Max ampacity of conductors

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jack christensen

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Are the tables for max ampacity in 310-16 overridden by the temperature maximum of 40 degrees C listed on the circuit breaker housing for a dedicated circuit? This question came up during a conversation directed at a 225 amp rating for 4/0 CU THHN. I say no, that the THHN should be 260 for 4/0 and according to the table 3/0 should be used. Thanks, Jack
 
Don't know where you are getting the 40 degree C but that listing dictates the ampage of the circuit and the size of the wire
 
Max wire ampacity

Max wire ampacity

The 40 degrees C is either stamped on the circuit breaker housing or is listed upon a label attached to the circuit breaker. If the amperage from the circuit breaker is passed through the conductor, is the maximum temperature for the wire ampacity regulated by the power supply limit (i.e., circuit breaker internal max temp)?
 
I believe the 40C is the ambient temp rating of the circuit breaker. The lugs are probably rated 75C.

The breaker should dissipate the heat (from the 75C lug) until the temperature in the panel exceeds 40C.

You can use 3/0 Cu. for a 200-amp load using the 75C column (providing you don't have other derating requirements).
 
The 40?C is the maximum ambient. The UL489 standard permits a 50?C rise over the ambient at the wire terminal with the circuit breaker loaded to 100%. Note that this test, as with most UL breaker tests, is conducted in open air, and the breaker is not subjected to heat from other breakers.
Don
 
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