Solar 120% Rule Calculator
Enter the panel busbar, main breaker, inverter output current, and proposed PV breaker total. The calculator checks the opposite-end 120% busbar rule and shows the breaker, current, inverter-capacity, inverter-count, and main-breaker limits implied by those numbers.
120% rule resultCompliant by this opposite-end 120% busbar calculation.
- Maximum allowable PV breaker total
- 40
- Required PV breaker total
- 40
- Breaker capacity remaining
- 0
- Maximum continuous PV output current
- 32
- Maximum theoretical inverter AC capacity
- 7.68
- Maximum count of selected inverter
- 1
- Largest compliant main breaker
- 200
Current and breaker values are in amperes; inverter capacity is in kilowatts.
This checks the NEC 705.12 opposite-end load-side busbar arithmetic only.
How to use this calculator
- Enter the busbar rating from the panel label and the main breaker or other utility-source overcurrent rating.
- Enter AC voltage, phase, inverter count, and each inverter or branch circuit maximum continuous output current.
- Choose whether the PV output lands on one combined breaker or on separate breakers.
- Enter the proposed total PV breaker rating, then read the pass or fail result and the reverse-sizing limits.
How the 120% busbar check is calculated
This calculator applies the load-side busbar arithmetic used for the NEC 705.12 opposite-end method. It is for a PV breaker or group of PV breakers located at the end of the busbar opposite the utility-source breaker. It does not decide whether that method is allowed for your exact panel or installation.
The maximum PV breaker total is:
H = 1.20 x B - M
- B is the panel busbar rating in amperes.
- M is the main breaker or other utility-source overcurrent rating.
- H is the largest PV breaker total permitted by the 120% busbar arithmetic.
The inverter current also has to be protected at not less than 125% of its maximum continuous AC output current. For one combined PV breaker, the calculator uses:
P = ceilStd(1.25 x N x i)
For separate breakers, each inverter or branch circuit is rounded up first and then added:
P = N x ceilStd(1.25 x i)
N is the number of identical inverters or branch circuits, i is the continuous output current for one, and ceilStd means the next standard ampere rating from the NEC 240.6(A) breaker table. The proposed PV breaker total passes this arithmetic only when Q >= P and M + Q <= 1.20 x B.
What changes the result most
The busbar rating and main breaker set the headroom. A 200 A busbar with a 200 A main has 1.20 x 200 - 200 = 40 A of PV breaker headroom. A 225 A busbar with the same 200 A main has 70 A of headroom, which is why panels with higher busbar ratings can accept more load-side PV backfeed.
The breaker arrangement can also matter. If several inverters land on separate breakers, each breaker is rounded up to a standard size before the ratings are added. That can produce a larger required total than one combined breaker sized after adding the inverter currents.
Reverse sizing limits
The calculator also solves backward from the busbar headroom. It divides the PV breaker headroom by 1.25 to estimate the maximum continuous PV output current. It then converts that current to a theoretical AC capacity using V x I / 1000 for single phase or sqrt(3) x V x I / 1000 for three phase. This is a current-to-power conversion, not a substitute for the inverter nameplate maximum continuous output current.
The maximum inverter count uses the selected inverter current and breaker arrangement. The largest compliant main breaker is rounded down to a standard breaker size after subtracting the proposed PV breaker total from 120% of the busbar rating.
What this calculator leaves out
This is not a full interconnection design. It does not evaluate center-fed panels, main-lug-only panels, subpanels, taps, supply-side connections, panelboard listing restrictions, conductor ampacity, terminal temperature ratings, service-load calculations, fault current, interrupting ratings, utility rules, or local amendments. Those items can be decisive even when the arithmetic shown here passes.
Worked example
Suppose a panel has a 200 A busbar, a 200 A main breaker, one single-phase inverter with 32 A of maximum continuous AC output current, and a proposed 40 A PV breaker at 240 V. The busbar headroom is 1.20 x 200 - 200 = 40 A. The required PV breaker is ceilStd(1.25 x 1 x 32) = 40 A, so the proposed breaker is compliant by this opposite-end calculation.
The remaining breaker capacity is 0 A. The maximum continuous PV current allowed by the busbar arithmetic is 40 / 1.25 = 32 A, which is 240 x 32 / 1000 = 7.68 kW of theoretical single-phase AC capacity. The selected 32 A inverter count limit is 1, and the largest compliant main breaker with a 40 A PV breaker remains 200 A.
Common questions
What is the solar 120% rule?
For the opposite-end load-side busbar method, the sum of the utility-source breaker and the PV breaker ratings is limited to 120% of the panel busbar rating. In formula form, the PV breaker allowance is 1.20 times the busbar rating minus the main breaker rating. The PV breaker also has to be large enough for 125% of the inverter continuous output current.
Why is a 200 A panel with a 200 A main often limited to 40 A of solar breakers?
The arithmetic is 1.20 x 200 A - 200 A, which leaves 40 A of PV breaker headroom. A 40 A PV breaker typically corresponds to 32 A of maximum continuous inverter output current after the 125% sizing factor. Other code and equipment rules still have to be satisfied.
Does a 225 A busbar with a 200 A main allow more solar?
Yes, by this busbar calculation it allows 1.20 x 225 A - 200 A, or 70 A of PV breaker headroom. That does not automatically mean every 225 A panel can accept every 70 A solar design. The panel listing, breaker locations, conductor sizing, and local approval still matter.
Should I use inverter watts or maximum continuous output current?
Use the inverter maximum continuous AC output current from the datasheet or listing. Watts can be useful for a rough sense of system size, but breaker sizing is based on current. This calculator converts allowable current back to theoretical kilowatts only as a secondary estimate.
How are multiple inverter or microinverter breakers added?
If the inverters are combined before one PV breaker, add their continuous currents, multiply by 125%, and round up once to a standard breaker size. If each inverter or branch circuit has a separate breaker on the panel busbar, round each breaker up first and then add the breaker ratings. Separate rounding can require more total breaker capacity.
Does the 120% rule apply to supply-side connections or main-lug panels?
Not in the same way. This calculator covers the opposite-end load-side busbar method only. Supply-side connections, taps, main-lug-only equipment, center-fed panels, and subpanels need their own code analysis and may follow different limits.