We are building a carriage house, with a 4 car garage and a basement underneath. The basement has steel I-beams and the metal b-decking that the garage floor is poured on top of as a ceiling. What is the recommended ways of running the electrical along/through/in this type of ceiling? The basement will be used a workshop so I want to be able to put plenty of lights up.

There is a gap where the b-decking is laid on the I-beams that I was thinking of running PVC conduit in for the wiring and breaking out into light/pull boxes every 10 ft of run (between each I-beam) or so. Is there a better/preferred method that anyone else has used?


I did a rehab on a warehouse this way, it's the way to go. Use metal EMT conduit and pull the common single-strand THHN wire. You don't have to pull a ground wire, the conduit is the ground (plus, it's screwed or clamped to the building).

Use 3/4" conduit wherever you can, it costs little more and is a lot easier to pull wires through especially add-ons. Generally they allow 4 circuits per conduit, so in congested areas, lay 2 or 3 conduits side by side - this is barely more work than laying one, at least at construction time. Check the electrical code for required conduit spacing.

Anywhere but a single branch, use 4" boxes, preferably deep ones. Don't be bashful about using 4-11/16 square boxes in places you expect a lot of wires to come together.

Be strategic about keeping bends down to a bare minimum between boxes, ideally just one 90 degree. This will make it easier to pull wires.

Once the EMT is in place, pulling wire is pretty easy. Feel free to use more colors of wire, e.g. blue for lighting, brown for outlets etc. You won't need any green wire.

  • "this way" being between the B-decking and I-beam or attaching to them more like gregmac above? – Rich Smith Jan 19 '16 at 14:08
  • I attached to the I-beams via beam clamps. amzn.to/1niYCcd – Harper - Reinstate Monica Jan 19 '16 at 18:23
  • Did you just use a threaded rod then through the bottom of the box with a nut to hold it up or some kind of clamp in addition to hold the conduit and or boxes. – Rich Smith Jan 19 '16 at 20:28
  • I didn't suspend it by a threaded rod, just used a short bolt or machine screw to attach the box directly to the clamp. – Harper - Reinstate Monica Jan 19 '16 at 22:32
  • I don't see anything wrong with going above the I-beams if you have the clearance. You have to support EMT conduit every 10 feet or 3 feet from any box (much more often for others.) If trapping it above I-beams will suffice, there you go. Otherwise I'd glue anchors to the steel rather than try to drill into steel+concrete. Use a nice big adhesion area like 2x3” or more, scuff sand steel surfaces etc. – Harper - Reinstate Monica Jan 20 '16 at 7:32

The common way of doing this is to use conduit and surface mount all boxes. The conduit provides mechanical protection (especially important for a shop) and the nice thing about this it keeps everything accessible, in case you want to add lights or a power drop for a tool or something later.

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Alternatively, you may also be able to use MC (armored cable) wiring, though with a quick search I can't find a specific NEC reference to this, so probably best to refer to your local inspector / authority having jurisdiction (AHJ) to check.

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  • 1
    Is there a reason you're recommending BX/AC? Straight type MC is usable under far more situations, and MCI-A type cable is just as easy to terminate and work with as AC is... – ThreePhaseEel Jan 19 '16 at 1:06
  • @ThreePhaseEel Sorry, old habits die hard. I really did mean MC (even my picture was MC). Edited. – gregmac Jan 19 '16 at 3:53
  • I believe that's type AC in the picture actually (the paper looks suspicious). Also -- an example of a type MCI-A product – ThreePhaseEel Jan 19 '16 at 5:05
  • Like the idea of flex but then I need more support than I'd like. Thinking about the EMT more now though too. Trying to get all of this done before they need to come in and spray a fireproofer on all the steel. – Rich Smith Jan 19 '16 at 14:22

Since it is a dwelling unit, and the metal is not a roof, then you can wire it like you would any other basement. PVC would be acceptable or MC cable. 1/2" to 1" PVC needs to be supported within 3 feet of a box and every 3 feet thereafter. MC cable needs to be supported within 12 inches of a box and every 6 feet thereafter. Although this may look pretty sloppy so I would support it more often. EMT could also be used and needs support within 3 feet of a box and every 10 feet thereafter so will need less supports/anchors.

Since the steel is full of concrete you will need a hammer drill and concrete anchors between the beams. (Or some similar method, maybe stud welding. We use this method often. Or a powder-actuated tool.) You may also risk cracking your garage floor if the anchors you use are too large/long.

Good luck!

  • How does the stud welding work? Weld the stud to the metal deck then then the electrical box attaches to it? Was thinking about using beam clamps and lengths of unistrut to support conduit if I can't run in the space between the I-beams and decking above. – Rich Smith Jan 19 '16 at 14:32
  • Stud welding is a form of spot welding where the end of the stud is the spot that gets welded. It takes a specific piece of equipment that would be expensive to buy for such a small job but may be available for rent or loan. We use ¼" studs that can be used to mount a box, straps or hangers. You can run in the hollows of the steel as long as it not a roof. If it is a roof you need rigid or IMC conduit to do this. – ArchonOSX Jan 20 '16 at 9:02
  • The unistrut idea is good. When we use strut with structural steel we use something called a "window clamp" to fasten the strut to the beams and columns if it is running perpendicular. Also, you can get cover for the strut that turns it into a raceway thereby killing two birds with one stone. Spring nuts for the boxes and chase nipples for wire entry to the back of the jbox and you got yourself a system. – ArchonOSX Jan 20 '16 at 9:18

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