CSLB C-46 Solar ContractorElectrical and Plumbing CodesHard
A solar installer is connecting a PV system to an existing residential service. The main service panel has a 200A busbar. The main overcurrent protective device (OCPD) is rated at 200A. The PV inverter output circuit breaker is rated at 40A. Using the 120% rule for load-side connections (NEC 705.12(B)(2)(3)), what is the maximum allowable continuous current rating of the PV inverter output circuit breaker that could be connected to this busbar without requiring an upgrade to the busbar or main OCPD?
- A20A
- B80A
- C60A
- D40A
Show answer & explanationAnswer & explanation
Correct answer: D. 40A
According to NEC 705.12(B)(2)(3), the sum of the ampere ratings of all overcurrent devices supplying power to a busbar must not exceed 120% of the busbar's rating. In this case, 120% of 200A is 240A. The formula for load-side connections is (Busbar Rating × 1.2) - Main OCPD Rating ≥ Inverter OCPD Rating. So, (200A × 1.2) - 200A = 240A - 200A = 40A. This means the maximum PV inverter OCPD rating without an upgrade is 40A.
Why the other options are wrong
- A. 20A is allowable but not the maximum permitted under the 120% rule for this scenario.
- B. 80A would significantly exceed the 120% rule (200A + 80A = 280A, which is > 240A).
- C. 60A would exceed the 120% rule (200A + 60A = 260A, which is > 240A).
NEC 120% Rule Calculation
A calculation method (from NEC 705.12(B)(2)(3)) to determine the maximum allowable PV inverter output overcurrent device rating when connecting to the load-side of a service panel, based on the busbar and main OCPD ratings.
- Formula: (Busbar Rating × 1.2) - Main OCPD Rating.
- Result is maximum PV OCPD rating.
- Ensures busbar is not overloaded by combined sources.
Memory trick: Busbar times 1.2, minus the main, equals PV's gain!