Solar 120% Rule Calculator

The 120% rule, step by step

A panelboard busbar is normally protected by a single main overcurrent device sized to the busbar rating. Backfeeding a solar or storage inverter into the same busbar adds a second supply, so the bus can now carry current from both ends. NEC 705.12(B)(3) keeps the busbar from being overloaded. The busbar loading term is 125 percent of the source output circuit current, because a PV or battery output circuit is a continuous load. Add the main overcurrent device rating and compare the total against the busbar.

Option (1) is the plain sum: 125 percent of the source current plus the main must fit within the busbar rating, and the backfed device can go anywhere. Option (2), the 120 percent rule, recognizes that when the utility feeds one end of the bus and the inverter feeds the opposite end, with the loads tapped in between, no single section of the bus ever carries the full combined current. That geometry lets the sum reach 120 percent of the busbar rating, provided the backfed device sits at the far end and the panel is marked so it stays there.

Worked example: 200 A busbar, 200 A main

1. The busbar is rated 200 A and the main breaker protecting it is 200 A.
2. Option (1): the main alone already equals the busbar, so there is no room for any source at 100 percent.
3. Option (2): 120 percent of 200 A is 240 A. Subtract the 200 A main and 40 A of busbar headroom remains.
4. The source term is 125 percent of the inverter output current, so the largest inverter output is 40 divided by 1.25, which is 32 A continuous.
5. The largest standard backfed overcurrent device at or below the 40 A headroom is a 40 A breaker, sized for that 32 A continuous output.
6. The backfed breaker must be at the opposite end of the busbar from the main, and the panel must be marked so it is not relocated.

Source current versus breaker rating

The 2020 NEC busbar term is 125 percent of the power source output circuit current, a current, not the backfed breaker rating. Earlier editions and many quick rules of thumb used the breaker rating, which is slightly larger because it is rounded up to a standard 240.6(A) size. This tool holds to the 2020 language: the pass and fail results and the maximum source output current come from 125 percent of the source current. It also reports the largest standard backfed overcurrent device on the conservative breaker basis, since the installed device is itself at least 125 percent of the source current, so a breaker within the busbar headroom always clears the code check. Both figures are correct; the breaker number is the never-over-permit answer.

Other 705.12(B)(3) options

Beyond the sum method and the 120 percent rule, 705.12(B)(3)(3) permits the sum of the ampere ratings of all overcurrent devices on the panel, both load and supply, excluding the busbar device, to equal the busbar rating; it is independent of the main rating but needs the full breaker schedule, so this tool documents it rather than computing it. Option 705.12(B)(3)(4) applies the same 120 percent arithmetic to a center-fed panelboard in a dwelling with the connection at one end only, and 705.12(B)(3)(5) permits multiple-ampacity busbars designed under engineering supervision with fault-current and busbar-load calculations. When no busbar method works, a supply-side connection under 705.11 has no 120 percent limit and is often the answer for a large system on a fully loaded service.

Frequently asked questions

What is the 120% rule for solar?

NEC 705.12(B)(3)(2): with the utility and the inverter at opposite ends of a busbar and loads between them, 125 percent of the source output current plus the main overcurrent device rating may reach 120 percent of the busbar rating instead of 100 percent. A 200 A busbar with a 200 A main leaves room for a 40 A backfed breaker.

How do I calculate the maximum backfeed breaker?

Take 1.2 times the busbar rating, subtract the main rating, and pick the largest standard 240.6(A) device at or below that headroom. For 200 A and 200 A that is 240 minus 200, so a 40 A backfed breaker.

Where must the backfeed breaker go?

For the 120 percent method it must be at the opposite end of the busbar from the main, with the panel marked so it is not relocated. If it can sit anywhere, the full sum must fit within 100 percent of the busbar under method (1).

Inverter current or breaker rating?

The 2020 code uses 125 percent of the source output circuit current, not the breaker rating. This tool computes pass and fail from the code exact current and reports the standard breaker separately.

What if my system fails?

Reduce the main rating, reduce the inverter output, use a higher busbar rating, use the sum-of-all-devices method, or move to a supply-side connection under 705.11, which has no 120 percent limit.

Related calculators

• Solar PV Wire Size Calculator: PV source and output circuit conductors and fuses from the module Isc per NEC 690.8 and 690.9.
• Solar Voltage Drop Calculator: PV string and array voltage drop against the 2 and 3 percent solar targets.
• Breaker Size Calculator: branch-circuit overcurrent device sizing per 210.20(A) and 240.6(A).
• Service Size Calculator: service-entrance conductor, disconnect, and grounding-electrode-conductor sizing from a known load.