Derating conductors for high ambient temp when only energized at a much lower temp

[Heat Trace]

I have an application where conductors will be installed in an area with an ambient temperature of 120F. Typically I'd de-rate the conductors for the elevated temperature but in this case the circuits will not energize until the temperature drops to 40F. Does the code still require the conductors to be de-rated for the 120F ambient condition?

 

[jumper]

So the ambient temperature will be 40F when the conductors are energized? Correct?

 

[Heat Trace]

Correct. They will be de-energized at any temperature above 40F.

 

[jumper]

If that is the case I believe that derating would not be required.

 

[Heat Trace]

That's my interpretation but I was unable to find anything definitive in the code or in any articles I have on the subject. I believe the code assumes conductors must be de-rated for the ambient temperature in which they will be energized. To me, that's the logical method but then - we are talking about the NEC!

 

[Heat Trace]

Thank you for the reply.

 

[Smart $]

I agree with the energized-ambient reasoning.

 

[iwire]

Are we confident enough to uprate the conductors due to the low temp?

 

[Smart $]

Are we confident enough to uprate the conductors due to the low temp?

Who said anything about uprating? hmy:

Regardless, you are correct in that "derating" at 40°F would result in an ampacity adjustment factor of 1.15.

 

[Carultch]

Are we confident enough to uprate the conductors due to the low temp?

Can you take credit for a colder than 30C ambient temperature, for equipment terminations per 110.14(C)?

As an example, 400 kcmil THWN-2, with a 75C temperature rating in a not to exceed 20C ambient temperature.

The 90C rated wire ampacity at the standard 30C is 380A, and it gets increased by a factor of 1.04 due to the 20C ambienet, and is thus 395.2A.

However, 110.14(C) requires us to consider 75C terminal rating as the possible "weak link", no matter what kind of wire in excess of a 75C rating. The ampacity of 400 kcmil in a 75C rated terminal is 335A. If you derate this by the 20C factor of 1.05, you get 351.75A.

I understand that for hot ambient temperatures and bundling, you don't apply the correction factors to the termination rating. You apply it to the wire rating.

So do temperature corrections of cold ambient apply to both wire and terminations in an example such as this?

 

[Carultch]

Are we confident enough to uprate the conductors due to the low temp?

The way I look at it, is prepare for the temperature maintenance system to work, and not to work. If this circuit is guaranteed not to run when the surrounding air exceeds 40F, then it makes sense to take credit for the cold ambient. This would also mean that if the wiring is in a room that is artificially heated significantly higher than human comfort, then it should be designed with an ambinet temperature of what we expect in the hot room during operation.

However, if the cooling system fails, and the circuit in question continues to operate, then the circuit in question should still have sufficient ampacity for the load at the ASHRAE 2% highest natural temperature it expected to experience.

 

[Smart $]

Can you take credit for a colder than 30C ambient temperature, for equipment terminations per 110.14(C)?...

That's a good question. I can rationalize taking the credit, but Code wise there is no statement to back up an implementation.... under current Code. Note the ambient correction factors were in Table 310.16 of the 2008 edition. They were made a separate table in the 2011 edition and remain so in 2014.

110.14(C) both then and now base the provision on Table 310.16 or Table 310.15(B)(16), as appropriate for the edition. So under 2008 and prior I believe the rationale could be implemented.

 

[iwire]

Who said anything about uprating? hmy:

I did, it's called a question.

 

[jumper]

Unless an EE speced/stamped it or I was wiring in Antarctica, I would not be uprating any conductors for ambient temperature.

 

[Carultch]

That's a good question. I can rationalize taking the credit, but Code wise there is no statement to back up an implementation.... under current Code. Note the ambient correction factors were in Table 310.16 of the 2008 edition. They were made a separate table in the 2011 edition and remain so in 2014.

110.14(C) both then and now base the provision on Table 310.16 or Table 310.15(B)(16), as appropriate for the edition. So under 2008 and prior I believe the rationale could be implemented.

Right. I would think you cannot "have your cake and eat it too" with this rule.

Given that the NEC doesn't require applying ambient temperature corrections to the termination ampacity for higher than 30C ambients, I would think that you wouldn't be allowed to take credit for a lower ambient temperature regarding terminations and 110.14(C).