A solar contractor is planning the installation of a grid-tied PV system on a multi-family dwelling. The system will utilize a single inverter connected to a dedicated breaker in the main service panel. According to NEC Article 705.12(B)(2)(3), what is the maximum permissible combined total of all sources of supply (utility and PV) that can be connected to the load-side of a service disconnecting means, expressed as a percentage of the rating of the service disconnecting means?
- A70%
- B100%
- C80%
- D120%
Show answer & explanationAnswer & explanation
Correct answer: D. 120%
NEC Article 705.12(B)(2)(3) (formerly 705.12(D)(2)(3) in older codes) specifies that the sum of the ampere ratings of all overcurrent devices supplying power to a busbar or conductor must not exceed 120% of the rating of the busbar or conductor. This '120% rule' allows for PV system interconnection without necessarily upgrading the main service panel, provided the PV contribution doesn't push the combined rating beyond this limit.
Why the other options are wrong
- A. This percentage is too low and not specified by the NEC for this calculation.
- B. This percentage is too low; 100% would mean no PV contribution if the service is fully utilized.
- C. This percentage is too low; 80% is often a derating factor, not the maximum combined source limit.
NEC 120% Rule (PV Interconnection)
A National Electrical Code rule (705.12(B)(2)(3)) that permits the sum of the ampere ratings of all overcurrent devices supplying power to a busbar to exceed the busbar's rating, provided the sum does not exceed 120% of the busbar's rating.
- Applies to load-side PV interconnections.
- Allows for PV systems without service upgrade.
- Maximum combined sources = 120% of busbar rating.
Memory trick: One twenty percent, the busbar's extent, for PV power, heaven-sent!