Wiring lights for multiple rooms on one circuit


Old 12-29-04, 12:58 PM
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Wiring lights for multiple rooms on one circuit

I understand how to wire a single pole switch to control lights in one room. I want to add a 2nd room with it's own switch, however, both rooms will be feeding off of the same circuit. In order to achieve this, do I simply split this circuit via a junction box and then have one lead go to the 1st room's light fixture/single pole switch and then a 2nd lead go to the 2nd room's light fixture/single pole switch?
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Old 12-29-04, 01:40 PM
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The simple answer to your question is "Yes", although splitting the circuit via a junction box may not be as easy as it sounds.

You need to have 6" of free conductor inside the junction box, for both ends of the cable once you have cut it. That means you need to have a bit under a foot of slack cable. I say "a bit under a foot", because if you put in a 4" square j-box, you automatically get about 2" of slack on each of the cable ends entering the j-box and would need to find another 8" or so of slack.

If you can't get the slack you need, you could:
  • Install a pair of j-boxes a foot or so apart. This would let you splice in a piece of cable so you had the required lengths of free conductor in your j-boxes.
  • Cut the cable and rerun the cable from where you're installing the j-box back to where the cable came from (assuming you have easy access to the entire cable run to do that). This latter option is more work, but would leave a neater installation when you're done.
FYI, you wouldn't have a "lead" going to each switch, you'd have a cable. This is assuming your house is wired with NM (aka "Romex") or BX (metallic armored cable). A "lead" implies a single conductor, at least to me it does.
Old 12-29-04, 01:59 PM
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Rather than trying to tap into the boxes in the attic, you would be better off finding where the power runs and pulling from there.

If the power runs into the switch box and is switched to the light, then tap into the switch box for your new light.

If the power runs to the light first and a switch loop goes to the switch then tap into the light for your power.

If you are running a new circuit then run it the best way that makes sense.
Old 12-29-04, 02:14 PM
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Tapping into existing boxes is fine, provided there's adequate room in the box. Assume (3) 14/2 NM + internal cable clamp + device. That's 20 cubic inches if I do the math right [(6 current carrying conductors + 1 grounding + 2 device + 1 cable clamp) * 2].

Unless the original EC used 22 cu boxes all over, I'd guess he doesn't have the box fill capacity to tap into an existing box.


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