Built for low-voltage DC — solar arrays, battery banks, van conversions, boats and RVs — where a single lost volt is a far bigger deal than on the grid.
Working in metric? The mm² calculator covers the same 12/24/48 V DC systems in millimeters and meters.
A 3% drop at 120 V costs you 3.6 V — annoying. The same 3% at 12 V costs 0.36 V, and yet 12 V electronics often misbehave below 11.5 V, chargers derate, and inverters alarm. Because the percentage budget represents so few absolute volts, DC systems need dramatically thicker cable per amp than household AC. Doubling system voltage (12 → 24 V) quarters the copper needed for the same power, which is why serious off-grid systems run 24 or 48 V.
| I | load current, amps |
| L | one-way circuit length, feet |
| R | conductor resistance, Ω per 1000 ft — NEC Chapter 9, Table 8 (75 °C) |
| Vd | volts lost in the wire, round trip |
The ×2 covers the return path (√3 replaces it for three-phase). Percent drop = Vd ÷ source voltage × 100. Compare against 3% (branch) / 5% (total).
Low-voltage disconnect. The cable drop plus battery sag pushes the terminal voltage below the appliance's cutoff. Upsizing the cable or shortening the run usually fixes it.
Yes — use the string's operating current (Imp) and the one-way distance from array to controller. For series strings the voltage is the sum of the panels, which helpfully shrinks the percentage drop.