Solar 120% rule calculator
Enter the panel's busbar rating and main breaker, then the inverter output current or the solar breaker size. The calculator checks the 120% rule for a load-side connection as you type. Nothing you enter leaves this page.
What the 120% rule says
When solar is connected on the load side, through a backfed breaker in the main panel, the panel's busbar can be fed from two directions: the utility through the main breaker and the inverter through the solar breaker. The 120% rule keeps the total within what the busbar can safely carry:
125% × inverter output current + main breaker ≤ 120% × busbar rating
The solar breaker goes at the opposite end of the busbar from the main breaker.
The calculator uses 125% of the inverter's maximum continuous output current, as the code does. If you only know the solar breaker size, it uses the breaker rating instead, which gives the same or a stricter answer, because that breaker is sized at 125% of the output or more.
An example
A common panel has a 200 A busbar and a 200 A main breaker. 120% of 200 A is 240 A; minus the 200 A main leaves 40 A. Divided by 1.25, that allows an inverter output of up to 32 A, about 7.7 kW at 240 V. The same panel with a 225 A busbar and a 200 A main leaves 70 A, enough for 56 A of inverter output.
When it doesn't fit
- A smaller main breaker. Replacing a 200 A main with a 175 A one frees 25 A, but only if a load calculation shows the home can run on the smaller main.
- A supply-side connection. Connecting between the meter and the main breaker avoids the busbar rule; it has its own requirements.
- A panel with a higher busbar rating, or a smaller system.
Which option is allowed depends on the code edition your jurisdiction uses and on your inspector. Center-fed panels and panels with several sources have their own rules.
Get the numbers right on the survey
The busbar rating and the main breaker size are both on the panel: the rating on the label inside the door, the main on the breaker handle. Photograph the label and the main breaker during the site survey so the designer doesn't have to guess.
Doing it in FieldPix
The built-in Solar Site Survey: Detailed in FieldPix asks for the busbar rating, the main breaker size, the service size and the open breaker spaces, each with a photo of the label, along with the attic, the rafters and horizon photos for shading. See the site survey checklist or FieldPix for solar installers.
Based on the 120% rule for load-side connections in NEC 705.12(B)(3)(2) (2020 and 2023 editions; 705.12(B)(2)(3)(b) in 2017). This is a quick check, not engineering or code advice; your design, your local code edition and your inspector have the final say.
