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  1. what's more common for light switches at home, 12 gauge or 14 gauge wire? how do I know what I have?
  2. I need to pigtail my ground and neutral wires to connect to my new leviton decora smart light switch. what do I ask at home depot to buy these pigtail ground and neutral wires?
  3. what does it mean by NM-B wire or THHN? I don't know which one to buy for my light switch, and don't know the exact specs to ask for. thanks
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    2. NO, you should never connect ground and neutral outside the main panel. – ratchet freak Sep 22 '17 at 10:02
  • I think FC may be misunderstanding what he has to do. I have a lot of Decora switches that come with built-in leads for hot, neutral, and ground; all you have to do to install them is pigtail those leads to your existing wiring. In which case, all you need are wire nuts of the appropriate size. – Robert Nubel Sep 22 '17 at 14:48
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  1. Turn off the breaker for the device. Is it a 15-amp breaker? Use 14 gauge for pigtails. Is it a 20-amp breaker? Use 12 gauge for pigtails.

  2. Buy a few feet of the appropriate color of solid THHN and wirenuts. (THHN also comes in stranded, in this case you don’t want that.) As noted in a comment if you mean that you are going to use neutral as a bootleg ground, that’s a big no-no.

  3. NM-B, or Non-Metallic sheathed cable is a multi-conductor wire with at least hot/neutral/ground, commonly called Romex. THHN is a single conductor wire.

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What you're probably looking for is a wire nut. If you have several loose wires in a box that need to be electrically joined, this will do that.

Get in the habit of thinking of wire nuts as an ordinary and routine way of joining wires. You should be making the right joins and not worrying about how to fit them on the available screws. And you shouldn't be tempted to use one of the horrible back-stabs.

enter image description here

A wire-nut attaching a pigtail to 2 other wires. ***

Another tool in your toolbox is the pigtail. This comes out of your receptacle and joins any number of wires with a wire-nut or other fastener. Some devices come with these, or you can install one yourself with extra wire.

What size wire to use? Look at the number on your breaker. 20 amp circuits must use 12 AWG or larger wire. 15A circuits can use 14 AWG, which is the smallest wire legal for mains/120V electrical. You're always allowed to use larger wire than required, so 12 AWG is a "universal donor" for 15-20A circuits. Don't work with two sizes, because then you risk mixing them up. I for one don't own any 14AWG wire.

What kind of wire to use? Your call. You can either buy THHN wire, which is sold one wire at a time, and is normally used in conduit. ** Or you can buy NM cable, which has several wires wrapped up in a jacket. Cut the jacket off and use the single wires.

You need multiple colors - white or gray for neutrals, bare or green for grounds, and any other color for any wire that could be hot. NM cable comes with all those.


** THHN it comes in solid and stranded. Solid is easier to attach to terminals, but stranded is a lot more flexible.

*** Looking at that picture, why did the installer do it this way? Other than the pigtail there are only 2 wires, and 2 available screws. Because this site is in an awkward position! The pigtails were fit at a comfortable workbench. Fit 3 wire-nuts and done - much faster than doing 5 screws.

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