Conductor rating question

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zam

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The General Contractor sent this request-for-information: "The engineer is saying the wire is rated for 90 degrees which is correct, but per NEC we can only use wire sizes off the 75 degree chart. 90 degrees is only good for de-rating purposes."

Is this sentence true? Can you please provide any feedback, if possible. Thank you.
 
I agree with Ron. NM cable uses 90° C conductors but their ampacity is limited to 60° C.
 
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zam said:
The General Contractor sent this request-for-information: "The engineer is saying the wire is rated for 90 degrees which is correct, but per NEC we can only use wire sizes off the 75 degree chart. 90 degrees is only good for de-rating purposes."

Is this sentence true? Can you please provide any feedback, if possible. Thank you.
Click to expand...

To be nit picky, the statement is not true. There is no prohibition on using the 90 degree ampacity if the conductors are so rated. However, as mentioned by others, nearly all equipment requires 75 degree terminations....BUT, nearly all splicing and connecting methods are rated 90 degrees so one can transition to 90 degree conductors and ampacity after the equipment.
 
A way to think about this issue is to consider a chain. The strength is limited by the weakest link.

If you have 90C wire connected to 75C terminals (an extremely common situation) then the ampacity at the terminal is set by the 75C limit. But in the middle of the run (where you might have lots of conductors in a single pipe) the 90C limit holds and can be used for derating.

If you had a 60C terminal, then at that location the 60A ampacity would apply.

After you figure out the ampacity of each location in the circuit, the lowest value is the ampacity of the circuit.

As electrofelon notes, if you correctly select components, you can use the 90C ampacity, but doing so is likely more trouble then it is worth.

Jon
 
winnie said:
As electrofelon notes, if you correctly select components, you can use the 90C ampacity, but doing so is likely more trouble then it is worth.

Jon
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Yeah considering voltage drop, that technique does seem to have limited practicality: if you really need to max out a conductor at it's 90 degree capacity, it seems like voltage drop may become a concern before too long. For a short run the conductor savings would be minimal, not to mention the hassle and cost of the splices. I have thought the same thing about using a100% rated breaker.
 
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