I am looking to add power to my detached metal (enclosed carport) garage. With that said I am trying to figure out the most economical solution to wiring the garage. I would like to run the wire like normal unfinished detached garages with the wire running behind the metal studs and between the outside wall with 12/2 Romex. Behind Metal Stud This would then run each span (leg) of the garage inside the rib of the metal to give me power plugs for various things like power tools. Between legs in ribs My questions are...

  1. Can this be done and be NES compliant?
  2. Would I have to cover the wall surface to be compliant? (plywood or similar)
  3. Should I run it in PVC conduit (1/2 or 3/4)? 3.a. Can I use Romex 12/2 in conduit or do I have to get THHN?
  4. At the subpanel do I need to connect a copper grounding rod for the sub?
  5. What is the better method GFI breaker or GFI plug at the start of each electrical run for safety?

Assuming when a house is put up for sale during an inspection a detached garage is not inspected does it really matter (within limits of safety) if I leave it exposed?

If you guys have any other advice on this please let me know.

  • 1
    I believe inspection of the building is based on the lender, but it would be best to complete your installation in a safe method. I don’t consider exposed nmb in a garage to be safe. 1/2” Sheetrock or plywood UL code legal to cover nmb. – Ed Beal Nov 6 '20 at 21:43

I need to say this: Be very wary of running to a particular wiring method merely because you are familiar and comfortable with it. Consider what you are wiring and where, and be willing to expand your skill set.

An example is you are trying out the idea of conduit, but want to stay with familiar NM cable. Most novices think this at first. But this gives a very difficult cable to pull, with no benefit whatsoever to doing so.

1a. Can you run NM unprotected in this structure?

First problem I see is the rule that NM cable needs guard plates if it's within 1-1/2" of a wall surface. The Romex running in the siding slots will be vulnerable to someone nailing or screwing into it from outside the building. Further, unless your plywood is offset to the inside of the metal framing, you'll need guard plates on the interior as well.

I consider PVC conduit to be pretty worthless as far as nail protection, and I'm rather accustomed to working with EMT metal conduit, which is compact and inherently provides nail protection, not to mention grounding if used as a wiring method. You'd have to check it, but it looks like 1/2" EMT conduit might be able to slip through that groove between siding and framing.

1b. Can I use the metal structure of the building as a conduit/raceway?

Equipment including conduits/raceways must be approved. NEC 110.2. It's a safe bet that if the raceway is UL-listed, the inspector will approve it. If not, it is up to the discretion of the inspector. So "maybe" but you might check first.

  1. Would you need to cover the wall surface?

For Romex (NM/UF) cable absolutely, to protect the cable from damage from things being leaned up against it and the like. You'd also need either a metal wire guard plate, or a surface setback 1-1/2" from the cable to protect from nail damage.

For EMT metal conduit, you're all set, and no further wall covering is needed. See why I love that stuff?

  1. Should I use conduit, what kind and size, and cable in conduit?

I would do the kaboodle in 1/2" EMT conduit unless you expect to be running big power somewhere. EMT allows use of normal size wires with up to 4 circuits per conduit (the equivalent of 8 if you use multi-wire branch circuits). 1/2" EMT is easy to work with, cuts with any hacksaw, deburrs with your multi-tool stripper, and the fittings are dirt cheap. Combine that with $1.00 Handy-boxes or 4x4 boxes, and it's easy going. You do need a bender and you need to get the knack for making offset bends, but it's pretty easy. You do not need to do compound bends; you can just cut the EMT at that point and have a coupler.

As far as wire in the conduit, it depends. If you fully connect the system with metal boxes, and the grounding is continuous, then I would run stranded THHN wire, it's unfairly easy to pull, and 2 wires is all you need. Pigtail your receptacles with solid wire; combining solid+stranded on a wire nut is easy but landing stranded on a receptacle screw is hard. The ground is in the conduit (and also the places it attaches to the building). If you are only using EMT as a damage shield and plan to exit the EMT and run as bare unprotected cable in places, then you need to run NM cable. However the conduit inside diameter must be 138% of the cable's width in the wide dimension, and 2 cables in 1 conduit are not allowed. (well the conduit size would be prohibitive).

  1. Does the subpanel need a ground rod?

The subpanel needs a proper Grounding Electrode Conductor as per Code. That needs to go to an Ufer ground cast into the concrete, a metal water supply pipe, or two ground rods set at least 6' apart unless one of them passes a test only an electrician can do.

  1. GFCI breaker vs GFCI recep?

Six of one, half-dozen of the other honestly. Besides price of course. With GFCI we're not normally concerned with protecting the wiring in the walls. AFCI is concerned with that, and it needs to be at the breaker, unless the run to the first recep is metal conduit.



Since the garage is detached yes a grounding rod is required. Last the ground wire from the rod is connected to the grounding buss and the metal frame of the box / the building. The grounded or neutral conductor is isolated from ground an insulated buss is normally available in a main lug panel or a main breaker panel, if the bonding screw or jumper is installed in a main lug panel it is removed.

Running cable behind metal studs of unfinished wall

Any NM-B or Romex running horizontally requires protection below 8 feet. In my jurisdiction even running down the side of the stud requires protection.

Although MC or metal clad is under the same requirements as NMB most inspectors will let this fly on an inspection so I would consider upgrading to MC wire it is a little more expensive but will look a lot better in my opinion than exposed NM-B.

Cable through conduit

Can you use NM-B or romex in conduit sure but why? It would be many times harder to pull (you cannot strip it) and if going to conduit THHN is cheaper and much easier to pull.

GFCI breaker or receptacle

As far as a breaker or first outlet it depends on the location to me because there is no difference in the operation or safety. The difference is in cost. Usually GFCI receptacles are cheaper than breakers. But again location comes up. By location if you have a separate room or a large area walking 30-50’ to reset a GFCI can become a pain when the receptacle is just a few feet away so many times I will use GFCI receptacles in a home and garage to save $ and have the reset close.

Hope I hit all your questions we normally ask that they be broken up but these are quite common for a detached building.

  • Nice spit-shine and polish Jeff. I almost thought I was looking at a ThreePhaseEel posting lol. +1 – Harper - Reinstate Monica Nov 7 '20 at 21:56

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