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I just had our house re-wired (by a professional, not myself). I'm looking into replacing a light switch with a fancy programmable version (Honeywell Econoswitch RPLS740B).

The location for the switch has two switches next to each other in the same box. They both have two black wires and and one bare copper wire ground connected to them. There is a bundle of 2 white wires nutted together behind the switches not connected to anything.

When I look at the manual for the programmable switch, it has three connections: black, blue (load), and white (neutral).

So my questions:

  1. For the black wires connected to the current switch, how do I know which is hot, and which is the heading off to the light (load)? Does it matter?

  2. For the white, do I just un-nut one of the wires behind the switches and hook it up?

  3. Why does the programmable switch require the white wire?

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up vote 4 down vote accepted
  1. Turn off the breaker, undo both wires from the switch, then turn on the breaker. Get a NCVT (non contact voltage tester) and see which of the wires is hot. That will be the one back to the panel, the other one will be the load.

  2. Yes, just connect the white to the other neutrals in the box, so they are all wire nutted together.

  3. The programmable switch requires a neutral because it needs current itself all the time. If you just had the 2 black wires, there would be no current flow through the switch when it was off, so no way to power the electronics.

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Actually when it's off it could use the Load side as a low current neutral. When it's on it would require an internal step-up transformer to get enough voltage for it to work. – Brad Gilbert Feb 3 '13 at 15:49

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