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Old 05-03-2020, 09:23 AM   #1
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3 way switch wiring


I have two lights on the front of my barn. They are currently each switched by their own single pole switches in the barn, with both switches being in one box and fed by one power wire from one circuit breaker. I am putting in LED bulbs so the current draw will be very small. I want to change their control to a pair of three way switches, with two in the current box and two in the house. Then I can control each light from the house or the barn. When I built the barn I put 4 extra 12/2 UF wires in the conduit connecting the house and barn and those wires are currently unused. I am thinking about using three of those 12/2 wires to connect the switches, emulating the function of 2-12/3 wires. I would indicate that the whites are current carrying conductors. I know it will work, but my question is will it be in compliance with the NEC? If not, can you cite the NEC passage that would disallow it? Thanks.
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Old 05-03-2020, 09:45 AM   #2
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Re: 3 way switch wiring


To me it will be a bit of a grey area. I can't site you the rule# but there is a rule stating that all current carrying conductors must be in the same cable jacket or conduit. By splitting the required 12/3 into two 12/2 you are breaking that rule. However it seems to be done that way quite often.
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Old 05-03-2020, 10:20 AM   #3
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Re: 3 way switch wiring


You might want to look at a wireless solution. I use Insteon but there are other products. 2 Insteon switches in the barn and an Insteon keypad in the house. At full retain about $150
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Old 05-03-2020, 12:42 PM   #4
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Re: 3 way switch wiring


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Originally Posted by joed View Post
To me it will be a bit of a grey area. I can't site you the rule# but there is a rule stating that all current carrying conductors must be in the same cable jacket or conduit. By splitting the required 12/3 into two 12/2 you are breaking that rule. However it seems to be done that way quite often.
Cable jacket “or” conduit I comply with, they are both in a 4” conduit between buildings. Cable jacket “and” conduit I do not comply with. 4 way switches are often wired with 2 separate 2 conductor wires, so I wonder if that complies. They are almost the same thing. I have no reservations at this point with its safety, but if it is prohibited there must be a reason.

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Old 05-03-2020, 12:57 PM   #5
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Re: 3 way switch wiring


Requiring all the conductors of a circuit in the same sheath is to prevent overheating. If the current goes out on 1 conductor and returns on a conductor in the same sheath or conduit, the EMF is cancelled out.
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Old 05-03-2020, 01:15 PM   #6
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Re: 3 way switch wiring


Same conduit implies single conductors. Like I said it is a grey area. Often two x/2 cable are used in place of an x/4 cable to bring neutral to a three way switch.
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Old 05-03-2020, 02:35 PM   #7
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Re: 3 way switch wiring


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Same conduit implies single conductors. Like I said it is a grey area. Often two x/2 cable are used in place of an x/4 cable to bring neutral to a three way switch.
True but technically a violation. If you got gigged you have no defense except beg for mercy.

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Old 05-03-2020, 04:26 PM   #8
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Re: 3 way switch wiring


Ok thanks. Do you happen to know where to find it in the NEC? I have a copy but I don’t use it enough to be familiar with where to find things but I will look.
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Old 05-03-2020, 05:21 PM   #9
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Re: 3 way switch wiring


Use one 12/2 to send power to the switch box in the house.
Use the other 2 12/2s as travelers for their respective 3 way switches.
Power 3 way switch from house end, and switch leg from barn end.
You will have a neutral in the house side box not needed (but required by 404.2) so it will be capped off.
The only difference between this and what you proposed is in the make-up of the wiring in each box.
Also, code compliant.


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Last edited by Shaneyj; 05-03-2020 at 05:34 PM.
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Old 05-03-2020, 06:55 PM   #10
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Re: 3 way switch wiring


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Originally Posted by Shaneyj View Post
Use one 12/2 to send power to the switch box in the house.
Use the other 2 12/2s as travelers for their respective 3 way switches.
Power 3 way switch from house end, and switch leg from barn end.
You will have a neutral in the house side box not needed (but required by 404.2) so it will be capped off.
The only difference between this and what you proposed is in the make-up of the wiring in each box.
Also, code compliant.


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That is actually what I planned to do. Two 12/s would each be a set of travelers for one 3 way switch and I would indicate that their whites are current carrying conductors. I will have one unused neutral in the power wire, which is the third wire. You said it better than I, as I forgot to say I was powering the switches as you described. Each switch will have less than 100 watts of LED lights on it.
My backup plan if there was a compelling reason not to do it as stated, was to put 2 pairs of 3 way switches in the barn. Then I could use all 4 12/2 cables as travelers for 2-4 way switches in the house.

Last edited by Old Thomas; 05-03-2020 at 07:14 PM.
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