Key takeaways
- The 120% rule lets 125% of the inverter’s continuous output current plus the main breaker reach 120% of the busbar rating, as long as the solar breaker sits at the opposite end of the busbar from the main. In NEC 2023 it is 705.12(B)(2).
- A 200 A busbar with a 200 A main leaves 40 A, which caps the inverter at 32 A of continuous output.
- The citation depends on the code cycle: 705.12(B)(2) in NEC 2023, 705.12(B)(3)(2) in NEC 2020 and 705.12(B)(2)(3)(b) in NEC 2017. Cite the one your AHJ has adopted.
- The code math uses 125% of inverter output current, not the backfed breaker rating, and an AC-coupled battery inverter on the same busbar counts too.
- When a panel fails, the fixes are a smaller inverter, a main breaker derate backed by a load calculation, a larger busbar, a supply-side connection under 705.11 or a power control system under 705.13.
The 120% rule for solar is the load-side connection method that lets a panel busbar carry 20 percent more than its rating when the solar breaker sits at the opposite end from the main breaker. Under NEC 2023 705.12(B)(2), 125 percent of the inverter’s output current plus the main breaker rating may not exceed 120 percent of the busbar rating. Below is the math, the right citation for each code cycle, and the options when a panel fails it.
What is the 120% rule for solar?
The 120% rule is NEC 2023 705.12(B)(2). Where a primary source and another power source feed a busbar from opposite ends and the busbar serves loads, 125 percent of the power source output circuit current plus the rating of the overcurrent device protecting the busbar may not exceed 120 percent of the busbar’s ampere rating.
The extra 20 percent exists because of where the current enters. With the utility feeding one end and the inverter feeding the other, current flows toward the branch loads from both directions, so no single point on the bus carries the full sum of both sources. Move the solar breaker up next to the main and that allowance is gone; the stricter 100 percent method applies instead.
The 125 percent multiplier treats inverter output as a continuous current, the same way the code sizes other continuous loads. The backfed breaker also has to stay where it was put, so the code requires a permanent warning label next to it. The exact wording, and where reviewers expect to see it, is covered in our guide to solar labels and placards.
On Avila Solar Drafting’s residential solar plan sets, the busbar calculation is written out on the single-line diagram, because that is the sheet a plan reviewer checks it against.
How do you calculate the 120% rule?
Multiply the busbar rating by 1.2, subtract the main breaker rating, and the result is the most that 125 percent of the inverter’s output current can be. Divide that figure by 1.25 to get the largest continuous inverter output current the busbar will accept.
Take the inverter’s rated continuous output current from its datasheet, not the backfed breaker size. The table below runs the math for common residential panel sizes.
| Busbar rating | Main breaker | 120% of busbar | Room for 125% of inverter current | Max inverter continuous output |
|---|---|---|---|---|
| 100 A | 100 A | 120 A | 20 A | 16 A |
| 125 A | 100 A | 150 A | 50 A | 40 A |
| 200 A | 200 A | 240 A | 40 A | 32 A |
| 200 A | 175 A | 240 A | 65 A | 52 A |
| 225 A | 200 A | 270 A | 70 A | 56 A |
Some checklists write the formula with the backfed breaker rating in place of 125 percent of inverter current. NYSERDA’s NY-Sun fact sheet on the 120% rule does, and it reports that main panel overcurrent protection on load-side connections is one of the top critical nonconformances its inspectors record. The breaker version is the conservative one, since the breaker is always at least 125 percent of the inverter current, so it never passes a panel the code text would fail.
Why does the 705.12 citation change with every code cycle?
The wording of the 120% rule has barely changed since NEC 2017, but its number has moved twice. NEC 2020 renumbered the busbar methods to 705.12(B)(3)(1) through (4), and NEC 2023 flattened them to 705.12(B)(1) through (4). The right citation is the one in the edition your AHJ enforces.
| NEC edition | 100% method | 120% rule | Sum of breakers | Center-fed dwelling panels |
|---|---|---|---|---|
| 2017 | 705.12(B)(2)(3)(a) | 705.12(B)(2)(3)(b) | 705.12(B)(2)(3)(c) | 705.12(B)(2)(3)(d) |
| 2020 | 705.12(B)(3)(1) | 705.12(B)(3)(2) | 705.12(B)(3)(3) | 705.12(B)(3)(4) |
| 2023 | 705.12(B)(1) | 705.12(B)(2) | 705.12(B)(3) | 705.12(B)(4) |
You can see the split in live submittal documents. The City of Orange, California residential PV checklist, built on the 2022 California Electrical Code, which is based on NEC 2020, lists the 120% rule as CEC 705.12(B)(3)(2) and requires the calculation on the single-line diagram sheet. SolarAPP+’s busbar compliance guidance cites the same rule as 705.12(B)(2).
A plan set that cites the wrong edition’s number is an easy correction comment for a reviewer to write. If you are not sure which edition a jurisdiction enforces, our walkthrough on finding your AHJ’s NEC cycle shows where to look, and the NEC articles that govern plan sets sets 705 alongside 690 and 706.
Worked example: a 200 A panel and a 10 kW inverter
A 10 kW inverter fails the 120% rule on a 200 A busbar with a 200 A main breaker, by 12.5 A. Four common fixes pass it: a smaller inverter, a 175 A main breaker derate, a 225 A busbar, or moving off the busbar altogether.
The house has a 120/240 V service with a 200 A busbar and a 200 A main breaker. The customer wants a 10 kW string inverter, rated in this example at 42 A of continuous output at 240 V. The check: 1.25 × 42 A = 52.5 A, and 200 A + 52.5 A = 252.5 A, against a limit of 1.2 × 200 A = 240 A. It fails.
| Fix | Math | Result | What it costs you |
|---|---|---|---|
| Downsize to a 7.6 kW inverter rated 32 A | 200 + (1.25 × 32) = 240 A | Passes exactly | Smaller system |
| Derate the main breaker to 175 A | 175 + 52.5 = 227.5 A | Passes | A load calculation, and a 175 A breaker listed for the panel |
| Replace the panel: 225 A busbar, 200 A main | 200 + 52.5 = 252.5 A, limit 270 A | Passes | A panel replacement |
| Supply-side connection under NEC 2023 705.11 | No busbar math | Not applicable | Utility and AHJ tap requirements, and the space to make the tap |
The derate is usually the cheapest of these, provided the load calculation holds up. For an existing dwelling, NEC 2023 220.83 or 12 months of demand data under 220.87 supports it. A house with a heat pump, an induction range and a Level 2 charger often will not support a 175 A main, and finding that out at inspection is the expensive way to learn it.
A fifth option is a power control system under NEC 2023 705.13, which limits the current the system can put onto the busbar so the calculation uses the controlled value. The PCS has to be listed for the purpose, and its setting has to appear on the plans.
A supply-side tap also changes the utility side of the job. Review the interconnection application before choosing it, because some utilities restrict taps on meter-main combinations.
Does a battery count toward the 120% rule?
Yes. An AC-coupled battery inverter is another power source on the busbar, so 125 percent of its output current is added to the solar inverter’s in the same calculation. A DC-coupled hybrid inverter counts once, at its own output rating.
This is where storage retrofits trip up. A 7.6 kW PV inverter at 32 A passes a 200 A busbar with a 200 A main exactly. Add an AC-coupled battery inverter rated 24 A and the sum becomes 200 A + 1.25 × (32 A + 24 A) = 270 A, well over the 240 A limit. The same pair passes a 225 A busbar with a 200 A main at exactly 270 A.
Many storage designs pass by using a power control system under 705.13 to cap the combined current, rather than replacing the panel. Either way, the storage sheets have to show the method. Our guide to what an energy storage plan set must show covers the rest of that package.
What do plan reviewers check on a load-side connection?
Plan reviewers check that the busbar and main breaker ratings match the panel label, that the solar breaker sits at the opposite end from the main, that the 125 percent math is written out, and that every power source on the busbar is counted.
- Busbar and main breaker ratings taken from the panel label, ideally with a photo from the site survey.
- Solar breaker location at the opposite end of the busbar from the main breaker.
- The calculation on the single-line diagram, with the citation for the adopted code cycle.
- Every power source counted: PV inverters, AC-coupled batteries and any existing system.
- The required warning label next to the backfed breaker.
- For center-fed dwelling panels, a connection at one end only, never both, under NEC 2023 705.12(B)(4).
Busbar math is one of the issues behind the rejections covered in why solar permits get rejected, and it belongs on any pre-submittal checklist.
When none of the prescriptive methods fits, 705.12(B) also allows a connection designed under a licensed engineer’s supervision. That path needs an engineer’s sign-off, which is the job of our solar engineering stamps.
Frequently asked questions
What is the 120% rule for solar panels?
The 120% rule for solar is a load-side connection method in NEC 2023 705.12(B)(2). It allows 125 percent of the inverter’s output current plus the main breaker rating to reach 120 percent of the busbar rating, as long as the solar breaker sits at the opposite end of the busbar from the main breaker and carries the required warning label.
How big a solar breaker can go on a 200 amp panel?
On a panel with a 200 A busbar and a 200 A main breaker, the 120% rule leaves 40 A. Under NEC 2023 705.12(B)(2) that 40 A is 125 percent of the inverter’s continuous output current, so the inverter can be rated up to 32 A, which is roughly a 7.6 kW inverter at 240 V.
Does the 120% rule apply to subpanels?
Yes. The busbar methods in NEC 2023 705.12(B) apply to whichever panelboard the solar connects to, whether that is the main service panel or a subpanel. The sum-of-breakers method in 705.12(B)(3) is often the easier fit for a subpanel that feeds few loads, and the busbars upstream still have to be checked.
Can I derate the main breaker to meet the 120% rule?
A main breaker derate is a common way to pass the 120% rule, but only when a load calculation shows the smaller breaker still covers the building. For an existing dwelling, NEC 2023 220.83 or 12 months of demand data under 220.87 supports the calculation, and the replacement breaker must be listed for that panel.
Does the 120% rule apply to a supply-side connection?
No. A supply-side connection taps the service conductors ahead of the main breaker, so no busbar carries the combined current. Since NEC 2020, supply-side connections are covered by 705.11, which sets its own rules for the tap, the conductors and the disconnect, and the utility often has requirements of its own.
If busbar math is holding up a permit, send us the panel label photo and the inverter datasheet. Avila Solar Drafting delivers standard residential solar plan sets in 2 to 3 business days, with AHJ and utility revisions free with no time limit and client change orders free for 3 months from data submission.
Create a free account at my.avilasolar.com and upload the job.
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