Aug 1 '11 at 16:47

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