Size AC breakers, disconnects, and conductors for solar inverter output. NEC-compliant calculations for residential and commercial solar interconnections.
Properly sizing the AC side of a solar installation is critical for code compliance and safety. This calculator handles the NEC requirements so you can quickly determine the right breaker, wire gauge, and busbar compliance for any residential or commercial solar interconnection.
Applies the 125% continuous load multiplier to inverter maximum output current for proper overcurrent protection device (OCPD) sizing.
Verifies the solar breaker meets NEC 705.12(B)(3) — the 120% busbar rule that limits total source ratings on a panel’s busbar.
Recommends minimum wire gauge for the AC output circuit based on OCPD rating and NEC ampacity tables.
Every grid-tied solar installation requires proper AC-side sizing for code compliance and safety. Use this calculator when:
Size the AC breaker, disconnect, and conductors for the inverter output circuit. Every solar permit requires these calculations documented on the plan set.
Before committing to a panel location, verify the main panel can accept the solar backfeed breaker under NEC 705.12(B)(3). This calculator checks instantly.
If the 120% rule fails, determine whether a main breaker derate, line-side tap, or full panel upgrade is the best path forward for the installation.
Enter your inverter’s maximum AC output current (found on the inverter nameplate or datasheet).
Select the AC voltage — 240V single-phase for residential, 208V or 480V three-phase for commercial.
Enter the main panel busbar rating (typically 100A, 150A, or 200A for residential).
Enter the existing main breaker size to check the 120% busbar rule.
Review the recommended breaker size, wire gauge, and busbar compliance status.
The minimum breaker or fuse size for the solar AC circuit — always ≥125% of maximum inverter output current per NEC 690.8(A)(1).
The next standard breaker size up from the calculated minimum (15A, 20A, 25A, 30A, 40A, etc.).
Total of all breaker ratings (main + solar backfeed) must not exceed 120% of the busbar rating. If it fails, you may need a main breaker derate, line-side tap, or panel upgrade.
The AWG conductor size required based on the OCPD rating and NEC 310.16 ampacity tables.
The maximum solar AC power that can be connected to this panel under NEC 705.12(B)(3).
This calculator follows National Electrical Code (NEC) requirements for sizing solar AC interconnection components. All calculations are based on the most current NEC standards.
Worked example: A SolarEdge SE7600H inverter outputs 31.7A maximum at 240V. NEC 690.8(A)(1): 31.7A × 1.25 = 39.6A minimum OCPD → standard 40A breaker. With a 200A busbar and 200A main breaker: (200 × 1.20) − 200 = 40A maximum solar breaker. This installation passes. Minimum conductor: #8 AWG copper per NEC 310.16.
Calculations sourced from SurgePV’s AC Size Calculator — surgepv.com/tools/ac-size-calculator/
Common residential inverter sizes with corresponding NEC-compliant breaker and wire requirements at 240V single-phase.
| Inverter Size | Max AC Current | 125% (NEC) | Breaker Size | Min Wire (Cu) |
|---|---|---|---|---|
| 3.8 kW | 15.8A | 19.8A | 20A | #12 AWG |
| 5.0 kW | 20.8A | 26.0A | 30A | #10 AWG |
| 6.0 kW | 25.0A | 31.3A | 35A | #8 AWG |
| 7.6 kW | 31.7A | 39.6A | 40A | #8 AWG |
| 10.0 kW | 41.7A | 52.1A | 60A | #6 AWG |
| 11.4 kW | 47.5A | 59.4A | 60A | #6 AWG |
| 15.0 kW | 62.5A | 78.1A | 80A | #4 AWG |
| 20.0 kW | 83.3A | 104.2A | 110A | #2 AWG |
| Busbar Rating | Main Breaker | 120% Limit | Max Solar Breaker | Max Solar System |
|---|---|---|---|---|
| 100A | 100A | 120A | 20A | ~3.8 kW |
| 125A | 125A | 150A | 25A | ~5.0 kW |
| 150A | 150A | 180A | 30A | ~6.0 kW |
| 200A | 200A | 240A | 40A | ~7.6 kW |
| 200A | 175A (derated) | 240A | 65A | ~12.5 kW |
| 225A | 200A | 270A | 70A | ~13.4 kW |
Solar inverters are continuous loads per NEC. The OCPD must be rated at 125% of maximum output current. A 31.7A inverter output needs a 40A breaker, not a 35A.
Discovering the 120% rule fails after contract signing creates costly change orders. Always verify the main panel’s busbar rating and main breaker size during the initial assessment.
If the 120% rule fails, replacing a 200A main breaker with a 175A or 150A breaker is often the cheapest solution. This frees up 25–50A of solar backfeed capacity without a panel upgrade.
A line-side (supply-side) connection per NEC 705.12(A) bypasses the 120% rule entirely. It connects before the main breaker, allowing any size solar system. Requires utility approval in most jurisdictions.
NEC 705.12(B)(3) states that the sum of the main breaker and all solar backfeed breakers must not exceed 120% of the panelboard busbar rating. For a 200A busbar with a 200A main breaker, the maximum solar breaker is (200 × 1.20) - 200 = 40A.
A 7.6kW inverter at 240V outputs 31.7A maximum. Multiply by 1.25 (NEC continuous duty) = 39.6A. The next standard breaker size is 40A. Your AC circuit needs a 40A breaker with #8 AWG copper conductors minimum.
You have several options: (1) Derate your main breaker — replace a 200A main with a 175A main to free up backfeed capacity. (2) Use a line-side tap connection before the main breaker — this bypasses the 120% rule entirely. (3) Upgrade to a larger busbar panel.
Yes, but the 120% rule still applies to the sub-panel’s busbar rating. The solar breaker must also be at the opposite end from the feeder breaker. Some jurisdictions require the solar connection at the main panel.
Wire size is based on the OCPD rating, not the actual current. A 40A solar breaker requires #8 AWG copper (NEC 310.16, 75°C column). A 30A breaker requires #10 AWG copper. Always verify with local code requirements.
NEC 690.13 requires a readily accessible disconnect means. Most jurisdictions require an AC disconnect between the inverter and the main panel. Some allow the backfeed breaker itself as the disconnect if it’s readily accessible.
Find the correct AWG gauge for solar AC and DC circuits.
Calculate voltage drop across any solar wiring run.
Verify busbar ratings for solar panel system interconnection.
Design optimal PV string configurations for any inverter.
Check NEC conduit fill compliance for solar wiring.
Calculate the right solar system size based on energy usage.
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