3

Relevant but didn't address the GFCI part of the question: Split a single branch circuit into two runs?

I'm wondering if I can split/tee/branch an existing circuit that uses wall mounted metal conduit. Existing wiring is:

                                      ______
                          ___________|(GFCI)|
                         |           |outlet|
                         |           |___B__|
  _________              |
 | Panel   |         _______
 |         |        | GFCI  |
 |     15A |--------| Outlet|
 |_________|        |___A___|
        

With 'A' being a GFCI plug that feeds 'B', protecting it as well

My hope is to do this:

                                      ______
                          ___________|(GFCI)|
                         |           |outlet|
                         |           |___B__|
  _________              |
 | Panel   |         _______                ______
 |         |        | GFCI  |              |(GFCI)|
 |     15A |--------| Outlet|--------------|outlet|
 |_________|        |___A___|              |___C__|
        

With 'A' connected to pigtails on the right side/downstream that connects to the existing lead 'B', and new lead 'C' using wirenuts

My questions:

  • Is this possible to do within code? I'm guessing my box for 'A' will be too small so I'll need to switch to a double gang just for space with all the wire. (wire in from box, grounds, pigtails to 2 out to B and C)
  • Is there any issues with tee/splitting off a GFCI like this? 'B' and 'C' are essentially sharing the GFCI in 'A'
  • Is this called tee-ing a circuit? I've been looking for information on it and haven't found much other than the link above. I assume its not common since it wouldn't ever happen in a new build?

Background: I've took a residential electricity class ages ago, and I'm competent enough to swap plugs and switches. If it requires adding a new circuit to the panel I'll either not do it, or hire someone. If its all downstream of the circuit then I'm confident I can wire it. I have a standard plug tester to check proper wiring and GFCI. I'm in MN in the US in a residential home built in the 70s.

3
  • 1
    As noted in both answers, you're probably just fine (code wise) with the existing single-gang box. However, I've found that the minimal additional cost of getting the largest (most cubic inches "CI") box I can find for the number of devices I need (1 in your case) is always worth the investment because it simply makes getting all the wires & the device(s) into the box much easier. There's much less wire origami required with the additional few CI. Especially the case with large devices like dimmers and GFCI outlets.
    – FreeMan
    Dec 1 '21 at 13:32
  • Thanks, @FreeMan. Yeah I think I'll get the bigger box for 'A' exactly for that reason.
    – martin
    Dec 1 '21 at 18:25
  • 1
    Teeing is not treated as anything special. Cable layouts are done in tree topology, nothing in Code requires the tree be a vine. Dec 2 '21 at 0:39
6

Yes, that’s perfectly valid. Everything downstream is in parallel either way, code does not care if you daisy chain or pigtail/tee.

As far as box full goes, you’re probably legal with a typical single-gang box and #14 wire. Six units for current-carrying wires, one for ground, two for the device gives 9 units, times 2.0 cubic inches for #14 gives 18 cubic inches required - which is one of the smaller size boxes available. But even if legal it may be a tight fit and upgrading to a bigger box is fine.

See How do you count the number of wires in a fill capacity of an outlet box? for box fill details.

2
  • Thank you for the answer and information on how to calculate space in the box!
    – martin
    Dec 1 '21 at 1:49
  • Oh no, they make Handy-boxes much smaller... 18ci is almost a 4x4 box at 21ci. Dec 1 '21 at 2:01
6

If it's called anything (it's usually not, and it's quite common in new as well as old work), it's "branching" - under US rules all your wiring is like branches on a tree - no "ring" or looped circuits like in the UK. A branch circuit comes off the breaker in the panel feeding it, and branches as needed to go where it is needed. But never back to itself. You could also call it a junction - it happens in a junction box, after all.

NOTE: You don't actually need 3 GFCIs here. If the breaker goes to the LINE terminals on GFCI outlet A, the wires to B&C can come from the LOAD terminals on GFCI A, and then A, B & C are ALL GFCI protected (B&C should have labels stating that applied - a bunch of those labels come with every GFCI.) If you want to throw money away, you can certainly use 3 GFCIs (in which case, don't use the LOAD terminals at all.) But one GFCI and 2 regular outlets, correctly wired, is just as safe.

Your box, assuming 14Ga wire to go with a 15A breaker, needs to allow for 6 wires terminating in the box, one device in the box which counts as 2 wires, and 1 ground wire. If there are internal clamps, add one more, and multiply by 2 cubic inches for 14Ga wire - 18 or 20 cubic inches.

Correction - you are in metal conduit which means you don't need any ground wire at all - the metal conduit is a perfectly acceptable equiment grounding conductor and takes up no room in the box. Likewise, you are in conduit, so there's no reason you'd have cable clamps, or cable. 16 cubic inches will do the job. Or 18 if you run a ground wire in the conduit anyway.

Pigtails are free, and more space than is the minimum required is fine, and may be easier to work in (a minimum box, especially with relatively large devices like GFCIs, can be quite packed.)

Many (most?) GFCIs have a screw clamped backwire setup that:

  • Is NOT backstab. Backstab (no screwing to hold wires, poke and pray) is bad, screw clamped wired from the back is good.
  • Supports having 2 wires on one clamp position (so you would not need to pigtail your B&C connections, you could just connect them to LOAD.)
  • You can just strip the wires; you don't have to bend a cute little hook to put under a screw (they are clamped between two flat plates BY a screw.)
1
  • 1
    Thank you! I should have made it more clear, in my diagram (GFCI) was my way of saying "normal plug that is protected by primary GCFI." Only A is currently a GFCI plug, and I'm trying to keep it that way.
    – martin
    Dec 1 '21 at 1:52

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