Solar Wire Size Calculator

Long low-voltage cable runs can waste power even when a conductor is large enough not to overheat. This solar wire size calculator estimates copper area for a selected voltage-drop target, then rounds up to a common American Wire Gauge size.

The formula doubles one-way distance because current travels out and back. K is a conservative copper resistance constant at elevated operating temperature. Allowed voltage drop is system voltage times the entered percentage.

The selected AWG addresses calculated voltage drop only. Code ampacity, insulation temperature, ambient correction, rooftop temperature, conductor bundling, wet-location rating, and terminal limits may require a larger conductor.

PV source circuits also require current multipliers and equipment-specific maximum-current rules. Never use this planning result as a permit-ready wiring schedule.

Assumptions and edge cases

The formula assumes copper conductors, a fixed constant K of 12.9 for copper at elevated operating temperature, a unity-power-factor circuit, and one continuous run without splices. Because K is constant, the result does not track real conductor temperature, which rises with load and rooftop heat. Two edge cases matter. On a very short run the required area falls below the smallest practical gauge, which means no voltage-drop constraint rather than permission to use undersized wire. On a long low-voltage run the area grows without bound, and a result past the 2/0 end of the table is a signal to change the design.

Worked example

At 20 A, 30 ft one way, 120 V, and 3% drop, the formula needs about 4,300 circular mils. Voltage drop points to 12 AWG after rounding up, though ampacity rules must also pass.

Second worked example

A second case shows that voltage, not distance, dominates. Move 40 A of charge current 80 feet one way at 24 V with a 2 percent target. Allowed drop is 24 × 0.02 = 0.48 V, so the requirement is 2 × 12.9 × 40 × 80 ÷ 0.48, about 172,000 circular mils — beyond the 2/0 end of the table. Carry the same power at 48 V and current halves to 20 A while allowed drop doubles to 0.96 V, cutting the requirement to roughly 43,000 circular mils, or 2 AWG. Doubling system voltage cut the copper about fourfold.

How to read the result

If the result is unexpectedly large, increasing system voltage or shortening the run cuts required copper. Have a qualified designer apply the adopted electrical code.

Read the gauge as a floor, not a specification. Within one gauge of the conductor already planned for ampacity, the design is comfortable. If voltage drop demands two or more gauges above the ampacity requirement, raising voltage, relocating the combiner or inverter, or splitting into parallel circuits is normally cheaper than copper. Losses scale with the drop allowed: 3 percent of a 5 kW array is roughly 150 W dissipated as heat at full output.

Planning inputs are deliberately editable because two homes in the same state can have different tariffs, roof conditions, equipment, and operating schedules. Save a scenario with “Copy link,” then change one assumption at a time. This makes the result useful for comparing decisions without implying false precision.

Formulas and defaults are documented in our data method. The principal references for this page are:

1. NREL — PVWatts Calculator documentation (2025) — Production modeling, array orientation, and system-loss context.
2. EIA — Electric Power Monthly, Table 5.6.A (2024 annual) — Residential electricity price benchmarks by state.
3. U.S. Department of Energy — Homeowner’s Guide to the Federal Tax Credit for Solar Photovoltaics (accessed methodology, 2025) — Background only; 2026 eligibility is never assumed.
4. NFPA 70, National Electrical Code (2023 edition) — Article 690 plus the 210.19 and 215.2 voltage-drop informational notes — Ampacity, PV current multipliers, and the advisory 3 percent drop figure.
5. NREL — PVWatts Version 8 documentation (2025) — Default system-loss budget and its wiring-loss component.

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