What the rule says
The 120% rule lets you connect solar to a panel’s busbar with a backfed breaker even though the main breaker and the solar breaker together add up to more than the busbar is rated for, as long as the total stays within 120% of the busbar rating and the two sources sit at opposite ends of the bus.
In the 2020 and 2023 NEC it is 705.12(B)(3)(2):
Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar.
The same rule sat at 705.12(B)(2)(3)(b) in the 2017 NEC and at 705.12(D)(2)(3)(b) in the 2014 NEC, so check which edition your jurisdiction has adopted before you cite it on the plan set.
The math
Two details catch people out:
- It is 125% of the inverter’s output current, not the solar breaker size. A 7.6 kW string inverter rated 32 A continuous counts as 40 A. Some jurisdictions ask you to use the breaker rating instead, which is more conservative; follow what your AHJ accepts.
- Every source that backfeeds that bus counts. A battery inverter or a second PV inverter on the same busbar adds its output current to the sum.
Worked example
A 200 A panel with a 200 A main: 1.20 × 200 = 240 A. Subtract the 200 A main and there is 40 A left for 125% of the inverter current, so the inverter can supply up to 40 ÷ 1.25 = 32 A continuous. At 240 V that is 7.68 kW AC, which is why so many 200/200 panels top out around a 7.6 kW inverter.
Quick reference: common panels
| Busbar | Main breaker | 120% of busbar | Room for 125% of inverter current | Max inverter output current | Max AC at 240 V |
|---|---|---|---|---|---|
| 100 A | 100 A | 120 A | 20 A | 16 A | 3.84 kW |
| 125 A | 100 A | 150 A | 50 A | 40 A | 9.60 kW |
| 150 A | 150 A | 180 A | 30 A | 24 A | 5.76 kW |
| 200 A | 200 A | 240 A | 40 A | 32 A | 7.68 kW |
| 200 A | 175 A | 240 A | 65 A | 52 A | 12.48 kW |
| 200 A | 150 A | 240 A | 90 A | 72 A | 17.28 kW |
| 225 A | 200 A | 270 A | 70 A | 56 A | 13.44 kW |
Maximum AC is the inverter’s continuous output current × 240 V. The breaker you install must also be a standard size at or above 125% of that current.
Conditions that come with it
- Opposite ends. The solar breaker has to be at the far end of the busbar from the main breaker or the main feed, so current from the two sources flows in from each end rather than stacking in one section of the bus.
- A permanent warning label next to the backfed breaker: “WARNING: POWER SOURCE OUTPUT CONNECTION — DO NOT RELOCATE THIS OVERCURRENT DEVICE.”
- Backfed breakers must be suitable for backfeed. A plug-in breaker fed by a listed interactive inverter doesn’t need the extra hold-down fastener that 408.36(D) otherwise requires; check the fastening provision in 705.12 for your edition.
- Center-fed panels have their own provision; the opposite-end logic does not map onto them directly, so check the section for the edition you are designing to.
When the 120% rule fails
If the numbers don’t fit, the usual options, roughly from cheapest to most involved:
- Downsize the inverter to fit the room the bus has, if the customer’s production target allows it.
- Derate the main breaker. Dropping a 200 A main to 175 A frees 25 A of room (65 A instead of 40 A), enough for a 52 A inverter. It only works if the NEC 220 load calculation shows the house fits under the smaller main.
- Use another busbar option in 705.12(B)(3), such as the 100% rule in (B)(3)(1) or the sum-of-breakers method in (B)(3)(3), where the panel qualifies.
- Limit the current with a power control system under 705.13 (2020 NEC and later), so a listed controller keeps the bus within its rating.
- Make a supply-side connection under 705.11: tap between the meter and the main, with its own fused disconnect. The busbar is no longer part of the math.
- Upgrade the panel or busbar, for example to a 225 A bus with a 200 A main (70 A of room).
How Ballast Solar checks it
Ballast Solar runs this check on every set under the NEC edition your jurisdiction has adopted, and when a panel fails it shows which fix the numbers support.
For jurisdictions that issue permits through SolarAPP+, see how SolarAPP+ works; for timelines by city, see solar permit requirements by jurisdiction.
Questions
Does the 120% rule use the breaker size or the inverter current?
The code uses 125% of the inverter’s output circuit current. Some jurisdictions accept or require the solar breaker rating instead, which gives the same or a more conservative answer.
What is the 120% rule for a 200 amp panel with a 200 amp main?
You have 40 A of room: 1.2 × 200 A minus the 200 A main. That allows an inverter of up to 32 A continuous output, about 7.68 kW at 240 V.
Does a battery count toward the 120% rule?
Yes, when its inverter can backfeed the same busbar. Add 125% of each source’s output current before comparing against 120% of the bus.
What NEC section is the 120% rule?
705.12(B)(3)(2) in the 2020 and 2023 NEC, 705.12(B)(2)(3)(b) in the 2017 NEC and 705.12(D)(2)(3)(b) in the 2014 NEC.
Can I derate the main breaker to meet the 120% rule?
Often, yes. A smaller main frees room on the bus, but the dwelling’s NEC 220 load calculation has to show the house fits under the smaller main breaker.
This guide explains the code for planning purposes. The adopted edition, local amendments and the AHJ’s interpretation govern; the contractor of record is responsible for the design.