Derating of high temp (Table 310.18) conductors

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tc_in_ky

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I'm familiar with the process for derating conductor ampacity for instance to connect a 90C conductor to a 75C terminal by using the appropriate columns in Table 310.16 or 310.17. How do you go about doing the same kind of derating for a higher temperature conductor such as Type Z from Table 310.18?
 
Would it not be the same? You derate from the higher temp but the final overcurrent protective device cannot be greater than the 75C assuming that is the weakest link
 
The problem I'm having is, there are no columns with ampacities for the lower temperature ratings. For instance in Table 310.18, 150C is the lowest temperature rating given for Type Z wire. How do I know what ampacity to derate to if I'm connecting this 150C-rated wire to a terminal rated for 75C? There is no 75C column for Type Z wire in Table 310.18.
 
I have never used type Z, and I never encountered that high an operating temperature range. So I can't answer your question. I will, however, offer one comment.
tc_in_ky said:
I'm familiar with the process for derating conductor ampacity for instance to connect a 90C conductor to a 75C terminal by using the appropriate columns in Table 310.16 or 310.17.
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That is not a "process for derating." It is a process for discovering the ampacity by looking at the right place in the book. I know I am splitting hairs here, but I think it is worth getting the concepts clear. The definition of "ampacity" includes the phrase "conditions of use." When you connect a #4 THHN wire (i.e., one with a 90C rating) to a termination that is rated for 75C, we are not "derating" the wire from 95 amps to 85 amps. Rather, we are taking into account that the "conditions of use" include the attachment of a 75C termination, and discovering that the assigned ampacity under those conditions is 85 amps.

I will take it a step further (and I expect some push-back on this one) and say that when we multiply the tabulated value by 80% because there are 4 current-carrying conductors in the same conduit, we are not "derating." Instead, we are applying an "adjustment factor." And when we multiply the tabulated values by 95% because the ambient temperature is 42 degrees C, that too is not "derating." Instead, we are applying a "correction factor." All of this is done in order to establish the ampacity. The term "derating" is merely a conversational slang term for the process.

Welcome to the forum.
 
tc_in_ky said:
The problem I'm having is, there are no columns with ampacities for the lower temperature ratings. For instance in Table 310.18, 150C is the lowest temperature rating given for Type Z wire. How do I know what ampacity to derate to if I'm connecting this 150C-rated wire to a terminal rated for 75C? There is no 75C column for Type Z wire in Table 310.18.
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Yes I see the issue now... I wonder if equipment that uses type z have terminals rated 150 C????? Wow,IDK
 
tc_in_ky said:
The problem I'm having is, there are no columns with ampacities for the lower temperature ratings. For instance in Table 310.18, 150C is the lowest temperature rating given for Type Z wire. How do I know what ampacity to derate to if I'm connecting this 150C-rated wire to a terminal rated for 75C? There is no 75C column for Type Z wire in Table 310.18.
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If you are using type Z with 75C terminals, the ampacity would be limited to the 75C column in T310.15(B)(16).
 
Dennis Alwon said:
How do we know that Z type isn't rated 60C-- I assume it is 75 C but that was my hesitation in suggesting 75C
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just a WAG here, but I would conclude if the final adjusted amperage is less than 100 amps use the 60C if its 100 amps or greater use the 75C
 
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