I have contractor saying that he can use THHN #3/0 Cu with a 225-amp circuit breaker for overcurrent protection. The terminals on the CB are listed as 75 C. However when I look at Table 310.16 THHN is not listed as a conductor in the 75 C column. I thought he would have to use a 200-amp CB based on the 75 C rating of the terminals. Bottom line can we use a 225-amp breaker with the #3/0 cable. Not sure how to answer his question correctly. The application is not a servicer entrance.
Tabel 310.16 75 C vs 95 C
- Thread starter brandtew
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You are correct about the terminal temperature ratings. #3/0 has an ampacity of 200 at 75C. Unless overcurrent protection at a higher level is allowed by 240.4(G), the 225A c/b with the 3/0 would be a violation.
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There is a piece of information missing. What is the load? If you are planning on being able to serve a 225 amp load, then you must use a conductor that has an ampacity of at least 225 amp. The ampacity of 3/0 THHN is 200, as David pointed out. That has nothing to do with overcurrent protection, nothing to do with the choice of feeder breaker.
If you were, as an example, to tell me that you did a load calculation, and that the total load (accounting for both continuous and non-continuous load) was 195 amps, then you could use a 3/0 THHN conductor. However, since it has an ampacity of 200, and since that value matches a standard overcurrent rating, and therefore you cannot use a higher rated breaker. Reference 240.4(B).
If you were, as an example, to tell me that you did a load calculation, and that the total load (accounting for both continuous and non-continuous load) was 195 amps, then you could use a 3/0 THHN conductor. However, since it has an ampacity of 200, and since that value matches a standard overcurrent rating, and therefore you cannot use a higher rated breaker. Reference 240.4(B).
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Also reference section 110.14(C) for applying conductors based on the temperature limitations at connectors.
winnie
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THHN is a 90C rated insulation. The ampacity of 3/0 THHN _conductor_ is 225A.
However the ampacity of the circuit is set by the lowest ampacity 'link in the chain'. If you have a 3/0 conductor terminated on a 75C terminal, then you must use the 75C ampacity.
If you can somehow provide a 90C rated termination on the 3/0 THHN, then you can use it with a 225A breaker.
For example, you could use a sufficient length of 4/0 between the breaker and a 90C rated splice, transition to the 3/0, and then do the reverse at the opposite end. An open question is 'how long' is a sufficient length; remember that copper is a good thermal conductor and you want to protect the breaker terminal from temperatures above 75C. In all but exceptional cases it would make more sense to simply pull the 4/0.
-Jon
However the ampacity of the circuit is set by the lowest ampacity 'link in the chain'. If you have a 3/0 conductor terminated on a 75C terminal, then you must use the 75C ampacity.
If you can somehow provide a 90C rated termination on the 3/0 THHN, then you can use it with a 225A breaker.
For example, you could use a sufficient length of 4/0 between the breaker and a 90C rated splice, transition to the 3/0, and then do the reverse at the opposite end. An open question is 'how long' is a sufficient length; remember that copper is a good thermal conductor and you want to protect the breaker terminal from temperatures above 75C. In all but exceptional cases it would make more sense to simply pull the 4/0.
-Jon
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