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So I purchased a fancy new programmable light switch. It has 3 wires coming out of it: load (blue), neutral (white), line (black).

I opened the face plate on my current switch to take it out, and it has only two (black) wires going to it. Behind the switch itself, I don't see any ground (ie. exposed copper) wires.

However, I think I possibly see the neutral lines, I see 3 white wires going into a red terminator cap (not sure if that is the correct term for it).

My questions: (1) Is it possible for me to replace my old switch with this new one? (2) If so, how should I wire the new switch?

FWIW, this switch will be controlling our outdoor porch light.

Here is a picture of the old switch, the new switch, and what's inside my switch box:

Here is a picture of the old switch, the new switch, and what's inside my switch box

  • Can you pull the white wires going into the red cap out and get us a better shot of them? (Do this with the breaker off of course.) – ThreePhaseEel Aug 18 '17 at 11:42
  • From what you are saying sounds to me you have all the right wires, but will it fit? Looking at your new switch you have leads coming from the front of the switch. How would you put it in a wall box and put a cover on it. It appears to me that it should go into a special enclosure of some sort. – Retired Master Electrician Aug 18 '17 at 17:01
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I think you can assume the white wires are the neutral associated with the line hot. Just make a 6" long pigtail of white #12 or #14 according to the gauge of the other wire and insert the stripped end into the wire nut with the other whites. The other end goes to the neutral terminal of the switch.

  • This should do it, white is the neutral and your fancy switch needs that to power its electronics. – Ed Beal Aug 18 '17 at 19:15
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Two blacks all but guarantee the neutrals are in the back of the box hiding. I am fairly sure I see them back there. Most residential North American work is done in cables. /2 cable is always black and white, you're stuck with that. When you see 2 blacks to a switch that usually means there are 2 cables coming into the box... and all their whites are nutted together in the back of the box, that would be neutral.

The smart switch will care which cable is the supply (or "line") vs the load. To ferret this out, remove the old switch and cap off both black wires, and get a non-contact voltage tester and see which one lights up. That is the LINE.

If that doesn't work, you can try powering up the smart switch off one black, then the other:

  • Add the smart switch neutral wire (noting it has built-in pigtails) to the netural bundle.
  • Hook the smart switch "LINE" wire (black on yours) to one of the black wires that came off the switch.
  • Cap the other one so it can't electrocute anyone.
  • Test. If that doesn't work, switch blacks.

Once you've identified LINE, tape it with black tape. Tape the other one (LAMP) with red or blue tape. (doesn't really matter, just so it's weird.) In new home construction, nobody tapes things because they're in a hurry. It's a shame, things are a lot easier if you identify wires.

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