NEC 705.30 overcurrent protection for interconnections

Overcurrent protection at the interconnection is where the PV system hands power to the building's electrical system, and NEC 705.30 is the section that sizes it. It answers a deceptively simple question: what breaker or fuse protects the conductors between the inverter and the point of connection? Get the number right and the rest of the interconnection design falls into place.

The 125% continuous-output basis

Inverter output is treated as a continuous load, so the overcurrent device must be rated at not less than 125% of the inverter's continuous output current. This is the same multiplier used for the PV output circuit in 690.8, and the two must agree — the interconnection OCPD and the output-circuit OCPD are usually the same device.

The workflow:

1. Start with the inverter's rated continuous AC output current (from the nameplate, not the peak). 2. Multiply by 1.25. 3. Round up to the next standard OCPD size in 240.6. 4. Confirm the conductor ampacity meets or exceeds that OCPD after any temperature/fill derating.

How it interacts with the busbar rule

705.30 tells you the breaker size; the 120% busbar rule in 705.12 tells you whether that breaker can land on a given panel. They are two separate checks that both have to pass:

• 705.30: the breaker must be at least 125% of inverter output.
• 705.12: on a load-side connection, main breaker + PV breaker must not exceed 120% of the busbar rating.

A 50 A PV breaker is legal by 705.30, but if it won't fit under the 120% math on a 200 A panel with a 200 A main, you either derate the main, use a line-side supply-side connection, or reduce the system. This tension between the two rules is one of the most common residential design puzzles.

Another placement detail from 705.12 carries over here: a backfed breaker used for the interconnection must be positioned at the opposite end of the busbar from the main when the 120% allowance is used, and it does not need the hold-down clip that some assume. Keeping the OCPD sizing (705.30) separate in your head from the placement and busbar limits (705.12) makes the whole interconnection easier to get right on the first submittal.

Conductor coordination

Sizing the OCPD is only half the job — the conductors it protects must carry at least the same current after derating. Rooftop and conduit-fill temperature corrections can pull the required conductor size up, so always verify against the 310.16 ampacity tables rather than assuming the raw ampere value. A 50 A breaker on conductors that only carry 45 A after derating is a rejection.

What a reviewer confirms

The reviewer reads the inverter nameplate current off your equipment schedule, checks that the OCPD is at least 125% of it, confirms the conductor ampacity supports the breaker, and then verifies the busbar rule at the point of connection. Any mismatch between the one-line, the equipment schedule, and the calculation is an easy correction to write.

OneLine Studio is a design aid, not a permitting authority. Confirm the enforced NEC edition and any utility interconnection amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the design before you file or energize. For a broader look at the sizing chain, start with the NEC reference hub.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.