conductor derating based on insulation rating

[mkhudson]

In conforming to NEC Table 310.15(B)(3)(a) to de-rate more than three THHN current-carrying conductors in raceway, we use the 90 degree C insulation rating in Table 310.15(B0(17). The Contractor is insisting on using the 75 degree C rating for this; we believe the 75 degree C rating only applies to overcurrent protection, not temperature de-rating. Which of us is correct?

 

[Dennis Alwon]

IMO, you are correct. BTW, welcome to the forum

110.14(C) Temperature Limitations. The temperature rating associatedwith the ampacity of a conductor shall be selected and
coordinated so as not to exceed the lowest temperature rating
of any connected termination, conductor, or device. Conductors
with temperature ratings higher than specified for terminations
shall be permitted to be used for ampacity adjustment,
correction, or both.

 

[charlie b]

Which of us is correct?

Neither.

110.14(C) says you can use the higher temperature column for both adjustments (meaning more that 3 CCCs in a conduit) and correction (meaning for ambient temperatures higher than 30C). So that makes the contractor wrong. But I don't know what you mean by "the 75 degree C rating only applies to overcurrent protection." So I can't say you are right.

 

[Dennis Alwon]

Neither.

110.14(C) says you can use the higher temperature column for both adjustments (meaning more that 3 CCCs in a conduit) and correction (meaning for ambient temperatures higher than 30C). So that makes the contractor wrong. But I don't know what you mean by "the 75 degree C rating only applies to overcurrent protection." So I can't say you are right.

I assumed he meant the terminals on the overcurrent protective device.

 

[mkhudson]

Thank you very much. We use the 75 degree C column to determine conductor overcurrent protection limitations, per table 310.15.(B)(16). But for de-rating, we use the 90 degree C column which is the actual insulation type.

 

[Dennis Alwon]

Thank you very much. We use the 75 degree C column to determine conductor overcurrent protection limitations, per table 310.15.(B)(16). But for de-rating, we use the 90 degree C column which is the actual insulation type.

You have it correct assuming 90C wire which just about everything is today.

 

[infinity]

In conforming to NEC Table 310.15(B)(3)(a) to de-rate more than three THHN current-carrying conductors in raceway, we use the 90 degree C insulation rating in Table 310.15(B0(17). The Contractor is insisting on using the 75 degree C rating for this; we believe the 75 degree C rating only applies to overcurrent protection, not temperature de-rating. Which of us is correct?

If the conductors are rated for 90° C then you can use that ampacity for derating purposes but you are not required to. If you use the 75° C ampacity you may end up using larger conductors and wasting some money in the process.

I believe that the correct table you're referencing is 310.15(B)(16) for conductors in a raceway.

Welcome to the Forum.

 

[Julius Right]

There are some differences between art. 110.14(C) and 310.15(B)(3)
310.15 Ampacities for Conductors Rated 0–2000 Volts.(B) Tables:
The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).
What “ampacity”?
110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Because we design power plants we do not use tables but calculate the ampacity.
In art.310.15(C) Engineering Supervision it is the formula:
I=√(Tc-Ta)/(Rdc(1+Yc)Rca)
Following Neher & McGrath and IEEE 835 we start with the rated conductor temperature [let’s say 90oC]
If we follow IEEE 835/1994 Annex A number of conductors is built-in Rca formula:
Rca=Ri+n(Rj+Rsd+Rc+Re) where:
Ri insulation thermal resistance [T.O.F.]
Rj jacket thermal resistance
Rsd =Cable to conduit
Rc=Conduit wall
Re=is thermal resistance of the earth or external thermal circuit (air, duct bank, etc.)
n=number of conductors within a stated diameter[Neher & McGrath Nomenclature]
That means-in my opinion- the derating purpose it is to adjust the ampacity in order to keep conductor temperature at 90oC.[it is not in order to reduce the conductor temperature].