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I have an existing 15a outlet in the wall of my garage (in-wall nm 14/2). I want that outlet to serve another area, so I've added junction box on top of the outlet and used EMT to run another direction (my "A" circuit in the picture).

I decided I needed a few extra circuits in my garage as well, so I'm running 2 new 20a circuits from the panel through metal junction boxes and EMT (the "B" circuit). This new run of thhn will include 2 hots, 2 neutrals, and 1 ground - all 12ga.

Now, the original outlet that was in the wall is where I actually outlets from the new circuits, so that box is acting as a pass-through for the original "A" circuit, but will have outlets tied to the new circuit ("B") there (top left, dotted area where the circuits collide).

TLDR; My question: How do I attach ground between these 2 disparate circuits (15a/14ga vs 20a/12ga)? Do I just pigtail them to the boxes and not worry about? Ultimately, I think my only real concern is a ground-loop situation.

circuit layout sketch

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  • Why run a ground with your EMT? The EMT will be a ground. Garages need GFCI protection. – JACK Sep 18 '20 at 15:27
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You're worried about a ground loop. The only way to do that would be to disconnect the ground wire in the 14/2 NM at the panel and in the 'pass through "box. Then the EMT becomes the ground for both circuits. Or just eliminate the 14/2 completely and add another #12 AWG to your conduit run and pull it all the way to your "A" junction box.

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  • I like this. I hadn't thought about disconnecting the ground from my "A" circuit completely and then letting it use the ground I provide in the new run (I used #12 on the 15a extension in case I wanted to upgrade later). This way I don't have to feel like I'm abandoning an entire run in the wall, just its ground wire. – KdiddyDawg Sep 18 '20 at 16:06
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You indicate that the "A" circuit just passes through that existing box at the top left and that you're adding a new box right next to it.

Assuming there's enough cable in that box, extend it to the new box and splice to your new cable run to the new box there. Do not connect any circuit A wiring in that box except to pass it through to a new piece of cable that carries on to the "A" box.

A better alternative, especially since you're already planning on installing a new box right there, is to use that existing box as the junction between the existing wiring and the new wiring and put a blank plate over it. Use the new box as the outlet box for the new circuit. This way there is no confusion whatsoever about how to wire and (for the future maintainer, maybe future you) there's no confusion about why wires are just passing through and, more importantly, no danger from having 2 circuits in one box and needing to remember to turn them both off for safe working in the box.

Also, I believe there's something code related about having a single outlet on a circuit (your A circuit), but I don't recall if it's a 15 or 20a breaker that has issues, so I'll leave that to the licensed electricians.

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    The "single outlet on a circuit" is as follows: 15A circuit: one or more 15A receptacles., 20A circuit: one or more 20A receptacles, plus zero or more 15A receptacles or 2 or more 15A receptacles. Another way to look at is: receptacle type should always match circuit type except that 2 or more 15A can work for a 20A circuit. What you can not do is: 20A circuit with one 15A receptacle and nothing else. Note that a typical "duplex receptacle" counts as two receptacles even though it is one "thing". – manassehkatz-Moving 2 Codidact Sep 18 '20 at 14:58
  • Thanks, @manassehkatz-Moving2Codidact. I didn't know that the installation of a duplex receptacle met the rules. Probably why this isn't a huge issue... – FreeMan Sep 18 '20 at 15:37
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    It is very much a "kitchen" thing. You are required to have 2x 20A circuits, but they can each be 2x 15A receptacles. The vast majority of residential plugin appliances are designed for a 15A circuit, so they fit a 15A receptacle. But having those receptacles on 20A circuits means you can plug a high current (e.g., toaster) appliance and a medium current appliance into the same circuit and generally not flip the breaker. The 15A receptacles (including those with GFCI) are designed to actually handle 20A passing through for this reason. – manassehkatz-Moving 2 Codidact Sep 18 '20 at 15:39
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You're in EMT!

All the EMT is a legit grounding path. You don't even need a ground wire, the EMT shell is the ground.

"Oh, but that doesn't seem like enough" is a thing I often hear. In your case, you have redundant EMT paths to all locations but the last, even if you ran no ground wires at all.

Note that if you do run a ground wire, you are required to tie that to the metal junction box, not the receptacle. You do not wire this like plastic boxes. However once the metal box is grounded, the recep can pick up ground via the yoke (or mounting screws if it is self-grounding).

The back of every metal box has a hole tapped #10-32 for a ground screw. The only place you need to use it is at the first box. You may need a ground clip there, as extension boxes often overlook the ground screw hole.

Box fill

Watch your box fill. Your 2-gang boxes with 4 wires in, 4 wires out and 2 receps will need 8 wires + 4 for the yokes = 12 "wires" x 2.25 = 27 cubic inches, or 29.25 if you insist on running ground wires. A common 4x4 steel box (21 c.i.) and domed cover (6.5 c.i.) is mighty close.

I do the 2-circuit thing all the time, and I have lots of places where I have to stack an extension box because I didn't think ahead and use a 4-11/16" box (slightly larger but twice the cubic inches).

For two GFCI devices abreast, don't waste your time with 4x4 boxes. I recommend 4-11/16" square boxes (reasonably priced at the electrical supply) and a domed cover. These domed covers are big enough that you won't need to mutilate the Decora cover-plate tabs.

Oh no! Unrelated circuits passing through EMT!

LOL I don't care. Totally routine.

This boils down to your careful wire marking. This sort of thing is why I own 10 colors of THHN wire. If you want to get er all dun with 2 colors of THHN,

  • Nobody says you have to use black & white. Gray is also allowed for neutral and hots can be any other color but green. So go to a real electrical supply and buy 1 spool of gray and 1 of blue or whatever floats your boat. (Home Depot won't have gray, purple, pink or brown).

  • Get your 5-pack of colored tape! Remember, with individual wires in conduit, there's no remarking to change purpose. A white wire with black tape is still a neutral. I like to mark wires with their partner's color.

Use stranded THHN and pigtail all your receps.

It installs even easier than solid wire. The reason for the pigtails is so you can use solid wire for the lands on the receptacle screws. I pigtail nearly 100% of my receps, simply because I don't want to fidget with 8 shepherd's hooks and fitting receps to cover while up a ladder or in some stress position. Assemble the lid and pigtails at a comfy workbench, then up the ladder, 4 wire nuts and done :)

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