US-Michigan—Subpanel install—Can I use PVC conduit and THHN in-wall to go from disconnecting means to panel inside the garage?

Question

Hello— I'm installing a 100 amp sub panel in my detached garage. The panel that I have does not have a disconnect breaker on it, and I'd prefer to locate it away from the point of entry in the garage for convenience. So I have a 100amp outdoor disconnect (basically a 100amp breaker in a box, listed as "service equipment") My intent is to run conduit from the house to the garage, terminate it in the disconnect, (I think a disconnecting means is required where it enters the garage) then run to the panel elsewhere in the garage.

My question is, can I use PVC conduit in-wall with THHN to go from the disconnect to the panel? Is there another way that I should do it? Using SE cable in wall and attic seems like a minefield, so I figured conduit might be best, but I'm not sure. If the conduit is surrounded by insulation, do I need to derate the wire inside it? If I use conduit, do I need to use Sch 80?

Any thoughts or caveats greatly appreciated!

Answer 1

Sch 40 is fine, this time at least

Since this conduit run is inside a wall, its generally not subject to physical damage, which means Schedule 40 PVC is fine for this job. I'd simply bring it out of the back of the box with a LB body, by the way, and then it can just pipe straight into the side of the indoor subpanel, or into the bottom or top with a LL or LR body if that is more convenient.

As to other means? PVC is really your best choice here, unless you want to go metallic instead, which puts you in EMT-land. (Rigid is a bigger pain to deal with due to threads, and SE cable is going to be harder to replace later if you want more power in the garage.) I'd just size it generously -- 2.5" is a good size for this.

Wire derating is not needed here

Insulation isn't an issue when it comes to derating for wire in conduit as long as the ambient temperature isn't roastingly hot (i.e. not greater than 30°C/86°F).

Don't forget the grounds

Since you're providing a feeder to a detached outbuilding, you'll need to both run an equipment grounding conductor back to the source of the feeder (it can be 6AWG copper) to provide a fault path, and run some more 6AWG copper back to a grounding electrode (ground rod) for the outbuilding to deal with lightning-type problems. Don't forget to pull the bonding screw in the disconnect while you're at it!