A commercial office space requires a new 20A, 120V branch circuit for receptacle outlets. The circuit conductors will be 12 AWG copper with 75°C insulation, and the overcurrent protective device (OCPD) is a 20A circuit breaker. The connected load is 16A continuous. What is the maximum continuous load permitted on this circuit, considering the OCPD and conductor ratings?
- A20A
- B16A
- C25A
- D24A
Show answer & explanationAnswer & explanation
Correct answer: B. 16A
According to NEC 210.20(A), where a branch circuit supplies continuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load. Also, the conductor ampacity must be at least 125% of the continuous load. For a 20A OCPD, the maximum continuous load is 20A / 1.25 = 16A. The 12 AWG copper conductor at 75°C is rated for 25A (Table 310.16). Its capacity for continuous load is 25A / 1.25 = 20A. The limiting factor is the OCPD. So, the maximum continuous load permitted is 16A.
Why the other options are wrong
- A. 20A is the maximum total load, but only 80% (16A) of it can be continuous for a 20A OCPD.
- C. 25A is the ampacity of 12 AWG at 75°C, but the OCPD limits the continuous load to 16A.
- D. 24A exceeds the rating of the 20A OCPD.
Continuous Load OCPD & Conductor Sizing
The requirement to size both the overcurrent protective device and the conductors for 125% of the continuous load to prevent overheating.
- OCPD rating must be at least 125% of continuous load (NEC 210.20(A)).
- Conductor ampacity must be at least 125% of continuous load (NEC 210.19(A)(1)).
- Continuous load is one expected to run for 3 hours or more.
Memory trick: Continuous loads need a 25% 'buffer' for both the breaker and the wires.