The short version: I moved into a condo in June and decided I wanted to add one electrical outlet to one of 3 15-Amp circuits. The new outlet would be right next to where the electrical panel is, so my plan was to remove this branch from the panel, us a short length of 12-3 wire to wire up the new GFI outlet, then reterminate the existing branch wire onto the new outlet. We'll call this Circuit A

No question which hot wire I needed to disconnect, but when it came to find the right white/common wire, my only strategy was to turn off most breakers, loosen a common wire from the bus, then check to see if the Circuit A was depowered (bathroom light gone out).

What I found was that two breakers were served by one common wire. I was using a trouble light on a long extension cord to another branch -- call this Circuit B) to see what I was doing, and when I loosened the bus screw on the common wire that serves Circuit A, I saw my trouble light get awfully bright. I said "Aha! This light is getting 220 volts!".

I think I can *maybe* puzzle out why that Circuit B was getting 220 with a loose common, but it did depower when I removed that common wire completely from the bus.

Furthering the mystery, Circuit B has a 30 Amp single breaker but the only thing on it, so far as I can tell, is a single outlet in a laundry closet. I am thinking that this circuit is either A) an artifact of an old 115 volt supply to a washing machine before they added 220V for washer/dryer. Anyhow, it looks like 14G wire coming off that 30A breaker. Too little wire for that.

What I can't understand is the shared common. Is this ordinary? I don't want to say "common"! Right now, all of the outlets test out OK. Circuit B doesn't really have any way to get loaded -- I just use it for a single lightbulb -- but I was leery of rerouting circuit A through this new outlet if there was something fundamentally dodgy about this.

Dodgy. That word sounds so....British!

Eh? Jim, have you moved to a country where there is 220V? If you are in England,then a seperate circuit with an earth leakage circuit breaker for a washing machine is a requirement. Also for an oven.

You can get 220V outlets in the states, but I think they're mainly for high power things like photocopiers. They also had different shaped/orientated pins on the connectors.

Well that would make sense then.

And full-size dryers are almost always 220V.

Maybe it's the dryer then. My dryer is 110V, but it's gas. I think she has electric.

actualy, it's farily common in US wiring to have a single white neutral wire for 2 breakers. Basicaly what you have is two 120v hot legs that give you a 240v circut for powering large devices like dryers, A/C units, stoves, etc.

however, having an improperly wired hot leg on a normal outlet is bad! 240v cirucuts should use a different outlet plug.

Jim,

From your description it sounds like you have two 110V circuits which share a neutral but have their hot sides on seperate breakers. Am I correct?

This is common. My kitchen outlets are wired this way. Half are on one leg (phase) and half on the other, all sharing a neutral and ground.

In this situation the wire used should be three conductor with ground (14-3WG or 12-3WG). The two hot wires (red and black) should connect to a "ganged" (two-pole, 220V) circuit breaker (15 amp for 14 guage wire). This ensures that the two circuits are not on the same phase. If the two circuits are on the same phase you can have 2X the current in the neutral wire (ie. 30 amps, with two seperate 15 amp breakers on the same phase). Not a good situation.

I would suggest correcting the problem and adding another breaker for your new outlet.

-Jeff

Thanks, everybody. Yes, I have 220/240 service coming in to feed an electric dryer, stove and baseboard heating.

Jeff said:

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This is common. My kitchen outlets are wired this way. Half are on one leg (phase) and half on the other, all sharing a neutral and ground.

Good to know!

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In this situation the wire used should be three conductor with ground (14-3WG or 12-3WG). The two hot wires (red and black) should connect to a "ganged" (two-pole, 220V) circuit breaker (15 amp for 14 guage wire). This ensures that the two circuits are not on the same phase. If the two circuits are on the same phase you can have 2X the current in the neutral wire (ie. 30 amps, with two seperate 15 amp breakers on the same phase). Not a good situation.

I'm not there right now, but circuit B's hot wire is red, IIRC, but it is definitely on a completely separate breaker.

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I would suggest correcting the problem and adding another breaker for your new outlet.

So in essence, the ganged breaker just knows how to give the right phase to each, or are there 2 separate bars back there for each phase? (the latter would make sense)

Given that they are independednt breakers and one is 30A, this *does* seem dodgy. Plus, this panel does not have a master breaker if it did overload -- I'd be relying on the main breaker in the basement. Thankfully, I don't think I'm overloaded yet.

A colleague pointed out the existence of mini tandem breakers, meaning I can get 2 separate 15 A circuits in the space of one traditional breaker. I hadn't a clue. This is how I think I am going to add the auxiliary outlet, though -- a whole separate (mini) breaker with it's own common/ground.

Back to it this evening!

when you say 'dodgy' and 'electrical', should you then automatically say, 'I'll get a professional' ?

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when you say 'dodgy' and 'electrical', should you then automatically say, 'I'll get a professional' ?

Well, if my post rate drops precipitously, you know what happened.

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when you say 'dodgy' and 'electrical', should you then automatically say, 'I'll get a professional' ?

I live! (I've done some of this before, but it has been years, and I wanted to be appropriately skittish in the presence of mucho voltos.)

Anyhow, maybe not as dodgy as I thought: Hard to tell, but the shared common looks like it may be 12G and the Circuit B hot wire looks like 12G. And I figured out that also feeds the water heater. Still undersized, I thought, for 30A breaker, but maybe not. I'm amazed at how skinny wires can be and still meet code.

I also solved my problem without radical measures. I looked and saw "D'uh -- that's a big dual pole 220 breaker for baseboard heat in bedroom that I will *never* use." Popped that out, popped in a single 15 A breaker, and I now have dedicated 15A circuit for alarm system and computer closet. Now if I can just figure out how the heck to fish my dang phone line into this closet for DSL/alarm circuit, I'll be all set.