Can you use a 500MCM cable on a 400A breaker?Yes. According to the "next size up rule," if the calculated load is under 380A, you can use a 500MCM cable. Even though a 400A breaker is not the "next size up" from a 380A load, you can still use the 500MCM cable because it is the next size up from the calculated load.

Must you use a 600MCM cable on a 450A breaker?Yes. Since a 450A breaker is not the next size up from a 380A load, you cannot use the 500MCM cable. You must use the next size up cable from the breaker rating, which is a 600MCM cable.

Must you increase cable size when a breaker is upsized for transformer inrush?No, not necessarily. The National Electrical Code (NEC) allows the use of a cable that is one size smaller than the breaker rating for transformer inrush conditions. This is referred to as the "tap rule." However, the cable must still be protected by the overcurrent device, which means that the cable ampacity must be greater than or equal to the breaker rating.Therefore, if you are upsizing a breaker for transformer inrush, you may be able to use a smaller cable size than the "next size up rule" would require. However, you must consult the NEC and the manufacturer's guidelines to determine the specific requirements for your application. Flag for review