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I need some help working out if the wiring in my recently-purchased house is dangerous. Quite a lot of stuff around the house, from the drywall to the flooring, was evidently done by the previous owner himself (he built the house) or by other non-professional friends of his, and it's possible this amateurism extends to the wiring also.

The two living room 3-way switches were working fine until I put new rocker switches in, after which I found that one of the switches must be on for the other one to work. Okay, so I must have mixed two wires up.

I did some reading on the interwebs, but everything I read failed to describe the situation I was facing. With both switches disconnected (everything but the grounds), they both showed a hot wire (according to my non-contact tester). Everything I'd read said that there should only be one hot wire out of six wires. Through trial and error, I was able to discover the combination that works; at the hall switch, the hot goes to common; and in the living room, the hot goes to one of the other terminals.

So, everything works fine now. What's worrying me though is that, in my reading about this issue, I learned that there are certain ways of wiring 3-ways ("California"/"coast"?) that are not safe. The fact that I couldn't find anything about having hot wires at both switches has left me concerned as to whether my wiring is okay or not.

What do you think? Is this enough information?

  • Using a California 3-way neither hot could possibly be connected to the common screw. I suspect the "hot" you connected at the living room was picking up some stray voltage that showed on your non-contact voltage tester. – Tyson Oct 4 '17 at 21:57
  • California/coast 3-ways are fine, it's the Carter system that is dangerous and against Code. Can you post photos of the insides of the boxes involved? – ThreePhaseEel Oct 4 '17 at 22:22
  • Thanks. Unfortunately, I didn't take photos before putting it all back together. – Graeme Wend-Walker Oct 19 '17 at 19:02
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The usual blunder is looking at wire position rather than purpose. Anytime you work with 3-ways, your first priority is identifying the two messenger wires going to the switch from the same cable.

However when novices change 3-way switches, they usually look closely at the physical position of the terminals on the switch, duplicating this on the new switch. The problem is manufacturers totally randomize which screws are in which positions. So the swap ends up shuffling the wires on the switch - and creates cases where the switch won't work in some positions.

The right way is to pause before removing the old switch, identify the messengers (typically on the brass screws of the old switch) and then mark them with colored tape, e.g. I like to use yellow for messengers (they can both be yellow since they are interchangeable.) Be sure to mark the wires the same on both ends of the wire.

The good news is there are only three possibilities - one wire is the common and the other two are the messengers - and often the common goes into a different cable than the messengers, which are always together.

That wiring

Homeowners doing their own wiring can be a problem because they can often get quite "inventive". Worst case, if you have to tear work out, they usually do give you enough wires to do it properly. If not, "smart switches" can often save the day.

You can exclude a California style wiring because it would involve /4 cable or (illegally) two /2 cables paired. The Carter system (serving hot or neutral, lamp in the middle, lamp lights when one is hot and the other neutral) is something to watch out for, not least because it is a wiring loop, which is bad. All wiring should be physically in a tree topology.

  • Thanks. Yes, that's exactly what I did: I tried to duplicate the wiring on the first switch. – Graeme Wend-Walker Oct 19 '17 at 19:01

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