Use this NEC 705.12 calculator to check whether a proposed solar interconnection satisfies the 120% busbar rule. Enter the busbar rating, the OCPD protecting the busbar, and the total maximum continuous output current of the contributing power sources to see available interconnection capacity and whether the proposed connection passes this 120% busbar method.
Who this is for: solar installers, PV designers, and electricians checking load-side interconnection capacity before permitting or utility application.
Calculator note: This calculator evaluates the common NEC 705.12 120% busbar method only. It does not determine whether an installation complies with all NEC, equipment listing, utility, or AHJ requirements. Other interconnection methods may apply. Confirm the NEC edition and local amendments adopted by the project AHJ, and verify the final source OCPD and circuit design against all applicable code provisions and equipment ratings before submitting for permitting.
How to Use the NEC 705.12 Calculator
1
Enter the busbar rating, in amps, from the panel nameplate.
2
Enter the rating of the OCPD protecting the busbar, typically the main breaker.
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Enter the combined continuous output current of all contributing power sources. Add currents together for multiple sources.
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Select your panel configuration, end-fed or center-fed dwelling panelboard, then review the pass or fail result and the available capacity.
​​What the NEC 705.12 Calculator Checks
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The calculator checks whether 125% of the total contributing power-source output current fits within the interconnection capacity remaining after the busbar's 120% allowance is reduced by the OCPD protecting the busbar. If the proposed source current fits within that remaining capacity, the connection passes this method.
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120% busbar limit = busbar rating × 1.20
Available 125% source-current allowance = 120% busbar limit − OCPD protecting the busbar
Maximum continuous source output = available 125% source-current allowance ÷ 1.25
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A proposed system passes this method when 125% of total continuous source output is less than or equal to the available 125% source-current allowance.​​
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NEC 705.12 Example Calculation
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Busbar rating: 200 A
OCPD protecting the busbar: 200 A
Total power-source continuous output current: 32 A
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120% busbar limit: 200 A × 1.20 = 240 A
Available 125% source-current allowance: 240 A − 200 A = 40 A
Maximum continuous source output: 40 A ÷ 1.25 = 32 A
Proposed 125% source-current value: 32 A × 1.25 = 40 A
Result: 40 A ≤ 40 A, so the connection passes this method
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This calculator covers the opposite-end busbar method. For the full range of NEC 705.12(B) interconnection methods, including the 100% rule and the sum-of-OCPDs method, see The 120% Rule for Solar: NEC 705.12 Guide for Installers.
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​​​How the NEC 705.12 120% Busbar Rule Works
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Under the opposite-end busbar method modeled by this calculator, NEC 705.12 permits a load-side connection when 125% of the total contributing power-source output circuit current plus the rating of the OCPD protecting the busbar does not exceed 120% of the busbar rating. The busbar rating and existing OCPD protecting it establish the starting interconnection capacity, so available capacity depends on both.
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Why does the OCPD protecting the busbar matter? A lower OCPD rating leaves more of the 120% allowance available for the source connection. Because of this, downsizing the main OCPD is sometimes proposed as a way to create additional interconnection capacity. Any change to the main OCPD must still be evaluated against the service, connected loads, equipment ratings, listing requirements, and the applicable code provisions before it can be relied on for interconnection capacity.
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This calculator applies the busbar method in NEC 2023 705.12(B)(3). Subsection numbering for this provision has changed across code cycles (705.12(B)(2)(3) in the 2017 NEC, 705.12(B)(3)(2) in the 2020 NEC). Confirm the correct subsection reference for the NEC edition adopted by your AHJ before citing it on a plan set.
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​​End-Fed vs. Center-Fed Solar Interconnection
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The center-fed dwelling provision discussed here applies specifically to center-fed panelboards in dwellings.
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For the standard opposite-end busbar method, the OCPD protecting the busbar and the additional power-source connection are located at opposite ends of the load-containing busbar, with the required warning label adjacent to the power-source OCPD.
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NEC 705.12 also includes a specific provision for center-fed panelboards in dwellings, which permits a power-source connection at either end of the busbar when the applicable 120% calculation is satisfied. This is a distinct provision from the general opposite-end method, and it applies specifically to center-fed dwelling panelboards, not to center-fed panels generally.
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Confirm which provision applies to your panel configuration before finalizing conductor and OCPD placement on the plan set.
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Where the 120% Busbar Calculation Appears on Solar Plans
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The 120% busbar calculation typically appears on the electrical portion of a solar plan set, alongside the one-line diagram, panel schedule, and source OCPD placement. During permitting, the AHJ or plan reviewer may verify the calculation against the panel nameplate rating, the busbar-protecting OCPD, the proposed source current, and the source OCPD location. Utility interconnection review may also require corresponding one-line diagram and service-equipment information.
Showing the calculation clearly, along with the busbar rating and OCPD values it depends on, helps reviewers verify the proposed interconnection and can reduce avoidable questions during permitting or utility application.
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Related NEC Calculators
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NEC 705.12 Calculator FAQs
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How do you calculate the NEC 705.12 120% busbar rule?
For the opposite-end busbar method, multiply the busbar rating by 1.20, then subtract the rating of the OCPD protecting the busbar. The result is the available 125% source-current allowance. Dividing that allowance by 1.25 gives the maximum continuous source output the connection can support under this method. This calculator performs that calculation and compares it against your proposed total source output.
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What is the 120% rule for solar interconnection?
The 120% rule is a load-side interconnection method under NEC 705.12 that allows a power source, such as a solar inverter, to connect to an existing busbar when 125% of the total contributing power-source output current, added to the rating of the OCPD protecting the busbar, does not exceed 120% of the busbar rating.
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How much solar can I connect to a 200A busbar?
For a 200 A busbar with a 200 A OCPD protecting it, using the opposite-end method, the 120% allowance is 240 A, leaving 40 A of available 125% source-current allowance. That corresponds to a maximum total continuous power-source output current of 32 A under this calculation. Actual available capacity depends on the project's busbar rating, OCPD rating, and applicable NEC method.
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What happens if a solar system doesn't pass the 120% busbar check?
If the proposed source current exceeds the available capacity under this method, possible design alternatives include evaluating another permitted NEC 705.12 busbar method, an applicable listed power control system, a service-side connection where permitted, or a main OCPD change that has been properly evaluated against the service, loads, and equipment ratings. A panel or service upgrade may also be needed. Each of these requires project-specific evaluation and is not calculated by this tool.
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What's the difference between end-fed and center-fed dwelling panelboard interconnection?
The standard opposite-end method requires the source connection at the opposite end of the busbar from the OCPD protecting it. The center-fed dwelling panelboard provision permits a source connection at either end of the busbar when the applicable 120% calculation is satisfied. Confirm which configuration applies to your panel before finalizing the connection point.
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Does this NEC 705.12 calculator guarantee code compliance?
No. This calculator evaluates the common NEC 705.12 120% busbar method only. It does not determine whether an installation complies with all applicable NEC provisions, equipment listing requirements, utility requirements, or AHJ requirements. Verify the final source OCPD and circuit design against the adopted NEC edition and all applicable requirements before submitting for permitting.
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Does a 200A electrical panel always allow a 40A solar breaker?
No. A 200 A panel does not automatically allow a 40 A solar breaker. For a 200 A busbar with a 200 A busbar-protecting OCPD under the standard opposite-end 120% method, the available 125% source-current allowance is 40 A. That corresponds to 32 A of maximum continuous power-source output under this calculation. Final source OCPD sizing must be determined separately under the applicable NEC requirements and equipment ratings.
Verify module and string counts against inverter voltage and current input limits, including cold-temperature Voc correction.
Check source-circuit conductor ampacity against the 125% continuous-current rule before it reaches the busbar.