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

A
ft

System voltage of the battery or series string.

Results

Voltage drop
5.93V
Voltage drop
49.39%
Voltage at the load
6.07V

Recommended gear

Engineering methodology

VD = (2 × 12.9 × I × L) / CM for copper at 75 °C (NEC Chapter 9 Table 8). L is one-way feet. Drop percent = (VD ÷ source voltage) × 100. Voltage at the load = source voltage − VD. This is Ohm’s law V = I × R with R taken from circular mils: R_round-trip ∝ 2 × L / CM.

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.

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