A solar contractor is preparing to connect a new PV system to the existing main service panel of a residence. The service panel has a 200A main breaker and a busbar rating of 200A. The new PV inverter has a 40A output circuit breaker. According to NEC 705.12(B)(2) (the 120% rule), what is the MAXIMUM allowable continuous current for this PV system's connection to the busbar without requiring an upgrade to the service panel?
- A50A
- B32A
- C40A
- D48A
Show answer & explanationAnswer & explanation
Correct answer: C. 40A
The 120% rule (NEC 705.12(B)(2)) states that the sum of the rating of the overcurrent devices supplying the busbar (main breaker + PV breaker) shall not exceed 120% of the busbar rating. So, (Main Breaker + PV Breaker) <= 1.2 * Busbar Rating. Given Main Breaker = 200A, Busbar Rating = 200A. Therefore, (200A + PV Breaker) <= 1.2 * 200A = 240A. This means PV Breaker <= 240A - 200A = 40A. The maximum allowable continuous current for the PV system's connection (which is the PV breaker rating) is 40A.
Why the other options are wrong
- A. 50A would also exceed the 120% rule: 200A + 50A = 250A, which is greater than 1.2 * 200A (240A).
- B. 32A is less than the maximum allowable.
- D. 48A would exceed the 120% rule: 200A + 48A = 248A, which is greater than 1.2 * 200A (240A).
NEC 705.12(B)(2) (120% Rule)
A National Electrical Code rule governing the maximum allowable current for PV systems connected to a service panel's busbar, ensuring the busbar is not overloaded.
- Sum of utility OCPD and PV OCPD must not exceed 120% of busbar rating.
- Applies to supply-side connections.
- Critical for preventing busbar overheating and ensuring safety.
Memory trick: Busbar's '120% Limit' keeps the power flow safe and sound!