Terminal derates for ambient temps

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SunFish

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Do I need to derate terminal ratings similar to conductor ratings if they are in ambient temperatures other than 30 deg C? For example if I have an AC disconnect with 75 deg C terminals mounted outdoors with a max ambient temp of 39 deg C and I land #6 THWN-2/THHN 90 deg C wire on that terminal do the 75 deg C terminal still have a 65 A rating (for #6 @ 75 deg C in table 310.15(B)(2)(b)) or do I need to apply the 0.88 derate to this terminal rating because it is in 39 deg C ambient temp?
 

Carultch

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Do I need to derate terminal ratings similar to conductor ratings if they are in ambient temperatures other than 30 deg C? For example if I have an AC disconnect with 75 deg C terminals mounted outdoors with a max ambient temp of 39 deg C and I land #6 THWN-2/THHN 90 deg C wire on that terminal do the 75 deg C terminal still have a 65 A rating (for #6 @ 75 deg C in table 310.15(B)(2)(b)) or do I need to apply the 0.88 derate to this terminal rating because it is in 39 deg C ambient temp?

As much as you might think that temperature derates apply to terminals, this is not the case in the NEC. I used to think the same thing myself.

The wire temperature rating may be used for both temperature correction factors, and bundling ampacity adjustments.
The terminal/equipment rating is required to be used prior to any "derate" factors.

It is no longer called a derate, even though the concept still exists.
 

GoldDigger

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As much as you might think that temperature derates apply to terminals, this is not the case in the NEC. I used to think the same thing myself.

The wire temperature rating may be used for both temperature correction factors, and bundling ampacity adjustments.
The terminal/equipment rating is required to be used prior to any "derate" factors.

It is no longer called a derate, even though the concept still exists.
They do apply, in a way, to motors however. The tables and percentages are not used but you have to look at temp rise and ambient temp relative to upper tolerable motor temp.
 

Carultch

Senior Member
Location
Massachusetts
Do I need to derate terminal ratings similar to conductor ratings if they are in ambient temperatures other than 30 deg C? For example if I have an AC disconnect with 75 deg C terminals mounted outdoors with a max ambient temp of 39 deg C and I land #6 THWN-2/THHN 90 deg C wire on that terminal do the 75 deg C terminal still have a 65 A rating (for #6 @ 75 deg C in table 310.15(B)(2)(b)) or do I need to apply the 0.88 derate to this terminal rating because it is in 39 deg C ambient temp?

See the final sentence of 110.14(C):
"Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment,correction, or both."

This really is the value of the 90C insulation rating, in most applications, in that it gives you flexibility for your calculations.

The other value to 90C insulation is much more obscure. It considers the rare event that you connect from field-installed connector to field-installed connector with 90C wire, and then to your equipment with 90C wire at 75C sizing. A good way to salvage a situation where you screw up, but not something you would initially intend to do.
 
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Carultch

Senior Member
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Massachusetts
They do apply, in a way, to motors however. The tables and percentages are not used but you have to look at temp rise and ambient temp relative to upper tolerable motor temp.

Good point. I'm not familiar with motor loads and their circuits, but I am aware that motors do change every circuit sizing strategy, from the standard practice for circuits in general.
 
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