For small air conditioner motor-compressors, fused protection may be used if the protective device rating is not more than what percentage of the motor's rated current?

For small air conditioner motor-compressors, fused protection may be used if the protective device rating is not more than what percentage of the motor's rated current?

• A. 40 amps if less than 3/4 hp or 90 amps if greater than 1 hp
• B. not more than six times the FLC
• C. not more than four times the FLC
• D. not more than 125% of the rated current

Answer: (D)

The key idea is how fuses protect a motor while letting it handle startup surges. A motor’s full-load current (FLC) is the running current the motor draws under normal conditions. For fused protection, the fuse rating must be high enough to tolerate the brief inrush or startup current, but not so high that a sustained overload goes unchecked. The standard guideline is to size the fuse at not more than 125% of the motor’s FLC. So if a motor runs at 8 A, you’d select a fuse up to 10 A (8 × 1.25). This balance prevents nuisance blowing during startup while still protecting the motor from overheating during a fault. The other options don’t reflect this widely used sizing rule.

The key idea is how fuses protect a motor while letting it handle startup surges. A motor’s full-load current (FLC) is the running current the motor draws under normal conditions. For fused protection, the fuse rating must be high enough to tolerate the brief inrush or startup current, but not so high that a sustained overload goes unchecked. The standard guideline is to size the fuse at not more than 125% of the motor’s FLC. So if a motor runs at 8 A, you’d select a fuse up to 10 A (8 × 1.25). This balance prevents nuisance blowing during startup while still protecting the motor from overheating during a fault. The other options don’t reflect this widely used sizing rule.