Wiring A Switch: One NM Run Or Two Best ?

050330 1256 - Robert11 posted:

It has been the practice to run just one romex cable to a switch: the white wire tied to the hot leads in the fixture box, and the return black from the switch goes to the fixture.

Look in your book under 1960's wiring methods. Renaming sucks.

Over the years, I have done a bit of relatively simple wiring around the house.

Have just finished reading the book by H. Richter, Wiring Simplified, 40th ed. with great interest. A very handy little volume.

Have the following question for those of you who might be doing this stuff for a living, please:

Whenever I have wired a new wall switch, I have always brought a piece of NM wire from the source to the switch, the black going to one terminal. Then I ran a second piece of NM with the black from the other switch terminal to the new light fixture, along with its white lead. (the whites would also be tied together for the two pieces of NM at the switch box).

I see in the book (p 110) where it is apparently permissible to just use a single piece of NM to the switch, with the white being re-marked black. This NM would then effectively be placed in series with the normal black wire from the source and to the fixture. (the white from the source also going to the light fixture).

The approach i have been using which I described in the first paragraph isn't even described in the book.

So, other than the first approach using twice the amount of NM, is there any advantage one way or the other ?

What do you folks do in a simple situation like this ? Why ?

Thanks, Bob

If you want power to be available at the switch box location (for example, for a combination switch/receptacle) you have no choice - you must run a cable from the source to the switch location and a cable from there to the light.

If the above is not the case, then the physical layout determines the best way.

An additional advantage (besides possibly using less wire) to a switch loop versus 2 cables is box fill. When you add a second cable, box fill calculation increases by 4 cu in for #14 wire, or

4.5 cu in for #12 wire. A small utility box is not code compliant for a switch and 2 cables.

Ed

Good point about the box fill. Also about needing power in the switch box. I've retrofitted my house with X10 switches, receptacles, etc. X10 switches (switches that can be controlled either manually or remotely via X10 commands) require a hot and neutral to the X10 switch and provide a switched hot out to the controlled fixture. I've been caught a few times when the existing switch had only one NM run, i.e. no neutral. My solution was to pull in a 12-3 plus ground. One NM cable with two hots, a neutral and a ground. The second hot (red) was used for the switched leg.

If I were wiring my next house and I didn't mind spending a few bucks, I'd think about running 12-3 to each switch.

Wiring A Switch: One NM Run Or Two Best ? Group: alt.engineering.electrical Date: Wed, Mar 30, 2005, 12:56pm (EDT-1) From: snipped-for-privacy@notme.com (Robert11) Hello: Over the years, I have done a bit of relatively simple wiring around the house. Have just finished reading the book by H. Richter, Wiring Simplified,

40th ed. with great interest. A very handy little volume. Have the following question for those of you who might be doing this stuff for a living, please: Whenever I have wired a new wall switch, I have always brought a piece of NM wire from the source to the switch, the black going to one terminal. Then I ran a second piece of NM with the black from the other switch terminal to the new light fixture, along with its white lead. (the whites would also be tied together for the two pieces of NM at the switch box). I see in the book (p 110) where it is apparently permissible to just use a single piece of NM to the switch, with the white being re-marked black. This NM would then effectively be placed in series with the normal black wire from the source and to the fixture. (the white from the source also going to the light fixture). The approach i have been using which I described in the first paragraph isn't even described in the book. So, other than the first approach using twice the amount of NM, is there any advantage one way or the other ? What do you folks do in a simple situation like this ? Why ? Thanks, Bob ~~~~~~~~~~~~~~>

I've come across your wiring method before, it leaves 2 uneccesary white conductors spliced in a box. it was a color code issue, and it's been a while since the NEC Geniuses along with Manufacturers, Electricians & Homeowner$ decided it was a foolish demand at that level.

I'd Just run a 2 conductor cable to the switch box it seems nobody even bothers covering the white wire black, as they say, if a guy doesn't know what wires run through a switch he shouldn't be in there to begin wih };-)

when you work with rigid conduit (emt) you'd run a red & a black only to the switch., it's nice nec manners to follow color codes when & if you practically can.

=AEoy

I mark white wires used as hot with a piece of heat shrink tubing. Never heard of anyone running two pieces of NM for this purpose.

It is done frequently, and is the most economical way in many cases. Here's an example: source==NM#1==Switch===NM#2==Light

Say from a receptacle box (source) to a switch directly above it to a ceiling light. Two NM cables enter the switch box, the whites are spliced and the blacks connect to the switch.

Ed