Why can THHN/THWN go in Schedule 40 PVC but NM cable (Romex) requires Schedule 80?

Question

NEC 334.15(B) says that exposed NM wire run along an interior wall should be protected, and they list various acceptable raceways, including Schedule 80(!) PVC, but not Schedule 40.

Why does NM cable require Schedule 80 when THHN/THWN requires only Schedule 40? Actually, I think THHN/THWN requires only Schedule 40, I couldn't find the prescription anywhere in the code. Home Depot carries only Schedule 40 in electrical.

Answer 1

Anything can go in anything. The differences come down to why something is going into something.

Individual wires (THHN/THWN) always have to go inside conduit. PVC Schedule 40 is sufficient above 8' indoors, buried underground, inside walls, etc. But if PVC Schedule 80 is required when subject to physical damage. That typically includes exposed inside a room (i.e., not hidden in walls) below 8' as well as certain places outdoors. For a typical run that means most of the conduit can be PVC 40 but very commonly some parts will need to be PVC 80.

On the other hand, non-metallic cables (a.k.a., Romex) never have to go inside conduit. However, when subject to physical damage they need to be protected. That does not need to be actual conduit - it can be anything that reasonably provides the protection. For example, you can use dimensional lumber or cut sections of plywood and screw the lumber or plywood in front of the cables to protect them from damage. However, if you want a ready-made solution then you can use any conduit which is rated for protection from physical damage, such as PVC 80. You can't use PVC 40 to protect cables from physical damage - it won't do any better than it does with individual wires. And when cable is not subject to physical damage it could be run inside PVC 40, but there is usually no reason then to use conduit at all.

Answer 2

That's a misconception.

What's actually true is

• wires in certain locations require protection from physical damage. What kind of wires they are is irrelevant.
• Schedule 40 PVC is inadequate to provide protection from physical damage, because it can't take a hit.
• Schedule 80 isn't much better, but they had to throw a bone to the plastics industry. For actually good protection, use EMT. Easier to work with anyway once you get the hang of it.

Home Depot carries only Schedule 40 in electrical.

Yet another argument in favor of EMT. But you're not doing yourself any favors by staying stuck in the big-box store rut. I rarely ever buy electrical gear there. I don't like their prices and I really don't like their selection.

I hear people say they think they're not allowed to shop at electrical supply houses... but that's not really it, since they also don't shop at their friendly locally owned hardware store, or local lumberyard either.