Why do the circuit breaker vendors have a dual rating for the temperature of conductors that can be used with their breakers? For example for one line of SquareD breakers from 10-30 Amps you can use 60C or 75C conductors. Above 30A you need to use 75C conductors. Is this dual rating (30 Amps and below)a result of some rule in the NEC? 110.14 doesn't seem to mention anything about the 30A cutoff. Thanks Tim.
Circuit Breaker Conductor Temperature Rating
- Thread starter tstrouse
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charlie
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Re: Circuit Breaker Conductor Temperature Rating
I am not sure that I understand your question so I may wander a bit.
The conductors through #1 are rated for 60C but may be rated 75C or 90C. If the terminations on both ends of the run are rated for 75C or 90C, you are permitted to use the conductor at its 75C ampacity. If the terminals are only rated for 75C, you would not be permitted to use a 60C conductor. If the terminals are listed only for 60C, you would not be permitted to use 75C wire at its 75C ampacity but you would be permitted to use it at the 60C ampacity. A dual rating takes care of the problem nicely on the overcurrent device end. Once you pass #1, the problem goes away since 1/0 and larger conductors are rated for at least 75C.
I am not sure that I understand your question so I may wander a bit.
The conductors through #1 are rated for 60C but may be rated 75C or 90C. If the terminations on both ends of the run are rated for 75C or 90C, you are permitted to use the conductor at its 75C ampacity. If the terminals are only rated for 75C, you would not be permitted to use a 60C conductor. If the terminals are listed only for 60C, you would not be permitted to use 75C wire at its 75C ampacity but you would be permitted to use it at the 60C ampacity. A dual rating takes care of the problem nicely on the overcurrent device end. Once you pass #1, the problem goes away since 1/0 and larger conductors are rated for at least 75C.
Re: Circuit Breaker Conductor Temperature Rating
Tim, The way I look at it when it comes down to the bottom line it's no big deal, 60 vs 75degC. Take a look at the 2002 NEC ART 240.4(D) Small conductors, #14,#12, and #10. Except for other derating factors up to 10 AWG size 75degC rated wire is applied as if were 60degC anyway. Isn't THHN 90degC rated anyway? It's still applied as 60degC. As such, there really isn't any advantage of using 75degC or 90degC rated wire because of the limitation as included in art 240.4(D) if derating factors aren't an issue.
Even so at the other end of the circuit is often a device such as a light fixture that calls for 90degC rated wire when 60degC won't work.
Also, keep in mind that larger breakers have been supplied with 75degC terminals for quite some time where 90degC rated THHN wire is applied at 75degC, again this is without any derating factors taken into consideration.
Tim, The way I look at it when it comes down to the bottom line it's no big deal, 60 vs 75degC. Take a look at the 2002 NEC ART 240.4(D) Small conductors, #14,#12, and #10. Except for other derating factors up to 10 AWG size 75degC rated wire is applied as if were 60degC anyway. Isn't THHN 90degC rated anyway? It's still applied as 60degC. As such, there really isn't any advantage of using 75degC or 90degC rated wire because of the limitation as included in art 240.4(D) if derating factors aren't an issue.
Even so at the other end of the circuit is often a device such as a light fixture that calls for 90degC rated wire when 60degC won't work.
Also, keep in mind that larger breakers have been supplied with 75degC terminals for quite some time where 90degC rated THHN wire is applied at 75degC, again this is without any derating factors taken into consideration.
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