Insulated conduit

[ClayC]

We have a project where the GC wants to spray fireproofing over our EMT conduits. Would this change our ambient temperature adjustment? If so, how would it be calculated?

 

[Fred B]

We have a project where the GC wants to spray fireproofing over our EMT conduits. Would this change our ambient temperature adjustment? If so, how would it be calculated?

I too have had similar questions related to NM and spray foam insulation, seems to me it would, but told not, no satisfactory answer has been given nor code/UL references either way. If someone with knowledge has definitive references that would be great.

 

[ClayC]

My gut reaction is not because I believe our conduit fill allows for air flow within the raceway. Ambient temp factors would apply based on room temp and I don't think it would change because of the insulation on the conduit. Just not sure!

 

[winnie]

It would certainly change the actual operating temperature of the conductors, but there is no easy way to calculate this.

My gut reaction is that unless you have conductors right on the edge of what is allowed, there won't be an issue because the normal calculations are so conservative. However such gut reactions are easily wrong.

What sort of conduit fill, with what sort of circuits and loading are we discussing?

Jon

 

[infinity]

There is no NEC requirement to even consider this, if there were then the NEC would provide a way to calculate it. Cables and conduits are run in insulation all of the time.

 

[winnie]

There is no NEC requirement to even consider this, if there were then the NEC would provide a way to calculate it. Cables and conduits are run in insulation all of the time.

IMHO there is a requirement to consider this. 310.10 basically says that you are not allowed to exceed conductor temperature limits.

This is rather like the 'basic speed rule', when driving you are not allowed to go faster than is safe for the conditions, no matter what the speed limit says.

So if you had a situation where a cable was buried in such extreme insulation that it would overheat even though it met NEC calculations, then that would be a code violation.

With all that said, as a practical matter, I stand by my previous statement and am in practical agreement with infinity. Unless you are talking about something loaded right up to the edge of NEC allowances, and buried in far more insulation than normally encountered, this is unlikely to be a problem and there is no easy way to calculate it.

Spray fireproofing is likely not that thick, and doesn't add much 'R' value. As an thermal insulator, it _must_ will slightly increase the operating temperature of the conductors, but do not believe this is likely to have any practical significance.

-Jon

 

[tkb]

What is the difference between a conduit in spray insulation and buried underground?

 

[winnie]

Same basic issues. Thermal insulation with thermal mass around a source of heat. If by 'conduits' you mean duct bank and by 'spray fireproofing ' you mean 3 feet of soil .... then there would be a significant derating issue

 

[tkb]

I don’t see a difference between a conduit buri in dirt and a conduit encased in insulation.
Same thermal affect.

 

[rnatalie]

My read is that unless the code says otherwise, it isn't a concern. There are only a few places where "free air" is listed as a concern and stuff in conduit follows different rules.

 

[winnie]

I don’t see a difference between a conduit buri in dirt and a conduit encased in insulation.
Same thermal affect.

As I agreed, same basic issue/theory. But a _significant_ difference in magnitude/importance.

An inch of fireproofing spray around a branch circuit conduit is a very different situation than a ductbank buried in feet of soil.

In one case you have a small circuit that is probably conservatively sized, covered with a small amount of insulation. In the other case you have a large circuit that is likely heavily loaded and because of its size it is more important to make efficient use of the installed capacity.

Additionally, the more expensive the circuit (larger capacity, larger conductors, etc) the more important it is to do more careful analysis to make best use of resources.

-Jon

 

[GerryB]

Does 334.80 (NEC 2017) apply to this question? Ampacity on NM cables with no spacing in thermal insulation.

 

[Greentagger]

NM and SER ampacity reduced to 60 chart if covered in insulation

 

[infinity]

EMT embedded in insulation or fireproofing would still be better than a cable when it comes to trapped heat. The typical run of EMT has a 40% or less fill meaning that 60% of the interior is filled with air. A cable has a minuscule amount of air and for single cables there is no ampacity adjustment required when embedded in fireproofing or insulation.

 

[Netiquette]

Yeah, this will be hard to calculate since there is no NEC requirement to even consider this.

 

[Hv&Lv]

I don’t see a difference between a conduit buri in dirt and a conduit encased in insulation.
Same thermal affect.

Different baseline temps

 

[sameguy]

The temperature is heat added to, not produced by.
Pipes on a roof, in high temperature areas,