A branch circuit is supplying a single hermetic refrigerant motor-compressor for an air conditioning unit. If a 30-amp fuse will not start the motor-compressor, the maximum rating of the branch-circuit short-circuit and ground-fault protective device may be increased to ___.

A branch circuit is supplying a single hermetic refrigerant motor-compressor for an air conditioning unit. If a 30-amp fuse will not start the motor-compressor, the maximum rating of the branch-circuit short-circuit and ground-fault protective device may be increased to ___.

• A. 35 amps
• B. 40 amps
• C. 45 amps
• D. 50 amps

Answer: (B)

When protecting a single motor, the protection device must tolerate the high inrush current that occurs when the motor starts, but still provide protection against faults. For a branch circuit serving one motor, you’re allowed to upsize the short-circuit/ground-fault protective device to a limit that accommodates starting current without compromising protection. This limit is typically 125% of the motor’s full-load current. In this scenario, the motor’s running current is such that a 30-amp fuse won’t start it, so you look to the maximum allowed by the rule. 125% of the motor’s full-load current yields a maximum protection rating of 40 amps, which is why that option is the correct choice. Going higher than 40 amps would exceed the allowed maximum for protecting a single motor, while 40 amps is the largest rating that still provides both start-up tolerance and protection per the applicable rule. The smaller values wouldn’t reflect the permitted upsizing to handle starting current.

When protecting a single motor, the protection device must tolerate the high inrush current that occurs when the motor starts, but still provide protection against faults. For a branch circuit serving one motor, you’re allowed to upsize the short-circuit/ground-fault protective device to a limit that accommodates starting current without compromising protection. This limit is typically 125% of the motor’s full-load current.

In this scenario, the motor’s running current is such that a 30-amp fuse won’t start it, so you look to the maximum allowed by the rule. 125% of the motor’s full-load current yields a maximum protection rating of 40 amps, which is why that option is the correct choice. Going higher than 40 amps would exceed the allowed maximum for protecting a single motor, while 40 amps is the largest rating that still provides both start-up tolerance and protection per the applicable rule. The smaller values wouldn’t reflect the permitted upsizing to handle starting current.