Solar Wire Size Calculator
Estimates the minimum copper wire gauge (AWG) for a solar DC run from current, one-way run length, system voltage and tolerable voltage drop, with a basic ampacity check. Use it as a starting point for code-compliant sizing before checking ampacity tables, conduit fill, and NEC 690 requirements.
Last updated: September 2026
Formula below · 3 sources (energy.gov, nfpa.org, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
The calculator finds the smallest copper conductor that keeps voltage drop within your limit. Required area (circular mils) = (2 × I × L × K) / (V × drop), where I is the current in amps, L is the one-way run in feet (the factor 2 covers the positive and negative conductors), K = 12.9 ohm·cmil/ft is the resistivity of copper at 75 °C, V is the system voltage (default 12 V; enter 24, 48 or your string voltage) and drop is the allowed voltage drop as a fraction (3% = 0.03). Copper areas (NEC Chapter 9, Table 8): 14 AWG = 4,110 cmil, 12 = 6,530, 10 = 10,380, 8 = 16,510, 6 = 26,240, 4 = 41,740, 3 = 52,620, 2 = 66,360, 1 = 83,690, 1/0 = 105,600, 2/0 = 133,100, 3/0 = 167,800, 4/0 = 211,600. The result is the smallest standard gauge whose area is at least the required area, i.e. the requirement is always rounded up to the larger wire. It also checks ampacity: the conductor must carry 125% of the entered current using the conservative 60 °C column of NEC Table 310.16 (14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 55 A, 4 AWG 70 A, 3 AWG 85 A, 2 AWG 95 A, 1 AWG 110 A, 1/0 125 A, 2/0 145 A, 3/0 165 A, 4/0 195 A), and it never returns smaller than 14 AWG. Sizes 1/0 and larger are shown as text (for example "1/0 AWG"). Ampacity must still be corrected for hot locations, conduit fill and the conductor insulation rating, and PV source circuits must use the NEC 690.8 design current; have the final size confirmed by a licensed electrician. Edge cases: long, low-voltage runs drive the size up very quickly — doubling the voltage halves the required area, which is why 24 V and 48 V systems are preferred for longer runs.
How to use
Example 1 — Short 12 V battery cable. 25 A, 20 ft one-way, 3% drop, 12 V. Required area = (2 × 25 × 20 × 12.9) / (12 × 0.03) = 12,900 / 0.36 = 35,833 cmil. 6 AWG (26,240 cmil) is too small and 4 AWG (41,740 cmil) is the first size that fits, so the result is 4 AWG. Ampacity check: 1.25 × 25 = 31.25 A, which 4 AWG (70 A) easily covers. Verify ✓. Example 2 — Longer 48 V run. 12 A, 100 ft one-way, 2% drop, 48 V. Required area = (2 × 12 × 100 × 12.9) / (48 × 0.02) = 30,960 / 0.96 = 32,250 cmil, so the result is again 4 AWG (41,740 cmil). Ampacity needs only 15 A. Verify ✓. At 12 V the same run would need four times the area (129,000 cmil, 3/0 AWG) — raising the system voltage is usually cheaper than buying heavier copper.
Frequently asked questions
How do I read the AWG result?
The result is a standard American Wire Gauge size for copper. Smaller AWG numbers are thicker wires: 14 AWG is the smallest size this calculator returns, then 12, 10, 8, 6, 4, 3, 2, 1, and the "aught" sizes 1/0, 2/0, 3/0 and 4/0, which are shown as text. The size already satisfies your voltage-drop limit and a basic 125% ampacity check, so you can use it or anything larger (smaller AWG number). For runs that would need more than 4/0, the calculator says so — use parallel conductors, kcmil cable or a higher system voltage. Aluminum conductors need roughly 1.6 times the copper area and are not covered here.
What system voltage should I enter?
Enter the nominal voltage of the circuit you are wiring: 12, 24 or 48 V for battery and charge-controller circuits, or the operating (Vmp) string voltage for a PV source circuit. If you leave the field blank the calculator assumes 12 V, the most demanding common case. Voltage matters a lot: at the same current and percentage drop, a 24 V circuit needs half the copper area of a 12 V circuit, and a 48 V circuit needs a quarter.
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.