DC cable voltage drop, explained.

Updated 24 October 2026 · Calculator v1.1.0, formula v1.0.0

Short answer: voltage drop = current × the resistance of the whole loop—the positive lead and the return lead. Resistance rises with conductor temperature, so a warm cable loses more. A low voltage drop does not prove a cable is safe: ampacity and protection are separate checks.

The formula

Loop resistance  R = 2 × L × r / 1000 × (1 + 0.00393 × (T − 20))
Voltage drop     V = I × R
Drop (%)         = 100 × V / minimum inverter-terminal voltage
Heat in cable    P = I² × R   (peak, at that current)

L is the one-way length in metres, r the conductor resistance in Ω/km at 20 °C from the cable’s datasheet, T the assumed conductor temperature in °C and I the current in amps. Example: 10 A through 1 m leads of 1 Ω/km at 20 °C gives R = 0.002 Ω, a 0.02 V drop (0.20% of 10 V) and 0.2 W of heat.

Length matters—both ways

12V class, 1000 W inverter load (100.1 A), 0.5 Ω/km at 20 °C, 75 °C conductor, 2.5% target:

Keeping the battery close to the inverter is often the cheapest improvement. Every crimp, busbar, switch and fuse adds resistance too; this model counts only the cable.

What voltage drop does not tell you

• Ampacity. Whether a conductor can carry the current safely depends on its insulation rating, installation method, bundling, ambient temperature and the applicable standard (for example NEC or ABYC). A qualified installer determines it.
• Protection. A fuse or breaker needs a current rating, a DC voltage rating and an interrupt rating suitable for the battery’s fault current. These are separate properties.
• Heat over a day. The heat figure is a peak at full load. Multiplying it by 24 hours assumes the load never stops and can double-count losses.

Choosing a target

Many designers aim for a few percent of drop on main battery-to-inverter cables because inverters shut down at low voltage and losses become heat. Pick a target with your installer and check it in the calculator; enter the length in feet or metres and the cable’s datasheet resistance.

Hardware to compare

Next: 12V vs 24V vs 48V · Safety checklist