Off-Grid Solar Calculators

Wiring and electrical safety

DC Voltage Drop Calculator

Use this when you already have a cable size and want to see the drop. Keep sensitive electronics and LED loads near 3% or less on 12 V systems.

Inputs

Units
A
ft

System voltage of the battery or series string.

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Imperial (copper 75 °C, NEC Chapter 9 Table 8): VD = (2 × 12.9 × I × L) / CM, where L is one-way feet and the 2 covers the round-trip conductor. Metric equivalent: resistivity ρ = 0.02145 Ω·mm²/m, so VD = (2 × ρ × I × L) / mm² with L in one-way metres. Drop percent = (VD ÷ source voltage) × 100. Voltage at the load = source voltage − VD. This is Ohm’s law V = I × R. IEC 60228 sizes are used in metric; 3% drop is a performance target, not IEC 60364-5-52 ampacity. Copper only — not aluminium.

Frequently asked questions

How do I check if my 12 AWG run is under 3% drop?

Enter current, one-way feet, system voltage and 12 AWG. Example: 20 A over 75 ft of 12 AWG copper is about 5.93 V, which is far above 3% of 12 V (0.36 V). You would need a much thicker size or a shorter run.

Does this voltage-drop math work for aluminum cable?

Not with K = 12.9. Aluminum has a higher resistance constant (NEC Table 8 uses a different K). This tool is copper only.

Why is load voltage lower than battery voltage?

The cable is a resistor. V_load = V_source − I × R. Excessive drop browns out inverters, LED drivers and MPPT sense circuits even when the battery is full.

More in this cluster

All tools in this cluster