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I relocated my dryer from the garage. The old duct is approximately 5 inches in diameter and runs under my house, through the cement slab, from the garage on the west to the outside of the house on the east. My main panel is on the east wall about 8 feet from where the duct exits. Can I run a metal conduit with the wires through this duct to add a sub panel?

Back story: I won’t be doing this myself. An electrician, Hired by a solar company, will be. I have been trying to get Solar co to tell me exactly how they plan to run wire to the sub panel, and they won’t/haven’t. I said I wanted to do it through the duct as I don’t have an attic and it’s not possible to hide a conduit on the front of my house (and will be very ugly). The solar co is telling me I can’t do this but won’t tell me why I can’t (and they haven’t come to look at it). I am trying to determine whether the company just doesn’t want to deal with doing this somewhat atypical install, Or if it really can’t be done this way and be to code… From my perspective, I don’t see the difference between running a conduit through walls, or an attic, or the outside of the house vs. through an unused, straight duct in a cement slab as long as the conduit will fit.

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  • If it was just the wires that be no, but electric conduit should be okay if it goes though. There might accessibly to the conduit that the vent might not provide.
    – crip659
    Nov 27, 2022 at 11:30

3 Answers 3

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If it was done properly ok - end to end closed conduit with proper fittings and wiring. The company doesn't want to commit to it because it's probably more labor-intensive and require more conduit than strapping it across the house. Many companies like cookie-cutter methods and don't want to think outside of the box. Also, is it a straight run? I couldn't see how you'd run conduit through a small opening that wasn't straight unless they use flexible armored raceway (much more expensive).

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  • You could make no coundulet in the front of the house as part of the contract. If it is not there write it and initial and date it.
    – Gil
    Nov 27, 2022 at 19:05
  • @Gil The risk, crazy as it sounds, is that if the company is busy enough (and right now, solar is big and Tesla has been backing out of the business as well) they could look at "special requests" and say "no, we're not going to do it". Nov 27, 2022 at 19:14
  • I had asked them about this numerous times before signing with them. The sales guy said no prob (in writing). They want to submit plans for permits and I don’t want to let them until I know how they’ll run it, as outside front of house isn’t an option. They just came back with what I think is a bs answer (“ we cannot run anything through an existing conduit due to wire fill requirements”). I asked size of wire (to calculate) and they said “#4”. Grrrrr… Nov 28, 2022 at 5:37
  • @brian. I think you’re right. The annoying part is it’ll be more conduit/work to run it outside. Nov 28, 2022 at 5:40
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If the duct is more than 10 feet long, probably not. EMT requires the least securing, and it must be secured and fastened every 10 feet. If your duct-in-slab is more than 10 feet long, I don’t know how you’d be able to fasten the conduit as required.

Other types (MC, PVC, etc) must be fastened even more frequently.

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  • It’s more than 10 ft. I’d guess 20-25? And no way to strap it… Nov 28, 2022 at 5:32
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There should be no issues if the proper wire is in the proper conduit. This is assuming there are no other issues with the duct, such as water intrusion, incursion/s that could damage the conduit, etc.

Given a choice, I would prefer electric under a slab rather than over my head.

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  • Don’t think there are issues with duct… and I agree about under vs over!! Nov 28, 2022 at 5:41

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