CSLB C-46 Solar ContractorPhotovoltaic System InstallationMedium
A solar installer is connecting a grid-tied PV system to a residential service panel. The system includes a string inverter with a maximum AC output current of 30A. The existing main service panel has a 200A busbar. According to NEC 705.12(B)(2), what is the maximum overcurrent protection device (OCPD) rating allowed for the PV inverter's AC output circuit if the main breaker is 200A?
- A30A
- B50A
- C40A
- D60A
Show answer & explanationAnswer & explanation
Correct answer: C. 40A
NEC 705.12(B)(2) states that the sum of the ampere ratings of all overcurrent devices supplying power to a busbar cannot exceed 120% of the busbar's ampere rating. This is often referred to as the 120% rule. The formula is: (Busbar Rating * 1.2) - Main Breaker Rating = Max PV OCPD. So, (200A * 1.2) - 200A = 240A - 200A = 40A. The maximum OCPD for the PV inverter output is 40A.
Why the other options are wrong
- A. 30A is the inverter output current, not the maximum OCPD allowed by code.
- B. 50A exceeds the calculated maximum OCPD allowed by the 120% rule.
- D. 60A significantly exceeds the calculated maximum OCPD allowed by the 120% rule.
NEC 705.12(B)(2) (120% Rule for Busbar)
Limits the total current that can be supplied to a busbar by both the utility and PV system to 120% of the busbar's rating.
- Ensures the busbar is not overloaded by combined utility and PV current.
- Formula: (Busbar Rating * 1.2) - Main Breaker Rating = Max PV OCPD.
- Critical for safe interconnection of PV systems to existing electrical panels.
Memory trick: Busbar's 120% limit, keep the total current fit!