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3 way switch -OK to drop down neutral like this to light through switch box via

3 way switch -OK to drop down neutral like this to light through switch box via

Old 12-21-11, 10:23 PM
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3 way switch -OK to drop down neutral like this to light through switch box via

I apologize for my diagram. The circuit is off that powers my computer with Photoshop, so I had to use paint.

I'm wondering if I can wire two three way switches and two lights as shown in the diagram. Although it's my first three way switch experience, I'm pretty sure it will work as diagrammed, but I want to also make sure wiring as shown doesn't violate NEC 2008 code -- or professional/good practice wiring.

Short story is that I'm replacing all the fabric wrapped 14/2 cables in my house that run lights. The hardest run for a new cable is between this set of three way switches and the basement light. Believe it or not, this is the one and only spot in the lighting circuits that already uses plastic romex cable, so if I can re-use the existing plastic romex cable, I don't have to do the tough run.

I wasn't interested in following the original wiring paths, so I ran the two circuits from the panel into the attic. I have a junction box in the attic, corresponding with each switch on the main floor. I'm only dropping down a 12/2 with ground to the switch boxes. Of course, the power source comes to the switch on a white remarked as black, and comes up to the light on the black. The neutral, of course, jumps from the attic junction box's source cable to the neutral on the cable running to the light.

That works great, except for this one light (the one in the basement in this diagram) that needs a neutral that I can't easily drop down a cable from the attic to.

I had to buy a small amount of 12/3 with ground to replace the old cables between the three way switches. I realized that I have enough 12/3 left over to run from the attic junction box to the three way switch that has the outgoing black common.

Assuming this wiring will work in the first place, is it OK to wire it this way?

Is it OK for me to deviate from my standard wring pattern, and drop a neutral down to the top three way switch box that just passes on to the basement light cable? Am I allowed to do this in the first place?

Is it OK to use a 12/3 cable for this purpose -- a neutral going down to branch off to a light, and the black and reds effectively going to the common screws on each of the three way switches?

Do I have the uses of red black and white done correctly, or is there a different way required by code or standard practices? (For example, I know that a white remarked as black wire in a 2 conductor cable to a switch has to act as the source for the switch - not sure if I've violated a rule like this in my diagram.)

FAQ's for my post:
* Michigan is on NEC 2008 still, so I'm not required to have a neutral at switches. I'm not concerned about future capacity for smart switches.
* I left the grounds off my diagram, since those are easy and don't really need to clutter up the diagram. But, all cables I'm using have a ground.
* I know I could have used 14 AWG wires instead of 12.
* I've put this circuit on a 15A breaker, rather than 20A. The original reason why is because the switches are only rated for 15A, and besides, two 15A circuits is overkill for 9 lights. But, since I'm running on a 15A breaker, using the existing 14/2 cable ran between one of the three way switches and the basement light will be OK. I'll mark in the box that the circuit must be 15A both because the switch limitations, and that 14/2 is used at that one spot in the circuit.

Old 12-21-11, 11:09 PM
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I think there is no problem wiring three way and one way switches in the same circuit. In your diagram, I would presume that the hot wire(source) is the black wire(12/2) connected to the red wire(12/3) that goes to the 3way switch that controls the light upstairs. I would assume also that the basement light is controlled by a separate light switch. Since all the three wires in the 3way switch are all hot and you connected the black wire for the basement light to the white wire(taped black) from the 3way switch; everytime the 3way switches are inactive (off), you won't have power going to the basement light. You can correct this by connecting the black wire for the basement light to the red wire in the 3way switch. By doing so, the power going to the basement light will always be active. I hope this will be of help. Good luck.
Old 12-22-11, 04:54 AM
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What size breaker is protecting this circuit?

You should not mix wire gauges in the same branch circuit.
Old 12-22-11, 08:28 AM
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Sometimes it is desirable to use a different gauge of wire somewhere in a circuit becuase that is a leftover piece of cable you have no near future use for and also it is inconvenient to have to go out and buy the correct size and perhaps end up with a lot of lefover in that size.

But if there is any 14 gauge wire in the circuit, the breaker may be no higher than 15 amps.

It is okay to have a 15 amp rated switch on a 20 amp circuit but the wire beyond the switch has to be at least 12 gauge.

It is suggested that (if permitted) if a 12 and 14 gauge circuit enters the breaker box as 12 gauge, that a short 14 gauge pigtail be put on the incoming hot conductor to connect to the (15 amp) breaker.
Old 12-22-11, 09:51 AM
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Thanks everyone!

tinoM - Yes, the hot source wire is the black wire in the 12/2 at the top left of the diagram. The basement light isn't controlled by a separate light switch, actually. I want the three way switches to control both the upstairs and basement lights, of course having them always in the same in or off state as each other. There's no light around the middle step, but if both lights are on, the entire stairway is nicely lit.

Wirepuller38 - Using a 15A AFCI combination breaker. I'd love to replace the 14/2 cable if it wouldn't be that bad to do, but that existing plastic romex cable happens on the be on the run that would involve tearing out part of the basement's stapled on fake drop ceiling. So, I'd love to use it for now so I can delay the basement ceiling project.

AllanJ - It scares me to have a 15A switch or 15A outlet on a 20A circuit, even though I see it's allowed and commonly done. If there's a malfunctioning light or device that pulls as much current as it can without tripping the 20A breaker, would the 15A switch or outlet start burning up? Or is there enough tolerance built into them that this wouldn't happen? I like the idea of using a 14 gauge pigtail in the panel, to make it very clear what's going on there. I'll make sure to put that in before I'm done.

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