A single-phase, 240-volt residential central air-conditioning unit has a compressor with an RLC of 28.9 amps, a 1/4 hp fan, minimum circuit ampacity 37.9 amps, and maximum fuse size 60 amps. The minimum size NM cable that can be installed (THHN/THWN insulation) is ___ AWG.

A single-phase, 240-volt residential central air-conditioning unit has a compressor with an RLC of 28.9 amps, a 1/4 hp fan, minimum circuit ampacity 37.9 amps, and maximum fuse size 60 amps. The minimum size NM cable that can be installed (THHN/THWN insulation) is ___ AWG.

• A. 10
• B. 6
• C. 12
• D. 8

Answer: (D)

When sizing a circuit for a motor load, you use the equipment’s minimum circuit ampacity (MCA) to determine conductor size. The MCA here is 37.9 A, so the wiring must have an ampacity of at least 37.9 A. Copper conductors with THHN/THWN insulation have ampacity ratings that make 8 AWG the smallest size that meets or exceeds 37.9 A (8 AWG provides about 40 A at common insulation ratings). The next smaller size, 10 AWG, would not meet MCA, since its ampacity is around 30–35 A depending on the table, while 12 AWG is even lower. The 6 AWG option would certainly exceed the requirement, but the question asks for the minimum size, so 8 AWG is the correct choice. The specified maximum fuse size of 60 A relates to overcurrent protection and does not change the conductor sizing in this case, since the conductor must be large enough to carry MCA regardless of the fuse.

When sizing a circuit for a motor load, you use the equipment’s minimum circuit ampacity (MCA) to determine conductor size. The MCA here is 37.9 A, so the wiring must have an ampacity of at least 37.9 A. Copper conductors with THHN/THWN insulation have ampacity ratings that make 8 AWG the smallest size that meets or exceeds 37.9 A (8 AWG provides about 40 A at common insulation ratings). The next smaller size, 10 AWG, would not meet MCA, since its ampacity is around 30–35 A depending on the table, while 12 AWG is even lower. The 6 AWG option would certainly exceed the requirement, but the question asks for the minimum size, so 8 AWG is the correct choice. The specified maximum fuse size of 60 A relates to overcurrent protection and does not change the conductor sizing in this case, since the conductor must be large enough to carry MCA regardless of the fuse.