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What makes a main-lug enclosure rated for 75 deg C NOT usable for 90 deg C conductors? Assuming you used 90 deg C conductors in an application where the design (e.g. feeder breaker) limited currents to the 75 C value, is that a problem?

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  • Keep in mind you can also round up on feeder breaker. For instance if you run #6 Cu or #4 Al THHN, which is 55A, 65A and 75A ... if you run it at 75C giving 65A, they don't make a 65A breaker so they let you use a 70A breaker. You can't plan to use more than 65A. Aug 30, 2022 at 4:32

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You're operating under a misapprehension.

75°C terminals are usable for 60, 75 and 90 °C wires.

The wire gauge (or overcurrent protection, based on wire gauge and temperature ratings) must be based on the ampacity of the lowest rating in play. Normally folks have a desired ampacity in mind, so it's the wire gauge you need to adjust to suit.

So for 60°C wire, the 60°C rating applies to the whole system

for 75 °C wire, the 75 °C rating applies

for 90°C wire, the 75°C rating applies

If the wires run through a hot location, the 90°C rating may offer some benefit on thermal derating of the wires.

The usual "education problem" we have is people taking the 90°C wire rating as a rating they can use with 90°C wire, when there are essentially no 90°C rated terminals in play, at least in non-industrial-scale equipment.

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