CSLB C-46 Solar ContractorElectrical and Plumbing CodesMedium
A solar installer is connecting a new grid-tied PV system to an existing residential service. The main service disconnect is rated for 200 A. The utility requires that the sum of the PV inverter output circuit overcurrent device and the main service overcurrent device does not exceed 120% of the service rating. If the PV system has an inverter output circuit breaker rated at 50 A, what is the maximum continuous current rating allowed for the main service overcurrent device, according to NEC 705.12(B)(2)(3)?
- A200 A
- B190 A
- C210 A
- D240 A
Show answer & explanationAnswer & explanation
Correct answer: B. 190 A
According to NEC 705.12(B)(2)(3) (the 120% Rule), the sum of the PV overcurrent device and the main service overcurrent device cannot exceed 120% of the service rating. The calculation is: Main OCPD <= (1.2 * Service Rating) - PV OCPD. So, Main OCPD <= (1.2 * 200 A) - 50 A = 240 A - 50 A = 190 A.
Why the other options are wrong
- A. This would be the existing main breaker rating, but it might exceed the 120% rule when combined with PV.
- C. This value exceeds the maximum allowed by the 120% rule when combined with the PV OCPD.
- D. This is the total 120% capacity, not the allowed main OCPD rating.
NEC 120% Rule (PV Interconnection Calculation)
A National Electrical Code (NEC) rule that limits the combined overcurrent protection device (OCPD) ratings for a service with interconnected power sources (like PV) to a maximum of 120% of the service busbar's continuous current rating, ensuring the busbar is not overloaded.
- Applies to supply-side and load-side connections.
- Prevents busbar overload from simultaneous utility and PV power.
- Formula: (1.20 * Busbar Rating) >= (Main OCPD + PV OCPD).
Memory trick: One-twenty percent of power protects the panel from peril.