Solar Panel Circuit Sizing Calculator

How to use this calculator

Select your nominal system voltage (12 V, 24 V, or 48 V), enter total array wattage at STC, and specify how modules are wired (panels in series per string and how many parallel strings). Add the one-way conductor run in feet and choose copper or aluminum. Click Calculate circuit size to see maximum circuit current, recommended AWG, minimum breaker size, and voltage drop percentage.

For accurate Isc-based sizing, use the short-circuit current from each module label. This tool estimates Isc from array wattage and voltage when nameplate data is not handy — always confirm with the manufacturer sheet before ordering wire or breakers.

Understanding your results

Max circuit current: Calculated as Isc × 1.25 per NEC 690.8(B)(1). Isc is estimated from array power and nominal voltage (conservative factor applied). This is the value used to size conductors and overcurrent devices on the DC source circuit.

Recommended wire gauge: Smallest standard conductor in NEC Table 310.16 (75°C) that meets the required ampacity for your selected material. Does not include additional derating for ambient temperature, conduit fill, or terminal limits — use our wire ampacity derating calculator for those adjustments.

Minimum breaker size: Next standard overcurrent protective device rating at or above the maximum circuit current, per NEC 240.6(A). PV OCPD must also comply with 690.9 — verify series fuse ratings and listed equipment limits.

Voltage drop: Approximate percent drop at maximum power-point current (Imp) over your entered run length. Values over 3% are highlighted in red; NEC informational guidance for branch circuits often targets 3% or less — consider upsizing one wire size for long DC homeruns.

NEC 690 solar circuit requirements

• 690.8 — Circuit sizing: Conductors and OCPD are sized for not less than 125% of the maximum current (Isc with applicable factors).
• 690.9 — Overcurrent protection: Fuses or breakers protect module strings and equipment; coordinate with listed disconnects and combiner ratings.
• 690.31 — Wiring methods: Use listed PV wire, USE-2, or raceway methods suitable for outdoor and roof exposure as required by the installation.
• 690.41 — Disconnecting means: Provide readily accessible DC disconnects for maintenance and emergency shutdown where required.
• 690.47 — Grounding: Equipment grounding conductors and grounding electrode system must meet Article 250 in addition to PV-specific rules.

Tips for solar circuit wiring

• Label all PV disconnects and combiner boxes per 690.56 — clear identification speeds troubleshooting.
• Keep homerun lengths as short as practical; DC voltage drop directly reduces charge controller performance.
• Use listed MC4 or manufacturer connectors — do not mix incompatible connector brands on the same string.
• Route conductors in sunlight-rated raceway or cable listed for exposure; avoid NM cable for outdoor PV runs.
• When paralleling strings, ensure fuse or breaker ratings match each string’s Isc contribution.

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