Solar Wire Size Calculator
Estimates the minimum copper wire cross-section needed for a solar DC run based on system current, run length, and tolerable voltage drop. Use it as a starting point for code-compliant sizing before checking ampacity tables, conduit fill, and NEC 690 requirements.
Last updated: May 2026
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About this calculator
The formula computes wire size from the voltage-drop equation rewritten for area: A (mm² or equivalent) ≈ (2 × I × L × ρ) / (V_drop × V_nominal). The factor 2 accounts for both conductors (positive and negative); the constant 0.017 is the approximate resistivity of copper (ρ ≈ 0.0171 Ω·mm²/m at 20°C); the divisor 12 stands in for the nominal DC voltage. Variables: Current is the operating amperage of the circuit (use the NEC 125% continuous-load multiplier for inverter output, but for module-string DC at STC current use the Isc × 1.56 NEC factor); Distance is the one-way run length; Voltage Drop is the tolerable percentage drop across the round trip (NEC suggests 2% for branch-feeder circuits, 3-5% for service-entrance; lower is better for performance). Edge cases: the formula returns a numeric area in 'AWG-equivalent' but is not a direct AWG number — converting cross-section to AWG requires a lookup table. The 12 V assumption hides system voltage: a 24 V system has half the voltage drop per percent at the same current; a 48 V system halves again. NEC 690 requires that conductors be sized for the larger of voltage-drop or ampacity — in long, low-current runs voltage drop dominates; in short, high-current runs ampacity dominates. Always consult NEC 310.15 ampacity tables and your local AHJ — this formula provides a starting point, not a final wire size. For DC arrays with module-level rapid-shutdown devices, the 'string current' may peak well above Isc during fault conditions; consult the inverter and rapid-shutdown manufacturer for design current.
How to use
Example 1 — Short 12V battery wire. 25 A current, 20 ft (6.1 m) run, 3% drop. (2 × 25 × 20 × 0.017) / (3/100 × 12) = 17 / 0.36 = 47.2 (formula's raw output). At 12 V DC and 25 A, you'd want at least 6-8 AWG copper for the ampacity alone — voltage drop usually drives slightly larger. Cross-check NEC table 310.15 for 12 V ampacity-with-derate before finalizing. Verify ✓. Example 2 — Long 48V solar string run. 12 A current, 100 ft (30.5 m) run, 2% drop, system is 48 V (not 12 V). The formula uses 12 as the denominator constant, so the raw output is off by factor 4. Manual fix: (2 × 12 × 100 × 0.017) / (2/100 × 48) = 40.8 / 0.96 = 42.5 AWG-equivalent. This corresponds to roughly 6-10 AWG actual copper depending on the lookup. Verify ✓. Always re-run the math at your actual system voltage and confirm with NEC 690 plus a proper voltage-drop chart.
Frequently asked questions
What is the actual AWG that I should use from this calculator's output?
The calculator produces a numeric figure that approximates wire cross-section needed for the voltage-drop constraint, but does not directly return an AWG gauge — AWG is an inverse logarithmic scale and requires a lookup table. For practical translation: AWG 14 = 2.08 mm², AWG 12 = 3.31 mm², AWG 10 = 5.26 mm², AWG 8 = 8.37 mm², AWG 6 = 13.3 mm², AWG 4 = 21.2 mm², AWG 2 = 33.6 mm², AWG 1/0 = 53.5 mm². Take the formula's output (treated as mm² equivalent), round up to the next larger AWG, and then also check NEC ampacity tables — the larger of the two requirements wins. For runs over 100 ft or systems above 48 V, use a dedicated voltage-drop calculator or PV-design software like PVsyst. Manufacturer tables for specific products (Schneider, Eaton, generic copper conductor tables) are the authoritative reference once you have a target cross-section.
Why does the formula assume 12V?
The constant 12 in the denominator is a hard-coded assumption for low-voltage off-grid systems where 12 V battery banks were the historical default. Modern solar arrays often operate at 24 V, 48 V, or hundreds of volts (string inverters, microinverters, central inverters). For a higher-voltage system, the formula significantly oversizes the wire because actual voltage drop per percent is much smaller at higher voltage. To use it at non-12 V, manually adjust the denominator: at 48 V, divide by 48 instead of 12; at 400 V (typical string inverter DC bus), divide by 400. The result drops by the same factor, meaning thinner wire suffices for the same percentage voltage drop.
Does this formula meet NEC code requirements?
Not by itself. NEC Article 690 (and 705 for utility-interactive systems) requires that wire be sized for both voltage drop AND ampacity, with NEC-specific derating for conduit fill, ambient temperature, and continuous-duty operation. The voltage-drop formula handles only the first constraint. For NEC compliance: (1) compute ampacity at NEC 310.15 table for the conductor type and ambient temperature; (2) compute voltage drop with this formula; (3) take the larger required cross-section. NEC also requires temperature-corrected current (multiplying Isc by 1.25 for irradiance variation × 1.25 for continuous-duty = 1.56 total for module-string current), so always design for the elevated rating. Your AHJ (Authority Having Jurisdiction) is the final word for code interpretation.
What is a reasonable voltage-drop target?
For residential PV: 2-3% on the DC side, 2-3% on the AC side, total system 4-6%. The NEC does not mandate a maximum voltage-drop value (it 'recommends' 3% for branch circuits in informational note FPN 210.19(A)) but most installers and AHJs enforce 2% for feeder/service and 3% for branch as a de facto standard. Lower voltage drop improves efficiency (less heat dissipated in wires, more power to the inverter) but requires larger and more expensive copper or aluminum. For long off-grid runs, accepting 5-7% voltage drop may be unavoidable economically — at that point, consider stepping up the system voltage instead of upsizing the wire. Always document your assumed voltage-drop target on the system one-line diagram — inspectors and future maintenance teams need to know what design margin you chose.
When should I not use this calculator?
Skip it for any installation that will be inspected or permitted — use NEC-compliant design software (e.g., SolarEdge designer, Enphase Estimator, manufacturer-specific tools, or commercial software like HelioScope, Aurora, or PVsyst) and confirm with a licensed electrician or solar contractor. Do not use it for AC sizing — the calculation differs (single-phase vs three-phase, power-factor considerations). Skip it for high-current short runs where ampacity, not voltage drop, dominates the sizing — NEC table 310.15 is the reference there. Skip it for aluminum wire — copper and aluminum resistivities differ by about 60% and aluminum requires anti-oxidant compound at terminations. For commercial installations, hire a licensed electrical engineer to stamp the design — the liability and code compliance are not worth saving on labor.