Solar design tools
Busbar 120% rule calculator
Will the backfeed fit on this panel? The bar beside the panel shows how much of the busbar's allowance the main and the backfeed breaker use. If it doesn't fit, you get every way to make it work — and you can drag the backfeed breaker to see why its position matters.
The panel
- Backfeed breaker you need
- Largest backfeed this panel takes
Show the math with your numbers
Section numbers follow the edition you pick — confirm the edition your jurisdiction adopted and any local amendments. Derating the main needs a load calculation that supports the smaller breaker. A design aid, not a sealed engineering document.
What the 120% rule says
When solar or a battery backfeeds a panel through a breaker, the busbar is fed from two directions at once: the utility through the main breaker, and the inverters through the backfeed breaker. The 120% rule allows the busbar to carry more than its rating in that situation, as long as:
- the main breaker rating plus 125% of the backfeed — the continuous output current of every inverter feeding the panel, solar and battery — is no more than 120% of the busbar rating, and
- the backfeed breaker sits at the opposite end of the busbar from the main.
| Edition | Section |
|---|---|
| NEC 2020 and 2023 | 705.12(B)(3)(2) |
| NEC 2017 | 705.12(B)(2)(3)(b) |
| NEC 2014 and earlier | 705.12(D) |
On a 200 A bus with a 200 A main, 120% of the bus is 240 A. Subtract the 200 A main and 40 A is left; divided by 1.25, the largest inverter output the panel takes is 32 A. For a fuller walkthrough with worked examples, see the 120% rule explained.
Why the backfeed breaker goes at the opposite end
With the utility at one end and the backfeed at the other, no single section of the busbar carries both currents at once — that is what makes the extra 20% safe. Plan checkers expect to see the backfeed breaker there, and the calculator always places it there, even when the panel passes on another rule.
The other ways to comply
- Within the busbar rating. If the main plus 125% of the backfeed fits within the bus rating itself, the 120% allowance isn't needed.
- The sum rule. If every breaker in the panel except the main adds up to no more than the bus rating, including the backfeed breaker, the panel complies.
- Derate the main. A smaller main breaker frees up room on the bus — only if a load calculation supports it.
- Supply-side connection. Connecting ahead of the main takes the busbar out of the question, with utility and AHJ approval.
- Upgrade the panel. The last resort, and usually the most expensive.