Rewiring Existing Receptacle With a Shared Neutral

ntl1991May 5, 2011

Hello there. I'm doing some rewiring in my circa-1948 home. Wiring is mostly the original 14-gauge 2-conductor with no ground, except for the rooms that I've already rewired.

Today I've rewired an outlet in my living room which ran off a single 15-amp circuit which powered my 55" Television, Surround Receiver, CD changer, DVD player, Tape Deck, Cable Box, VCR, two floor lamps, and wall sconces. I rewired it because I will be using an 11,000 BTU window A/C this summer, and the AC blows the fuse when the TV is on and the compressor kicks on. (I have a dedicated 240V 20A receptacle under this window, but I don't have a 240V AC, and the smallest I can find is 18,000BTU which is way too large for my 1000 square foot apartment.)

So, I rewired the existing outlet (an end-of-the-run receptacle) with 12 AWG wire and fed it back to the fuse box along with a 20A fuse. The problem is that this outlet also had a lone neutral wire running from it up in the wall where it provided a path back to the fuse box for wall sconces over my fireplace. The hot wire for the sconces comes from one of the other three living room outlets.

Is it acceptable to connect this neutral to the receptacle of my air conditioner's (now dedicated) 20A circuit as it was before? I'm sure the 12 gauge wire is large enough to handle the current traveling back to the panel from the two 9W CFL bulbs along with the air conditioner.

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You say both house and apartment so I'm not sure where you're living but just as a reminder, unqualified persons are generally prohibited from doing electrical work in multifamily dwellings.

As for the shared neutral, absolutely not. Imagine someone is working on that new circuit and those wall scones happen to be on. They pull the fuses and assume the power is off to that A/C circuit. As they're handling the neutrals, their hand comes into contact with the metal junction box (or in any way becomes a path to ground) they're going to get zapped.

You need to fix that neutral. The easiest course of action would have been to change that 240v circuit for the A/C to a 120v circuit.

    Bookmark   May 6, 2011 at 4:58AM
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You cannot "Shared Neutral" in the manner you describe.

All circuit conductors must be run together (in the sme cable/conduit) and it is likely this neutral may be carrying more current than allowed.

There are 'shared neutral' circuits, but they are called multi-wire circuits and MUST use opposite legs of the 240 V to avoid overloading the neutral wire.

It sounds like a real hack job.

    Bookmark   May 6, 2011 at 9:33AM
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I'm not sure why it would be called a "hack job" as it's the original wiring. It was correct when the outlet was running off the living room circuit, because that is the circuit that supplies the hot side of the sconces.

So, could I pull some 14/2 from the box where the air conditioner is back to the other living room receptacle that previously powered this outlet, and connect the neutrals together, providing a neutral path for the sconces back through the living room circuit rather than the new air conditioner circuit. Correct?

Also, I didn't think about rewiring the 240V receptacle, but it's certainly an option if I cannot use the method above... The 240V outlet is run off of a separate breaker box external to the fuse panel, and has 2 20A breakers, providing the 240V. Can I just cap one leg of that 240V circuit, disable that one breaker, and connect up a 20A duplex receptacle?


    Bookmark   May 6, 2011 at 10:24AM
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"I'm not sure why it would be called a "hack job" as it's the original wiring. It was correct when the outlet was running off the living room circuit, because that is the circuit that supplies the hot side of the sconces. "

Still a hack job since the circuit conductors have been required to be run together all the way back to knob & tube.

    Bookmark   May 6, 2011 at 1:15PM
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Do you have a neutral wire in the 240V circuit?

    Bookmark   May 6, 2011 at 6:27PM
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Aha. I thought those sorts of things were allowed back then... Apparently not. I have true shared neutral somewhere in the living room circuit (I think it's in the front stairway or front entry light) because, about a year ago, when I replaced the first 2-prong non-grounding receptacle on the living room circuit with a GFI to allow grounding receptacles downstream, every time I would turn on the sconces or front hall lights, it would trip the GFI. So, I had to install a separate GFI at every outlet in the living room.

There was no neutral in the 240V circuit, a hot black and hot white along with a ground going back to the GE Load Center which has two 20A breakers.

I rewired the white white in the breaker box from the breaker to the neutral bus bar. In the receptacle box, I removed the 240V receptacle, and installed a 20A 120V receptacle.

I ran the #12 wire that I fished to the living room outlet over to a plastic junction box (due to no ground wire) where I tied it back into the 2-wire Romex. I had to replace the GFI in the outlet because there is no ground path back to the fuse box through the original Romex. One day I will rewire the living room so all of the outlets are one 20A circuit. Not today, though...

Because I will be getting a gas range (and have already had the plumber stub out a gas line behind the range), I had to run another 20A circuit to a new receptacle behind the range. Because I had a free 20A breaker in the GE Load Center where the 240V AC plug was originally installed, I tied the circuit in there, next to the 120V AC plug.

Thanks for all the help today! I sure do appreciate it.

    Bookmark   May 6, 2011 at 7:04PM
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"Do you have a neutral wire in the 240V circuit?"

A straight 240 V circuit does not require or use a neutral.

The load is across the two hot legs.

A 120/240V load DOES need a neutral for the 120 V portion.

A 240 V circuit is 3-wires, hot-hot-ground.

A 120/240 V circuit is 4-wires, hot-hot-neutral-ground.

    Bookmark   May 8, 2011 at 9:43AM
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