Page 63 of the 2008 Bussmann SPD book says this:
"So why would anyone ever want to use a conductor with a 90?C or a 105?C rating if they can?t be applied at their ampacity ratings for those temperatures? The answer lies in the fact that those higher ampacity ratings can be utilized when derating due to ambient conditions or due to exceeding more than three current carrying conductors in a raceway."
My question is, where in the NEC does it allow you to use the higher ampacity rating than the terminal in the case of temperature or # of conductors derating?
The SPD book goes on to give an example:
"Assume that an ampacity of 60A is needed in a circuit with a 75?C termination at one end and a 60?C termination at the other end, where the ambient is 45?C. First, since one termination temperature rating is higher than the other, the lowest one must be used, which is 60?C. The first choice might be a 4 AWG TW conductor with an ampacity of 70A at 60?C. However, in the NEC? the Correction Factors table at the bottom of conductor ampacity Table 310.16 reveals that the 70A ampacity must be derated, due to the 45?C ambient, by a factor of 0.71. This yields a new ampacity of 49.7, which is less than the required 60. This is where a conductor with a higher temperature rating becomes useful. A 4 AWG THHN conductor has a 90?C ampacity of 95A. Again, looking at the table at the bottom of Table 310.16, a factor of .87 must be used, due to the 45?C ambient. This yields a new ampacity of 82.65, which is adequate for the required 60A ampacity.
Could a 6 AWG THHN conductor be used in this application? Its 90?C ampacity is 75A. Using the factor of 0.87 for the 45?C ambient gives a new ampacity of 65.25, which seems adequate for a required ampacity of 60A. However, a 6 AWG conductor of any insulation rating could never be used in this application because the 60?C terminal requires that the smallest amount of copper is a 4 AWG for a 60A ampacity (simple rule 2 in previous paragraphs). The amount of copper associated with a 4 AWG conductor is required to bleed the right amount of heat away from the terminal. The use of less copper won?t bleed enough heat away, and therefore overheating problems could result."
"So why would anyone ever want to use a conductor with a 90?C or a 105?C rating if they can?t be applied at their ampacity ratings for those temperatures? The answer lies in the fact that those higher ampacity ratings can be utilized when derating due to ambient conditions or due to exceeding more than three current carrying conductors in a raceway."
My question is, where in the NEC does it allow you to use the higher ampacity rating than the terminal in the case of temperature or # of conductors derating?
The SPD book goes on to give an example:
"Assume that an ampacity of 60A is needed in a circuit with a 75?C termination at one end and a 60?C termination at the other end, where the ambient is 45?C. First, since one termination temperature rating is higher than the other, the lowest one must be used, which is 60?C. The first choice might be a 4 AWG TW conductor with an ampacity of 70A at 60?C. However, in the NEC? the Correction Factors table at the bottom of conductor ampacity Table 310.16 reveals that the 70A ampacity must be derated, due to the 45?C ambient, by a factor of 0.71. This yields a new ampacity of 49.7, which is less than the required 60. This is where a conductor with a higher temperature rating becomes useful. A 4 AWG THHN conductor has a 90?C ampacity of 95A. Again, looking at the table at the bottom of Table 310.16, a factor of .87 must be used, due to the 45?C ambient. This yields a new ampacity of 82.65, which is adequate for the required 60A ampacity.
Could a 6 AWG THHN conductor be used in this application? Its 90?C ampacity is 75A. Using the factor of 0.87 for the 45?C ambient gives a new ampacity of 65.25, which seems adequate for a required ampacity of 60A. However, a 6 AWG conductor of any insulation rating could never be used in this application because the 60?C terminal requires that the smallest amount of copper is a 4 AWG for a 60A ampacity (simple rule 2 in previous paragraphs). The amount of copper associated with a 4 AWG conductor is required to bleed the right amount of heat away from the terminal. The use of less copper won?t bleed enough heat away, and therefore overheating problems could result."
