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I have Siemens QP breakers, but I believe this generalizes well.

Let's take the QP breakers as an example:

Siemens QP Circuit Breaker

There are some temperature ratings marked on it where it reads

CU-AL 60/75°C WIRE

CONDUCTOR 60/75°C

Why are two temperature ratings indicated? Is it simply to indicate compliance with either 60°C usage cases (ex. NM-B) or 75°C usage cases (ex. THWN)? Does it depend on whether the wire is copper or aluminum (ex. 60C for Cu and 75°C for Al)?

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60/75°C Wire — All circuit breakers rated 125 A or less are marked for use with 60° C, 60/75°C or 75°C only wire. This marking indicates the proper wire size for termination in accordance with Table 310.15(B)(16) of the NEC . It is acceptable to use wire with a higher insulation rating if the ampacity is based on the wire temperature rating marked on the breaker. For breakers rated more than 125 A, the proper wire temperature rating is 75°C and it is optional for the breaker to bear this marking.

REF: UL Marking and Application Guide - Molded Case Circuit Breakers 2016 Page 10 Terminations (20)

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