The 120% rule for solar and battery breakers

What the code requires

Section 705.12 covers connecting solar, batteries or other power sources on the load side of the service disconnect, usually through a breaker in a panel. The math uses the source's maximum circuit current under 705.28(A). On a feeder, the feeder's ampacity must be at least 125% of the source's output current, with added protection rules when the connection is not at the end opposite the primary overcurrent device.

For panel busbars, the 2023 NEC gives six options. Under 705.12(B)(1), 125% of the source current plus the rating of the breaker protecting the busbar may not exceed 100% of the busbar rating. Under (B)(2), the 120% rule, that sum may reach 120% of the busbar rating, but only when the source breaker and the main are at opposite ends of a busbar that serves loads. The busbar must also be sized for its connected loads under Article 220, and a warning label telling people not to move the source breaker is required.

Under (B)(3), the sum rule, all load and supply breakers on the busbar except the main may add up to no more than the busbar ampacity, the main may not exceed the busbar rating, and a label warning of multiple sources is required. (B)(4) covers center-fed dwelling panels, where a connection at either end is allowed at 120%. (B)(5) covers feed-through conductors, and (B)(6) allows any other setup only under engineering supervision with fault-current and busbar-load calculations.

On the job

• On a 120% rule installation, the solar or battery breaker must be at the opposite end of the busbar from the main, and the do-not-relocate label must be in place.
• Where an energy management system controls the source, its current setpoint is the source current in these calculations under 705.28(A)(2) (see 705.13).

Exceptions that matter

• These busbar options are written for equipment without a specific listing for combining power sources.

What changed in 2023

The 2023 NEC restructured and renumbered the busbar options, so the 120% rule is now 705.12(B)(2) instead of 705.12(B)(3)(2), limited them to equipment without a specific listing for combining sources, replaced the power control system setting with the energy management system setpoint in 705.28(A), and moved the marking, backfeed and fastening rules to 705.30(C) through (E).

In California

The 2025 Energy Code, which works alongside this rule, requires new single-family homes with utility service over 125 A to have a main panel busbar of at least 225 A unless a battery system is installed (see 706.10).

Related sections

• NEC 705.11: Supply-side (line-side tap) solar and battery connections
• NEC 705.13: Energy management systems (formerly PCS) for solar
• NEC 705.28
• NEC 705.30
• NEC 706.10: California battery-ready rules for new homes
• NEC 408.36: Overcurrent protection for panelboards

Where this comes from

• NFPA: 2026 NEC (A2025) CMP-4 First Draft public input report, reprinting 2023 text
• Mike Holt (EC&M): NEC Requirements for Interconnected Power Sources, Sep 16 2024
• Electrical Contractor: Concluding the 2023 Code Review (M. Earley), Sep 11 2023
• California Energy Commission: 2025 Building Energy Efficiency Standards (CEC-400-2025-010-F), 150.0(s)
• Read the official NEC free at NFPA