Solar 120% Rule — NEC 705.12 Busbar Check

NEC 705.12(B)(3)(2) lets the main overcurrent device and the backfed PV breaker together total up to 120% of the busbar rating, on one condition: the PV breaker must be at the opposite end of the busbar from the main supply. A 200 A busbar with a 200 A main therefore allows 240 A of total supply, leaving exactly 40 A for solar. That 40 A breaker backs a 32 A inverter — roughly 7.6 kW at 240 V.

Because of where the current goes, not how much of it there is. With the main at one end and the PV at the other, power flows inward from both ends toward the loads in between. No single section of busbar ever carries the arithmetic sum of the two devices — the worst-case section carries the larger of them. The opposite-end placement is not a formality; it is the entire engineering basis of the allowance, which is why 705.12(B)(3)(2) makes it a condition and why the panel needs a permanent label saying so.

With a 200 A main on a 200 A busbar, 32 A of continuous inverter output — about 7.6 kW at 240 V. The arithmetic: 200 × 120% = 240 A allowance, minus the 200 A main leaves 40 A, and a 40 A breaker backs 40 ÷ 1.25 = 32 A of inverter. Going one step up to a 40 A inverter needs a 50 A breaker and lands at 250 A, which fails. Those two amps are the whole difference.

Three routes, roughly in order of cost. Derate the main breaker — dropping a 200 A main to 175 A on a 200 A busbar opens up a 60 A PV breaker, and most houses never come close to 175 A of actual demand. Use a supply-side (line-side) tap under 705.11, which connects ahead of the service disconnect and bypasses the busbar rule entirely. Or fit a smaller inverter. A main derate is usually cheapest, and NEC 220.87 measured demand is how you prove it is safe.

Both, in order. The rule compares breaker ratings, so the number that goes into the sum is the PV breaker. But you size that breaker from the inverter's continuous output current at 125% per 690.8(A) and 705.28 — a 32 A inverter needs 32 × 1.25 = 40 A. Using the inverter's nameplate kW without the 125% factor is the most common way this calculation goes wrong.

Yes. NEC 2020 rewrote Article 705 to cover all power production sources, and energy storage systems interconnected on the load side face the same busbar arithmetic. A battery inverter that can export is a source. Note that a system able to both charge and discharge may need consideration in both directions, and that 705.13 power control systems are an increasingly common way around a busbar that will not otherwise take the interconnection.

No — and treating it as the only one is why some interconnections get refused unnecessarily. 705.12(B)(3) contains several paths, including the sum rule (all breakers excluding the main not exceeding the busbar rating), a centre-fed panelboard allowance, and a path under engineering supervision. This calculator models the 120% option only, because it is the one nearly every residential job uses. If it fails here, the others are worth reading before reaching for a service upgrade.

At the opposite end of the busbar from the input feeder conductor or main breaker. In a typical residential load centre with the main at the top, that means the very bottom pair of spaces. Not near the bottom — the last position. If a full-size breaker already occupies it, that breaker moves. Inspectors check this specifically, and the required warning label spells the rule out for whoever opens the panel next.