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Can I run wires from two separate circuits through the same box, if one of those circuits is just passing through (no connections)?

I have a switch in the basement which is currently wired to outlets in the room and I want to change it to switch the overhead light instead.

I know how I would go about wiring it. The diagram shows how the room is currently wired. What I will do is remove all wires from the switch twist the two black (live) wires together with a nut and remove the pigtailed ground from the switch (and the box). That leaves those outlets all normally powered without a switch.

enter image description here

This wiring diagram belongs to do-it-yourself-help.com. The original can be found here: https://www.do-it-yourself-help.com/outlet-switch-electrical-diagrams.html

Now my question is, can I just leave those wires in that box even if I am going to run wires from the light into that box, and to that switch? The light is powered from wires on a different circuit, so I will have two sets of wires from two different circuits running through that box.

It seems okay to me, although it's not clear if I am breaking regulation codes by having two circuits so close together. I know you will likely tell me it will depend on where I am (Alberta), but I would rather err on the side of safety. Would you do this, yourself? If not, how would you properly route the existing wiring around the switch? I know that wherever I choose to splice those wires, they need to be in an accessible box.

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  • At most you would need to put in a bigger box to make room for all the wires. – ratchet freak Feb 1 '17 at 8:58
  • Why are your cables marked as 2-wire while they're obviously 3-wire? – Agent_L Feb 1 '17 at 10:15
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    @Agent_L ground doesn't count – ratchet freak Feb 1 '17 at 11:10
  • One of the grounds is counted in box fill. – Ed Beal Feb 1 '17 at 19:00
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The answer is yes you can have 2 separate circuits in the same box (they can have a splice also but not needed in your case). The only concern would be the total box fill. Based on the NEC the wire fill would be calculated at 2.0 for each conductor for 14 gauge wire and 2.25 for 12 gauge wire. The device or switch counts as 2X for the largest wire connected to it or each in the box. All the ground wires count as 1X of the largest ground in the box. If the box has clamps they are counted as 1X of the largest wire. With the numbers added up they need to be smaller than the listing on the box for cubic inches. If more information is provided on the size of the wires entering and exiting the box and total number of devices we can help with the calculation.

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  • Okay, the box is 2x3x2.5". The wires would include: a 2-wire to the source + a 2-wire to the plugs (with 2 or 3 nuts to splice these two together) and 1 more 2-wire to the light (no nuts since this will by connected to the switch). I'm not sure if we count grounds when calculating this? – Octopus Feb 1 '17 at 18:02
  • The box is listed as 12.5 cu inch. NEC 314.16.A. We would only count 1 of the ground wires. (What size wire ?) we don't count the wire nuts they are counted in the splice. If I read this correctly there would be a total of 7 wires and a device (the switch) with 14 gauge wire the multiplier would be 2 (2.25 for 12 gauge). The device is 2x the wire size, with 7 wires and 1 device using 14 gauge the box would need to be 18 cubic inches without clamps 20 with clamps. – Ed Beal Feb 1 '17 at 18:58
  • This is good stuff.+ – JACK Jan 16 '20 at 17:40
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There is, of course, no problem with putting a larger box into the wall so that you have more room for all the wires and nuts you plan to put inside.

There might be a problem with the presentation of the finished work. You wouldn't want to put a double gang faceplate on your wall when you only have one switch.

The solution is to use what's called a plaster (or mud) ring. This could cover say, a two gang box and leave only a single gang exposed through the drywall.

enter image description here

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    I didn't know there was such a thing. Thank you oddly named DIYer. – Octopus Feb 2 '17 at 5:44

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