Code compliance of custom wooden surface “conduit” in garage

Question

TL;DR: Would it be code compliant and safe to build a narrow wooden box to run along the base of a garage wall to protect runs of numerous electrical wires?

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I'm going to be adding a load center subpanel inside my garage (fed from a 50 amp breaker in the main panel with a 4x 6-gauge cable) to make it convenient and modular to add electrical outlets for shop tools. Initially I'd like to design the new subsystem to accommodate six independent 20-amp GFCI outlet circuits and overhead lighting, but I don't picture adding them all at once, so I want to design it so each addition will be easy and have minimum cost as I can get to it. Although it's a lower priority, it would also be nice to have it easy to swap the 6/4 feeder cable for a larger gauge if I ever want to feed the subpanel from a larger amperage breaker.

For context, the main panel is combined with the service meter, on the exterior of the attached garage. Where I want to locate the subpanel is on a new piece of plywood over the surface of the drywall, on the opposite side of that wall, inside the garage. It will probably be a few stud bays over from where the main panel is affixed.

Obviously, I want my solution to be safe and code compliant. My location is Lehi, UT. I do have a specific right/wrong question related to a specific solution idea (though, of course, I also welcome alternative ideas in the comments).

I prefer surface mount for both the boxes and the subpanel to minimize the need to remove and rehang the drywall or to fish wires through studs.

I have already begun a design using a single PVC conduit for running wires (It's inexpensive and easier to modify, and I'm less concerned about toxicity of burning PVC in a garage than I would be in the house). I like the idea of a single conduit running along the wall base to carry the wires for all future outlets. I have found charts (such as [this one][1]) that give appropriate diameters for the number of wires, but at those sizes it seems more difficult to find the right conduit bodies at a low cost. This difficulty is aggravated if I factor in the large feeder cable for the subpanel--I don't mind a separate conduit for it, but because of details of the situation that I won't get into here, a single solution would be nicest.

So an idea I've just had is to build a long narrow wooden box along the base of the wall where I'd like to run the wires. I've picked up a notion that wires and cables outside of conduit can be permissible in spaces such as a garage, but that code does require that the wires not be located where they could easily be damaged. So this box would be a solution for running the wires as if outside of a conduit, but also protected. I would use conduit to protect short runs outside the box (such as for the outlets and for the feeder wire to the subpanel). I could either build the box on the surface of the drywall, or cut out the drywall and give the box access to the (insulated) stud bays (I just worry if that would bring code sections to do with framing and fire brakes into play). I would make the cross dimensions of the box large enough to accommodate all the outlet wires, the ground wire, and the 6/4 feeder wire all at once. I could unscrew the front of the box for working with the wires, but otherwise it would remain secured. I would secure the wires inside at regular intervals, essentially as though they were inside walls, just without studs and corners in the way.

So, my question is: Can anyone foresee points of applicable electrical codes that this would violate? Aside from the letter of the code, can you foresee safety issues with this solution?

Answer 1

This sounds like a good use for a heavy-duty surface raceway

The situation you're describing (several circuits following the same path, with future expansion as a consideration, while wanting to keep things neat and tidy vs. the "unfinished" or "industrial" look of a typical conduit installation) sounds like it'd be a good use-case for what's known as surface raceway (called "Wiremold" by electricians after Legrand's trade name for the stuff). However, as you have noticed, the Wiremold found in home improvement stores (typically the 500 series, or equivalent nonmetallic product) is designed for low-profile work in finished spaces, and thus only has the capacity for a couple of circuits maximum.

In particular, since this is a garage, you'll probably want to use a metal surface raceway system for durability's sake. Legrand has a suitable product in their 2400 series, although you'll need to mount your receptacles in device boxes instead of inline in order to get all your wiring to fit. If you want the "in-line" look for your receptacles, you'll need to upgauge to the 3000 series Wiremold, which provides enough space for well, just about anything you wish to do inside it.

Note that you'll want to review NEC 386.22 for fill and derate requirements, as they are different for the larger-capacity surface raceways than they are for ordinary conduits. In particular, raceways with a cross-sectional area upwards of 4 in² are allowed to contain up to 30 current-carrying conductors without invoking thermal (ampacity) derates, unless you exceed 20% of the raceway's area with your wirefill. Also keep in mind that inside surface raceways, you will want to run individual stranded THHN wires in order to keep fill and working manageable, instead of using big, stiff NM cables within the surface raceway.

Answer 2

In my jurisdiction, wiring can be exposed where it is 5ft or higher from the ground.

As for conduit boxes, bulk heads, and raceways, you have to make sure that the wires are not touching each other. This has to do with thermal buildup. And where they do, to get around tight corners for instance, you have to put insulation (e.g. left-over fibre glass insulation) between them.

So if you are used to cramming a bundle of low voltage data wiring through a conduit, this cannot be done with power wiring. You have to keep the wires separate and neatly parallel by nailing them against a joist or a board.

Further, once the wire is concealed (as by a bulk head) it has to be 1.25in away from any finishing (wood, drywall etc...). Metal conduits or AC/BX wires do not have this restriction.

Another issue is duct/conduit penetration between garage and house, where some form of fire retardant sealant is required within the duct. This applies to electrical runners/conduits and it is stipulated in the building code and not the electrical code.

I am sure there are professionals here who can spell it out better for you (or set me straight) for your location (Lehi, UT.), but I hope this gets the discussion started for you.