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Old 07-18-2010, 11:22 PM   #1
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3-way switches, share neutral?


I am wiring 2 separate 3-way switches to power two groups of overhead lights. I'm going to run the 12-3 wire from one 2-gang box to the other and then use 12-2 to go to all the lights. My question is this: instead of running 2 separate 12-3 wires between the switch boxes, can I run one 12-3 wire and one 12-2 wire and share the neutral from the 12-3 with the other run, using the white wire from the 12-2 as the second hot for the switch?

These switches will both be on the same circuit with power coming through the first switch box. Thanks in advance guys.

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Old 07-21-2010, 12:15 PM   #2
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3-way switches, share neutral?


joside

Just saw your post went unanswered are you still there?

If so I'm understanding you have two double gang boxes with two 3-ways in each box. Your wanting to know if one nm cable run can be 12/2g between two of the switches and the run between the other two 12/3g. You then want to use the neutral in the 12/3 to serve both sets of lights operated by the 3 -way switches.

Is that a correct understanding?

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Old 07-21-2010, 12:23 PM   #3
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3-way switches, share neutral?


Stubbie,

Thank you, I figured my question was lost in the shuffle. Yes, that is exactly what I am asking. Maybe I should add that both these switches are getting their power from the same 12-2 wire at the 1st switch box.
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Old 07-21-2010, 01:01 PM   #4
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3-way switches, share neutral?


Quote:
Originally Posted by joside View Post
Stubbie,

Thank you, I figured my question was lost in the shuffle. Yes, that is exactly what I am asking. Maybe I should add that both these switches are getting their power from the same 12-2 wire at the 1st switch box.
Technically the electrical code requires that a circuit or a branch in the circuit have all wires associated with that circuit inside the same nm cable.

Personally I don't see any of the issues that occur by not having a neutral in each nm cable in your situation.

But I'd just run 12/3 to the switches to avoid any code issues.

There are other ways to do what you want but the wiring method is not a sheathed cable..ie... flexible conduit. Or you would need to go to a supply house and get a sheathed power cable with 5 conductors and ground. Probably more trouble than it is worth.

You might wait to see if some of the other electricians here have a difference of opinion.
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Old 07-21-2010, 01:27 PM   #5
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3-way switches, share neutral?


I recently had a very similar situation. 2 - 2 gang switch boxes but only 1 - 3 way switch in each box, the other switch in each box is a single pole feeding 2 other lights. Ran 12-3 between the switch boxes. The feed to the light was in switch box 1. Due to the way we had pulled wiring for this rewire ( rewired kitchen, laundry room and attached garage) there was a hot and neutral in each switch box from the same circuit. As I was installing the switches (in box 1) I made up the grounds and the neutrals (connecting the neutral to the light with the neutral from the feed), connected the black from the light to the common of the 3 way, and the red & black of the 12-3 to the switch travelers. Then I realized that there was no place to connect the white of the 12-3 cable to the other switch box. Normally that white would have been used to bring the neutral from the other switch box and connect to the neutral to the lamp. Scratched my head for a second and realized I was using the same circuit neutral. Capped the white from the 12-3 in each box and let it go. Seems like all the conductors are in the same cable and certainly I did not mix neutrals from 2 different circuits.
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Old 07-21-2010, 01:44 PM   #6
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3-way switches, share neutral?


I as a do-it-yourselfer would have risked a failing inspection by answering the OP's question "yes", based on the following scenarios from other forums neither of which had a (NEC) code reference link supplied to back it up.

1. Two switch locations, each with 2 gang box for three way switches to control light and fan independently. Power comes to box 1, light/fan unit connected to box 2. But the light/fan unit has only one neutral, a black wire for the light, and a blue wire for the fan. If you run separate 3-wire cables for the 3 way switches, you must connect both neutrals, one from each 3 conductor traveler cable, to the single white wire at box 2 going up to the light/fan unit, thus producing a downstream combined neutral situation which is normally abhorred. Or leave one 3 conductor cable's white wire unused which brings us back to the "yes" answer to the OP's original question.

2. Power to single gang box 1 for 3 way switch. Two 2 conductor cables strung to double gang box 2 where there is a 3 way switch and also a single pole switch for a different light. No power to box 2. Diagram on another forum showed one of the cables providing the travelers and the other cable providing neutral for both lights and unswitched hot for the single pole setup.


Quote:
Originally Posted by rjniles View Post
I recently had a very similar situation.
Two 2-gang switch boxes, each fed with power. Single pole switch feeding light connected to same box, in each box. Three way switch setup using both boxes and feeding yet another light.

The same power feed (box 1 OR box 2) should provide both hot and neutral to the light controlled by the three way switches. So you will need to use all three conductors in the 12-3 cable going between the switch boxes. As you described it, the white you capped off would be the unswitched hot from the feed in box 1 to the common terminal of the 3 way switch in box 2.
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Last edited by AllanJ; 07-22-2010 at 07:33 AM.
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Old 07-22-2010, 02:49 AM   #7
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3-way switches, share neutral?


Share neutral?

First, normally neither 3-way switches nor 3-wire cables between these sws have any neutral [to share]. The insulated cable conductors in NM MI MC AC and similar cables are all hot in most situations:
1 common
2 travellers
3 total (+1 uncounted grd wire)

If a neutral is needed alongside too (not usually the case), an additional wire is needed somehow, which can be in a larger cable, conduit, or additional cable.

In this case though, with two 3-way sws at each end of the same run, you could use the "common" in one cable for all 4 sws I believe; and use the white in the 2d 3-wire cable as a shared neutral to connect the line neutral with the load neutral at the two bxs (neutral bypassing all sws).

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Old 07-22-2010, 07:30 AM   #8
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3-way switches, share neutral?


With just one 3-way setup, it is not unusual for it to be arranged as: power-switch-switch-light. Then the white wire in the (3-conductor) cable between the switches is the neutral and there is no constant (raw; unswitched) hot.
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Old 07-22-2010, 11:26 AM   #9
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3-way switches, share neutral?


VRP,

I'm not sure exactly what you're getting at but when I say "netural" I'm referring to the white wire that doesn't get connected to any of the switches but just carries through the circuit by itself from the power to the first lights.

The run of lights is not going to be between the switches because I don't want to run double lengths of 12-2 wire (we are talking about 9 lights here). It will save me a ton of wire to just go: power>switch1>switch2>lights.

When I say "share" I simply mean that only one 12-3 wire would carry the neutral wire between the switches then it would split back into the two neutrals needed for the two separate runs of light.

Electrically this should be fine because the neutral has no connections with any of the switches: it runs straight from the power source to the first lights.

My question was really whether the code allows it and per Stubbie, it does not because "Technically the electrical code requires that a circuit or a branch in the circuit have all wires associated with that circuit inside the same nm cable."
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Old 07-22-2010, 04:34 PM   #10
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3-way switches, share neutral?


Quote:
Originally Posted by AllanJ View Post
With just one 3-way setup, it is not unusual for it to be arranged as: power-switch-switch-light. Then the white wire in the (3-conductor) cable between the switches is the neutral and there is no constant (raw; unswitched) hot.
Allan, what you describe is NOT proper 3-way wiring, as it cannot possibly get both sws to control the light(s) under various conditions. The "common" connections on the sws, and wire run between the sws, aren't optional and may not be ignored. There MUST be TWO traveller wires and the common wire between the switches for everything to work correctly.

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Old 07-22-2010, 05:32 PM   #11
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3-way switches, share neutral?


Quote:
Originally Posted by VRP View Post
Allan, what you describe is NOT proper 3-way wiring, as it cannot possibly get both sws to control the light(s) under various conditions. The "common" connections on the sws, and wire run between the sws, aren't optional and may not be ignored. There MUST be TWO traveller wires and the common wire between the switches for everything to work correctly.

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Quote:
Originally Posted by AllanJ View Post
With just one 3-way setup, it is not unusual for it to be arranged as: power-switch-switch-light. Then the white wire in the (3-conductor) cable between the switches is the neutral and there is no constant (raw; unswitched) hot.
This is a very common wiring for three way switches. Power in to Common at SW1, blk and red as travelers and the whites splice thru as a neutral. At SW2 the black from the fixture to the Common, whites splice thru and again black and red to travelers.
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Old 07-22-2010, 06:18 PM   #12
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3-way switches, share neutral?


Given further thought, I admit that I erred and need to withdraw my earlier comments on this thread. (Gosh it's been a while since I did one of these from scratch!) Yes, the "common" on SW1 is the line; on SW2 the load. My apologies.
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Old 07-22-2010, 06:22 PM   #13
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3-way switches, share neutral?


Obviously I'm not an electrician but everyone else here seems to be so I'm guessing there is just some confusion between what you guys are saying. What is confusing me is when Vic said, "There MUST be TWO traveller wires and the common wire between the switches for everything to work correctly." These seems to suggest there need to be 3 wires (excluding ground) between the switches to work.

As far a I can figure, for the power>switch1>switch2>light scenario, you only need 2 traveler wires between the switches and the neutral just passes through. You could just as easily run this neutral as a separate wire straight from power>light and the circuit would work (although doubtfully be code).

Then again, I'm not an electrician so maybe I have no idea what I'm talking about...

**UPDATE** I apparently posted simultaneously with Vic so never mind the above confusion.
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Old 07-22-2010, 08:35 PM   #14
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3-way switches, share neutral?


Here is a code compliant way which is what you were asking in the first post.

In the other way you would have one 12/2 g between switch boxes using a white in the 12/2 as a traveler and the white in the 12/3 as the neutral to the lights. In the box with the 12/2 to the lights you wold join the two neutrals of those cables to the neutral of the 12/3.
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Old 11-13-2010, 10:10 PM   #15
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3-way switches, share neutral?


Stubbie... he knows that. He wants to know if he can legally omit one of the neutrals between the switch boxes, replacing one of the pieces of 12/3 with 12/2 instead.

It'll work electrically. I don't think it's legal.

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