Help me understand existing wiring to three way switch
I want to replace the working switches in a three way light switch arrangement. I do not understand the wiring. With both existing switches removed i tested to isolate the hot wire .I found that all the wires in one box were hot and two in the other box were hot. Also one box had a red wire and the other box did not.My understanding was that I should only have found one hot wire and that the two would have been travellers?
Replies
Dan is right. Use an old C-7 christmas tree light with some decent test leads to check the wires. There are 2 basic ways to wire a 3 way loop. In one way, you feed the switch, then the travellers in one box and the travellers feed the second switch with the light out the other end.
In that case the neutral (white) will be connected straight through.
You will have 2 cables in each box, one 2 wire, one 3 wire.
The other way to do it is a switch loop where one box will only have the 3 wire cable coming in. Power goes down on the white wire (reidentified to a different color) and back on the travellers. The neutral in the incoming and outgoing cables in the source box will be connected and there will be 3 cables in it.
You can have other configurations but those are the two most popular
At least part of your confusion is due to the fact that you used some sort of high-impedance tester to determine which wires were "hot". If you have a "hot" wire (connected to the breaker in the breaker panel) and an unconnected wire running through the same cable, the unconnected wire will test "hot" with a neon tester and some other forms of circuit testers (such as those with LCD displays). You need to use something like a regular incandescent bulb to reliably test in this situation.
(Gory details: The capacitance in the wires between the "real" hot wire and the "pretend hot" one is large enough that a charge is transferred between the two wires. The amount of current that flows with this charge is miniscule, but, eg, a neon tester needs very little current to light up. An incandescent bulb (even, say, a 7w Christmas lamp) needs much more current and will "drain" the charge from the "pretend hot" before enough current flows to light it up.)