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Can you explain how to size 90°C conductors terminating on 75°C rated equipment?

Conductors are to be sized using their ampacity from the insulation temperature rating column of 2011NEC Table 310.15(B)(16) that corresponds to the lowest temperature rating of any terminal, device, or conductor of the circuit [110.14(C)].

Unless the equipment is listed and marked otherwise, conductor sizing for equipment terminations must be based on Table 310.15(B)(16) in accordance with (a) or (b):

(a) Equipment Rated 100A or Less [110.14(C)(1)(a)].
Conductors must be sized using the 60°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(1)].
Conductors terminating on terminals rated 75°C are sized in accordance with the ampacities listed in the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(a)(3)].

(b) Equipment Rated Over 100A.
Conductors must be sized using the 75°C temperature column of Table 310.15(B)(16) [110.14(C)(1)(b)(1)].

Separate Connector Provisions [note: this does not apply to circuit breakrs]; Conductors can be sized to the 90°C column of Table 310.15(B)(16) if the conductors and pressure connectors are rated at least 90°C [110.14(C)(1)(b)(2)].

Source:  Mike Holt Newsletter


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