The rating of cord- and plug-connected room air conditioners must not exceed what percentage of the branch circuit rating when lighting units are also supplied?

• A. 25%
• B. 50%
• C. 75%
• D. 100%

Answer: (B)

This question tests how much of a branch-circuit’s capacity a cord- and plug-connected room air conditioner may use when the same circuit also feeds lighting. The rule stops the air conditioner from taking too much of the circuit so there’s still room for lighting and to tolerate the air conditioner’s startup surge.

When lighting is supplied on the same branch circuit, the air conditioner’s rated input must not exceed 50% of the branch-circuit rating. So on a 15-amp circuit, the air conditioner should be limited to about 7.5 amps; on a 20-amp circuit, about 10 amps, and so on. This balance protects the circuit from overloading and reduces the risk of nuisance breaker trips due to the air conditioner’s inrush current while lighting remains on the same circuit. The other percentages would either underutilize the circuit more than necessary or fail to provide adequate headroom for lighting and inrush.