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scatkins 12-02-2008 02:21 PM

PVC Conduit use in Attic? What is accepted Ambient
I'm considering using listed Grey PVC schedule 40 conduit in an attic. NEC recommends above ground use only in ambients less than 122F (50C). [The conduit vendors tell you that service limit of PVC is 140F]

So I want to use PVC in the attic, but is there an NEC other acceptable reference on assumed max ambients by geographical region? Yes the standard answer I get is the non-specific "it depends" or ask your local regulatory authority. Realize LA's are very helpful in some areas, not mine so I would rather understand the background before I talk to him.

Clearly if you are in Dallas your attic will hit greater than 122F in the summer. Probably not if you are in Fairbanks.

I for example I am in coastal California. It gets to 100+ once a year or so maybe. So I'm sure the attic will see >122F once a year or so for a few hours? But if I use that max as the design guidline just about everywhere in the continental US will see 122F in their attic at one time or another, which would imply one should ever use PVC in an attic.

So what is the advice? Avoid PVC in attics in geographical areas that are typically very hot for summer (Desert areas, Southwest etc). The rest is probably fine assuming standard derating is utilized?

What I was really hoping was that somewhere the NEC had a region map with temperatures assigned to regions as guidance. (I believe I've seen a lot of this in the HVAC industry for other uses).

Any thoughts or practical advice is appreciated.

Also, in asking around on this issue I seem to get a lot of people giving me anticdotal stories about how attics can get to 160F. Maybe, but I'm not really buying that for a typical residence. When I start poking at that it always seems to source back to vendors of attic venting and fan products and claims are for improperly vented spaces in the desert. So, as a baseline I'll assume good ventalation is provided. I would have a hard time beliving greater than 150F. (At 115 outside and a 30 degree rise that still is only 145 (which admittedly is really hot but....).

J. V. 12-03-2008 11:33 AM

Check schedule 80 PVC and see if the temp allowance is higher. IMO I cannot see any issue with schedule 40. Just sagging, that can be remedied by strapping it more frequently.

If you must use conduit, your next option would be EMT.

scatkins 12-03-2008 01:07 PM

I think Sch 80 is the same 140F service limit. Seems that it is an inherent to the nature of PVC itself. Looks like other variants (CPVC) have higher temps, but I don't believe there are any NEC listed products.

On a slighly different note, I'm looking at whole house Vac system installation as well which also uses schedule 40 PVC (or thinner) through the attic. Although it isn't creating its own internal heat like cable in an electrical conduit, it would face the same ambient limitations, but no one seems to have issues with that.

Agreed probably some sag is the worst that will happen. Probably best to be careful with support and be a little more conservative than normal on the conductor fill as well.

Based on feedback from folks I've asked (rather lack of feedback) it seems that there is not a lot of consensus or guidelines on attic temperatures. Seems to be more of a case by case based on common sense.

My real concern wasn't really safety, rather getting beat up by an inspector in the future for doing something out of the norm. But it doesn't seem like there is a lot of norm to compare against.

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