What is the maximum standard size overcurrent protective device allowed for THW conductors rated at 75°C?

In electrical installations, the ampacity of conductors is determined by their construction and the temperature rating of the insulation. For THW conductors, which are rated for a maximum temperature of 75°C, the calculation of the maximum allowable overcurrent protective device involves looking at the ampacity table provided in the National Electrical Code (NEC).

According to NEC guidelines, the maximum size of an overcurrent protective device for conductors must not exceed the allowable ampacity rating of the conductors under their specified temperature rating. For THW conductors rated at 75°C, the standard ampacity value is typically higher than for lower temperature ratings.

When calculating, a 75°C rated conductor can generally handle up to 250 A for its maximum rating. This is why the maximum allowable standard size overcurrent protective device for THW conductors rated at 75°C is indeed 250 A. This ensures both safety and compliance with electrical codes by protecting the conductors from overheating due to excessive current.

In the context of the other choices, lower ratings like 70 A do not utilize the full capacity of the conductors, while values such as 175 A or 350 A would either be too low or exceed safety limits for THW conductors