Low-voltage DC punishes thin wire: at 12 V, the 3% budget is just 0.36 volts. Enter the load and run; the solver returns the smallest real conductor that survives both the drop budget and the ampacity table — for solar, marine, vans, and automotive work.
The same drop in volts is a much bigger percentage at low voltage — 0.9 V lost is 0.4% at 240 V but 7.5% at 12 V. That's why DC wire sizes look shockingly large to AC-trained eyes, and why doubling system voltage (12→24 V) lets the same wire carry the same power four times farther. The DC drop calculator is this tool's sibling for checking an existing wire; per-voltage sizers: 12 V · 24 V · 48 V.
Generic wire-size tools solve ρ·L/A and report a theoretical gauge — famously including sizes like "13 AWG" that don't exist at the supply house, with no check that the wire can carry the current thermally. This calculator returns only real, purchasable conductors, enforces the NEC ampacity floor for the load, applies the drop budget, and shows the verdict — wire you can actually buy and legally install. Every result comes with the fan chart, upgrade economics, and a PDF report.
| Vd(max) | budget volts = limit% × source voltage |
| Rmax | largest acceptable resistance, Ω/kft |
| answer | smallest gauge with R ≤ Rmax and Table 310.16 ampacity ≥ I |
Two gates, not one: a gauge must pass the drop budget AND carry the current thermally. Physics-only calculators stop at the first gate.
For contrast, the pure area formula gives A = 2ρLI/V = 2 × 0.0225 × 6.1 × 30 ÷ 0.36 ≈ 22.9 mm² — a size that doesn't exist at any supply house. Real conductors, real gates.
Work it in three lines: budget = 3% × 12 V = 0.36 V; R(max) = 0.36 × 1000 ÷ (2 × 25 × 30) = 0.240 Ω/kft; smallest gauge under that with ampacity ≥ 25 A is 2 AWG (0.194 Ω/kft, 115 A). Actual drop: 0.29 V = 2.42%.
On 12 V at a 3% budget: 4 AWG copper. The same load on 24 V needs only 8 AWG, and on 48 V, 12 AWG (ampacity-limited) — the calculator above shows all three in two clicks.
3% for voltage-sensitive loads (fridges, electronics, charge circuits) per common practice and ABYC guidance for critical circuits; up to 10% is tolerated for non-critical loads like resistive heaters and some lighting.