Electrical Question Unrelated to Woodworking

I wanted to borrow some electricity to install a light in a closet. A 3-way switch for the hall light was conveniently located on the opposite side of the same wall. In the box for the 3-way switch I found 4 each two conductor w/ground romex wires. The grounds were all tied together. Three of the insulated wires went to the switch as expected. Three neutrals were tied together, and two hots were tied together. But the two hots that were tied together were not on the same breaker with the light switch. They were on another breaker on the other side of the box, making 240 volts between the switch and the other two hots. So I had two different circuits on different breakers sharing a common neutral that is apparently the only neutral path back to the box. This is all original wiring in a house built in 1976. I know it's all original because the wires were spray painted along with the walls.
Is this a common practice I haven't run across before? Does the code allow it?
I realize that since the circuits are on different phases, the common neutral carries only the DIFFERENCE in the loads on the two circuits, not the combined load of both. But it still doesn't feel quite right to me. What if somebody moved one of the circuits to a breaker on the same side of the box. Then the common neutral would carry the combined load and overheat.
DonkeyHody "Every man is my superior in that I can learn from him." - Thomas Carlyle
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try alt.home.repair
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It sounds like a multiwire circuit. Essentially you are using a 240v circuit for two 120v circuits. It saves a little wire, and cuts the voltage drop a bit. It is perfectly legal if done properly.
I found exactly what you feared in my circuit box. Both circuits were on the same leg; if they were ever heavily used it could have caused a fire. Many people use a 240v breaker, both for the reason you describe (with a 240v breaker you have to wire it correctly) and to prevent surprises from just shutting off one of the circuits and thinking both are off, but it is not required.
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Actually, I think the current code DOES require a two pole breaker now for a multiwire circuit. For both of the reasons you describe.
- - LRod
Master Woodbutcher and seasoned termite
Shamelessly whoring my website since 1999
http://www.woodbutcher.net
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very common around here........ Mark H.
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snipped-for-privacy@bellsouth.net (DonkeyHody) writes:

Yes and yes.

yes, and probably not. How often are both current carrying legs at full capacity, especially in a house?
scott
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In at least some areas, it is against code to have a light fixture in a closet. It may be allowed in some walk-in closets. You may want to check into it before going ahead.
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That means the closet needs to be pretty deep to have a light.
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Thanks to all who replied. This is the only closet in the house without a light, and it's deep enough to meet the rule (which I didn't know about, thanks). I had planned to install the light (slimline flourescent shielded) above the door to be sure it doesn't get bumped by a box going onto the shelf.
DonkeyHody "Every man is my superior in that I can learn from him." - Thomas Carlyle
snipped-for-privacy@aol.com (Greg) wrote in message

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wrote:
|
| |>I wanted to borrow some electricity to install a light in a closet.| |In at least some areas, it is against code to have a light fixture in a |closet. It may be allowed in some walk-in closets. You may want to check |into it before going ahead.
Depends on the size of the closet. I have two light fixtures and a skylight in mine. BTW, a skylight is a bad idea in a closet, at least in sunny AZ. SWMBO had a brand new red silk blouse stored for several months before deciding to wear it. One sleeve was badly faded from the sunlight. I wound up putting some window tinting film on a sheet of plastic and using it as a filter. |
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It is against code to have a bare light bulb type fixture in a closet, in fact in any area inhabited area. A fixture where the bulb is protected from damage is another story.

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This is called a multiwire circuit and is fairly common. Since the two hots are 180 degrees out of phase with each other, the current flow through one hot to neutral tends subtracts form the current flow from the other hot to neutral. Normally these should be on the same 2 pole breaker or two adjacent breakers that are tied together so that when turning off one circuit the other circuit is off as well.

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Bob, I was just wondering what happens if the neutral that is feeding the 2 opposing (out of phase) hot legs were to be disconnected? 240V across both of the legs (not each)? Which IF the loads were of equal resistance, would result in 120V across each leg. But I'm guessing that that would be a long shot to have each legs loaded the same. Does this make sense?                         Mark L.
Bob Peterson wrote:

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Thats about the size of it. its not unheard of for a neutral wire to come loose and end up with 100 volts on one leg and 140 on the other. The symptom tells you its probably an open neutral.
By the way, someone said something about 4 neutrals being bonded together. Presumably the poster meant they were wire nutted together somewhere out in the field wiring, as I beleive the code requires that neutral bar connections can only be a single wire. Ground connections on the other hand are allowed 2 or 3 wires per connection point (depends on label on panel).

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snipped-for-privacy@bellsouth.net (DonkeyHody) wrote in

If I understood you correctly, there's three neutrals tied together in the box. Since the neutrals are all tied together at the breaker box too, it doesn't make any difference which side the 3 hots come from - regardless of how you slice it, there's 3 return paths for the 3 loads.
John
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Not necessarily. He had 4 cables entering the work box:
Assumptions:
12-3 feeding power from panel to work box on two breakers 12-2 providing common for 3-way switch 12-2 to light fixture 12-2 to string of outlets.
One of the 12-3 current conductors feeds the switch, the other feeds the string of outlets. Here there will be three grounded (neutral) conductors bonded together, but only the 12-3 grounded conductor is a return path to the panel (well, the grounding conductor is, too).
scott
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snipped-for-privacy@slp53.sl.home (Scott Lurndal) wrote in

Ah - yes, I wasn't reading what he wrote as being configured like that, but your assumptions sound a lot more likely. I would not be pleased with wiring like that (and would probably rewire it, which would take longer than anticipated & require more bad language to be invested).
John
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John, My terminology might not be entirely accurate. I'm refering to everything on the breaker side of the load as "hot" and everything on the return side of the load as "neutral". Four cables enter the box. Cable #1 carries two "hot" wires coming from another 3-way switch controlling the hall light, but they alternate which one is hot depending on the position of the other 3-way. Cable #2 carries the "hot" from the switch to the hall light and provides a neutral return from the hall light back to the switch box. Cable #3 brings in a hot from the opposite phase in the breaker panel and provides the only neutral return to the breaker panel from the switch box. Cable #4 carries power from cable #3 to a receptacle (or two or three) and provides a neutral return back to the switch box. All 4 bare ground wires are tied together. All 3 neutral wires are tied together, but 2 of these are returns from loads #2 and #4 to the box leaving only the third neutral as a path back to the breaker panel.
I forgot to mention that ALL WIRES ARE #14, on 20 Amp breakers, which increases my level of discomfort with having this shared neutral. I'll at least tie the two breakers together, since they are conveniently adjacent, but rewiring would be cussworthy because 1)This closet is on an outside wall where headroom in the attic drops to nothing, and 2) I had an additional 8 inches of insulation blown in over the tops of the ceiling joists, making both the joists and the wires hard to find.
Thanks to everybody for your help.
DonkeyHody
"We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore." - Mark Twain

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(DonkeyHody) wrote:

All is OK so far, since (as you stated) the two breakers are on opposite legs of the service.

This is emphatically NOT OK. For safety and code compliance, 14-gauge wire *requires* 15 amp breakers.

Actually, the shared neutral is the least of your worries in this circuit. With the two breakers being on opposite legs, the shared neutral never carries any more current than the *difference* in current between the two hot legs. For example, if one hot leg is drawing 9 amps, and the other is drawing 7, the current in the neutral is only 2 amps. In the worst case (with no load at all on one side), the neutral carries the same current as the loaded hot leg. That's still bad, of course, because the breakers are oversized for the wire.

Tying them together is good -- but replacing them with breakers of the proper rating for the wire (i.e. 15 amps) and tying *those* together is better. Installing a 15 amp double-pole breaker is even better still.
This is important for at least two reasons:
1) It keeps the two breakers on opposite legs. If they were ever placed on the same leg, the current in the shared neutral would equal the *sum* of the currents on the separate hots. In the worst case, even with breakers properly sized to the circuit conductors, this could be *double* the current that the neutral conductor is rated to carry -- a serious fire hazard.
2) It ensures that *all* conductors in the shared circuit are de-energized at once. If the two breakers are independent, it's possible to shut off only one leg, leaving anyone performing maintenance on the _other_ leg exposed to an electrocution hazard.

Yep -- so replace the breakers with 15s, tied together, and everything will be fine.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
Get a copy of my NEW AND IMPROVED TrollFilter for NewsProxy/Nfilter by sending email to autoresponder at filterinfo-at-milmac-dot-com You must use your REAL email address to get a response.
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snipped-for-privacy@bellsouth.net (DonkeyHody) wrote in message

Whew!!!!!! I thought "Wood related electrical question" he was going to ask the impedance and dielectric constant of wood.
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