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I'm building a new garage, mostly on my own. I hired an electrician to run cable and install a sub-panel, which was completed yesterday. Today it passed inspection. Next step is running cable for lights, switches, and outlets, which I will be doing myself.

From my understanding of the Wisconsin Electrical Code (which generally follows NEC 2017), I have three options:

  1. Run 2" PVC conduit from the top of the sub-panel up to the ceiling (either straight up, or use two 45 degree elbows to enter the wall cavity and run up through the top plate). Run all cables through this conduit into the ceiling, then distribute from there.
  2. Run each cable through its own knock-out in the top of the panel, with a clamp. Drill holes in the plywood to feed the cables behind, and up through the cavity (I'll need to extend the plywood the rest of the way to the ceiling). Cables will need to be stacked and stapled along the center of the studs, within 6" of the panel, then every 24". Drill a hole through the double top plate to bring the cables into the ceiling, then distribute from there.
  3. Same as (2), but instead of using knock-outs on the top of the panel, use knock-outs on the back.

Are these options correct? Are there other, better solutions I've missed? Any missing/wrong details?

Surface-mount sub-panel in garage

Details which may be important:

  • I will not install drywall at this time, but plan to at some unknown future date. Wiring must meet code for open wall cavities, but also allow the possibility of adding insulation and drywall in the future.
  • This is a small detached garage which will mostly be used for storage (no welders or any other sort of two-phase load).
  • I opted for surface mount so I can insulate behind the panel later on.
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    Gotta admit I'm really perplexed: as you've noticed, the choice to bring that feeder up between those studs and surface mount the box causes a lot of unpleasant ripple effects in the remainder of the wiring and also the room finish. I'd find a way to remove that stud on the right side of the conduit and re-do the installation flush.. – Greg Hill Jun 4 at 23:19
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    Why the heck is this panel surface mounted on plywood? Why not mount it between the studs? It would be a LOT cleaner to route your cables in the cavity between the studs. Just drill a series of 7/8" holes in the top plate with up to 3 cables per hole coming out of the top of the sub-panel. I see no reason to mount the panel the way your "electrician" did. – George Anderson Jun 4 at 23:39
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    A series of sub-optimal choices led me here... 2" conduit through a 2x4 bottom plate means a cut in the plate, so it needs a stud on either side of the cut. Should have brought the conduit in through the floor instead of the wall, or used a 2x6 bottom plate. I opted for surface mount panel to allow for insulation behind the panel. – LShaver Jun 5 at 3:07
  • While you may not have need for one now, you or a future owner might well want an electric vehicle charger (thus, typically a 240-volt 2-pole load) but that would likely be served by mounting it on the plywood by the panel with a short section of conduit. – Ecnerwal Jun 5 at 11:18
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I'm amending this answer to correct a misconception.

There is nothing wrong with this installation

I realize there's a trend these days in society where people want to destroy or conceal all utility space. As if they're shameful or something. Fine, then, go be Amish. For the rest of us... utilities are a miracle.

Surface mount is particularly correct for a garage, because stuff in garages changes - that's precisely why you got such a large panel and surface mounted it in the first place. Since the garage is utility space, burying the panel in a wall is just dumb.

The problem, really, is the Code requirements to physically protect wire/cable from damage. And this is where you've "split the switch". You adopted the surface mount/accessible/maintainable philosophy which is correct, except you are still "married to" concealed wiring methods (NM/Romex) which rely on the walls for physical protection.

The pro way to proceed from here is multiple conduits. (Because of the "4 circuits per conduit" rule). Normally I'd run 1/2" or 3/4" conduit, but this is where that NM/Romex wiring method gets in the way. Cable takes a lot of room in conduit, because it's treated as a single wire of the widest dimension (since it twists). So NM-B cable is 0.40" wide as far as conduit fill is concerned, and that requires fat conduit.

Keep splitting the switch: run a hybrid THHN/Romex setup

One very elegant answer is to run several smallish conduit (1/2" or 3/4") to locations of your choosing, and proceed from those locations with normal Romex/NM. The box becomes like a "mini panel" - you don't have to reach the panel with the Romex, only one of these boxes.

Inside the pipe, use wires made for pipe - THHN individual wires. This is a fantastic way to work, as the individual stranded wires are flexible and supple, not the gator-wrestling of working with Romex (can you imagine trying to thread Romex through a hole in a stud!? Ha ha, who would put up with that?) You do have to "do the pipe-fitting" to get the conduit up there in the first place, but the wiring is easy street.

And from a novice's perspective, it's easy to fit the pipe, check it out, shoot photos/have it inspected, revisit if needed, and have the physical installation be perfect. Then add the wires, which is straightforward work.

A 1/2" conduit can fit 4 common circuits (that's the max anyway due to thermal limits) and 3/4" can fit big stuff (and other circuits too). Metal conduit also serves as the ground "wire" so no need for wires proper.

EMT metal conduit is also a great way to deal with outlets or devices that are "right next to the panel" e.g. on the same piece of plywood. A short EMT nipple, steel junction box, two THHN wires and you're done!

On certain upgrade-friendly runs, run conduit/THHN all the way

Especially if you wind up walling the wiring off behind drywall.

You would do this where you might have a variety of shop tools, e.g. a saw that you might upgrade to a 240V saw someday, or an EVSE (go big on that conduit - Tesla chargers can draw 80A!) Then, upgrading the wires to that location will be a snap.

Working with the Romex

Your method #2 will require a way to guard the NM cable as it exits the panel and goes in to the plywood.

Your method #3 will require special cable clamps that can affix from one side (since you can't access the backside). My concern is the cable clamp won't work properly unless the hole-behind is larger than the knockout, and how can you accomplish that??? (I imagine 10 minutes of grinding with a Dremel or something).

I think you have the right idea with the conduit. There's only one snag: You're only allowed 4 circuits in a conduit before a thermal derate becomes necessary. The thermal derate means bumping one wire size for 5-10 circuits in the same conduit, and two wire sizes for 11-20 circuits.

The cure to this is to run multiple smaller conduits. NM-B cable in conduit takes a lot of space, though, as it counts as a single wire of the wide dimension.

14-2 NM-B is 0.36" across. 14-3 NM-B is only 0.31" across (it's basically round).
12-2 NM-B is 0.40" across. 12-3 NM-B is 0.37" across (again rounder).

Punch those numbers under "custom" in a conduit fill calculator and it'll tell you what pipe sizes will work. Generally it looks like 1-1/4" is the "univeral donor" and 1" will work with three /3 or #14 (since those are smaller).

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  • #3 can be done with an old work NM clamp like an Arlington White Button or Raco Insider that can be fitted into the KO from the inside of the box... – ThreePhaseEel Jun 5 at 0:00
  • The garage is 12x20, so 5-10 circuits is probably all I will need. – LShaver Jun 5 at 3:12
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    This is more like it! When I read your first answer I thought "huh. I was sure Harper would suggest conduit and THHN to a j-box in the ceiling or attic, and NM from there." And now here it is. :-) – Greg Hill Jun 5 at 17:53
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Arguably better: EMT on the surface, with THHN and 4 or less circuits per tube (i.e. not heading for the ceiling unless that actually makes sense for the circuit.) Or head for the ceiling and junction-box to NM-B there, but that's not much better, at least in the "it's a garage, rodents happen, metal is good insurance against teeth" sense.

Couldn't ask for a better place to learn to bend EMT. Not overly difficult, really, but being a garage, if it's less than perfect, no big deal.

Or, correct your "electrician" screwing the panel up by making a chase from the top of the panel to the ceiling by attaching additional 2x4s (and have a bump in the wall if you get to drywall.) Which might well be the cleanest and simplest solution to this [cough] massive boneheaded manever [hack, cough]

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  • I like the chase option, sounds straight-forward, clean, and easy to access in the future. Any special considerations there? – LShaver Jun 5 at 3:12
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    I'd use a screwed on piece of plywood for a cover, rather than drywall, with an eye to "access in the future." – Ecnerwal Jun 5 at 11:11
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    Might also want to go ahead and insulate behind that section NOW. Will be much easier before you run wires. Then you can extend the current plywood to the ceiling and have a surface to attach your cables to at the back of the chase, rather than having to run them along the studs - unless the plywood is within 24" of the ceiling now? Or you could just put a board across where needed to have a stapling surface. Would have been better to insulate those stud bays before plywood, really, if insulation is in the plan for the future. – Ecnerwal Jun 5 at 11:25

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