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placebo 03-29-2011 05:03 PM

Puzzled by kitchen wiring
 
I want to install GFCI outlets in the kitchen of a house built in the mid-60s, so yesterday, I experimented a bit to determine which breakers control which outlets. It turns out above-counter outlets in the kitchen are hot unless two 20-A breakers are both off. I assumed the two circuits somehow ended up wired in parallel at one of the outlets.

Today, I took apart one outlet to split the chain of outlets into two segments. There were two black-white pairs as you'd expect for an outlet in the middle. As expected, turning on one breaker caused one segment to become hot and left the other segment off. The other breaker, however, had absolutely no effect. If it's on and the other is off, none of the outlets has power.

Frankly, I'm at a loss to explain what I'm seeing. Any ideas?

gregzoll 03-29-2011 06:33 PM

You would have a MWBC or in layman terms "Multi-Wire Branch Circuit".

Leah Frances 03-29-2011 07:46 PM

Quote:

Originally Posted by gregzoll
You would have a MWBC or in layman terms "Multi-Wire Branch Circuit".

I'm not sure if this is laymans terms..... :)

Wildie 03-29-2011 07:52 PM

Quote:

Originally Posted by placebo (Post 619460)
I want to install GFCI outlets in the kitchen of a house built in the mid-60s, so yesterday, I experimented a bit to determine which breakers control which outlets. It turns out above-counter outlets in the kitchen are hot unless two 20-A breakers are both off. I assumed the two circuits somehow ended up wired in parallel at one of the outlets.

Today, I took apart one outlet to split the chain of outlets into two segments. There were two black-white pairs as you'd expect for an outlet in the middle. As expected, turning on one breaker caused one segment to become hot and left the other segment off. The other breaker, however, had absolutely no effect. If it's on and the other is off, none of the outlets has power.

Frankly, I'm at a loss to explain what I'm seeing. Any ideas?

It seems that you have two, 20 amp breakers feeding the two outlets on the receptacle. The receptacle would have both the neutral and the hot tabs removed.
To maintain this arrangement, you will have to use 2-20 amp GFCI breakers in the panel.
My advice would be to remove one of the #12X2 feeders from the box and install the GFCI receptacle.
The #12x2 that is removed would then be moved to a new box and GFCI.

AllanJ 03-29-2011 07:59 PM

Are both halves of the (duplex) receptacle in one outlet box live at the same time and dead at the same time, or does turning off one breaker make one half dead and the other half is still alive?

With just two black/white pairs coming into the outlet box, usually one is the feed from the panel (possibly daisy chainng through other outlet boxes) and the other is the continuation to other outlet boxes further on.

placebo 03-29-2011 09:35 PM

Quote:

Originally Posted by Leah Frances (Post 619586)
I'm not sure if this is laymans terms..... :)

Heh, that's what I thought too. Fortunately, there's Google. :) Greg nailed it. There's a shared neutral. I took an outlet apart again but disconnected only the hot side. Now one breaker controls the outlets on one side of the outlet and the other breaker controls the other side.
Quote:

Originally Posted by AllanJ (Post 619595)
Are both halves of the (duplex) receptacle in one outlet box live at the same time and dead at the same time, or does turning off one breaker make one half dead and the other half is still alive?

With just two black/white pairs coming into the outlet box, usually one is the feed from the panel (possibly daisy chainng through other outlet boxes) and the other is the continuation to other outlet boxes further on.

Right now, if either breaker is on, all the outlets on all the receptacles are live, so it seems there should have been one receptacle where one socket was on one branch and the other on the other branch. I suspect the two branches were joined when we replaced the old outlets long ago and didn't notice one of the tabs should have been removed.

Wildie 03-29-2011 10:23 PM

Its lucky that the circuits are on the same leg or you would've had 220 applied across the tab on the receptacle.
I think that it can be argued that both the hot and neutral tabs should be removed so that each circuit uses its own neutral.

placebo 03-29-2011 10:28 PM

Yeah, it was very fortunate. I'm going to have to track down why the neutral for one branch doesn't seem to be working.

AllanJ 03-30-2011 08:45 AM

Two hot feed wires may not share the neutral unless both hot wires are in the same cable or conduit as the neutral.

For separately breakered hot lines, the two must be connected to breakers on opposite sides of the 120/240 volt circuit and furthermore, in no place both hot wires back to the panel are connected to the same duplex receptacle with the tab between the screws unbroken.

It is not unheard of to have separate 2 wire cables entering an outlet box to feed the two halves of a duplex receptacle separately. Here both the neutral tab and the hot side tab must be broken off. Here, also, an unusually large box is needed if a third and/or fourth cable are provided to daisy chain to receptacles further on.


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