# Wire Ampacity Derating. Terminal temp rating vs Wire temp Rating



## ceb58 (Feb 14, 2009)

Carultch said:


> I am using 90C rated wire (THWN-2) and I have source and destination enclosures with terminals rated for only 75C.
> 
> There are 4 current-carrying wires in my conduit between enclosures. Per NEC Table 310.15 (B)(3)(a), this gives a "number of wires derate" of 0.8.
> 
> ...


 You have too much crap going on. For derating you use the 90 deg. column. 310.15(B)(4). So for a #6 Cu THHN it is good for 75 amps. Derate factor of 80% so you wind up with max ampacity of 60 amps. So your OCP could not exceed 60 amps. But what are the 4 wires? 2 ungrounded, 1 grounded and 1 EGC? 3 ungrounded and 1 EGC? You may not have a derating situation.


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## Carultch (May 14, 2013)

Thank you so much for your reply.

I think I am understanding that #6 is good. No need for #4.

Let's suppose that my 4 wires are ALL "current-carrying conductors", each carrying identical current. I am not counting any grounds of any kind, either GEC or EGC, as part of my derates. These count for conduit area fill, but not derates.

The three most common cases for a situation like this are as follows:
A: 3 live phases and a neutral. I am aware that neutral carries zero current if balanced, but if somehow it isn't, neutral will carry current.
B: 2 live DC conductors, and 2 "neutral" DC conductors carrying the return current. Perhaps of unrelated circuits, or perhaps of parallelled wire sets.
C: 4 live DC conductors, of a "bipolar" or otherwise ungrounded system. Perhaps of unrelated circuits, or perhaps of parallelled wire sets.


If I understand correctly, "number of wires" derate applies to the ampacity of the wire, and NOT the ampacity of the terminal. That is what I originally thought.

It may be common for calculation convenience, to use the 75C terminal rating derated, as a "just to be safe". And I'd like to understand if it is necessary or not. To me, that sounds like upzising a hook, just because the rope broke a strand, several feet away from the hook.


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## HARRY304E (Sep 15, 2010)

Carultch said:


> I am using 90C rated wire (THWN-2) and I have source and destination enclosures with terminals rated for only 75C.
> 
> There are 4 current-carrying wires in my conduit between enclosures. Per NEC Table 310.15 (B)(3)(a), this gives a "number of wires derate" of 0.8.
> 
> ...


Yes read 110.14 (C) all the way through.

Welcome to the forum..:thumbup:


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## HARRY304E (Sep 15, 2010)

Carultch said:


> Thank you so much for your reply.
> 
> I think I am understanding that #6 is good. No need for #4.
> 
> ...


That's right it apply s to the wire first.


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## Carultch (May 14, 2013)

Thank you. I think I am getting this.:thumbup:

It is my understanding now, that wire bundle derates apply exclusively to wire amapcity, and not to terminal ampacity.

Terminal ampacities would always be treated as individual wires. Perhaps derated for ambient temperature/direct sunlight, where applicable, but not for wire bundles.

It is a practical rule to understand correctly, because 75 C terminals are very common, and 90 C wire is what we use by default. If the terminal temperature makes the whole wire only rated at that temperature, then there'd be no purpose for 90 C wire.


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## HARRY304E (Sep 15, 2010)

Carultch said:


> Thank you. I think I am getting this.:thumbup:
> 
> It is my understanding now, that wire bundle derates apply exclusively to wire amapcity, and not to terminal ampacity.
> 
> ...


 That's right,The 90 C wire gives you some room for your calculations..


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