Voltage Drop Calculator
The Voltage Drop Calculator estimates how much voltage a conductor loses over its length when carrying current, using the resistance of common copper wire gauges and a multiplier for aluminium. It returns the volts lost, the drop as a percentage of the supply, and the voltage remaining at the far end of the run. This helps you check that a cable run keeps the load within an acceptable drop, typically 3% for a branch circuit.
Formula
Vdrop = I × R_ft × L × 2; %drop = Vdrop / Vsupply × 100
- I
- Load current in amperes
- R_ft
- Conductor resistance per foot (×1.6 for aluminium)
- L
- One-way run length in feet
- Vdrop
- Voltage lost across the round-trip conductor
How it works
- Enter the supply voltage, the load current in amps, the wire gauge (AWG 14, 12, 10, 8, or 6), the one-way run length in feet, and the conductor material (copper or aluminium).
- Each gauge has a copper resistance per foot; choosing aluminium multiplies that resistance by 1.6. The drop accounts for both conductors (out and back) of the circuit.
- Voltage drop = current × resistance-per-foot × length × 2 conductors. The percent drop is the drop over the supply voltage, and the end voltage is the supply minus the drop.
Worked examples
A 120 V circuit drawing 20 A through 100 ft of #10 AWG copper.
- #10 copper resistance ≈ 0.00124 Ω/ft.
- Vdrop = 20 × 0.00124 × 100 × 2 = 4.96 V.
- Percent drop = 4.96 ÷ 120 × 100 = 4.13%; end voltage = 120 − 4.96 = 115.04 V.
About 4.96 V dropped (4.13%), leaving 115.04 V at the load.
The same circuit using aluminium instead of copper for #10 AWG.
- Aluminium resistance = 0.00124 × 1.6 = 0.001984 Ω/ft.
- Vdrop = 20 × 0.001984 × 100 × 2 = 7.94 V.
- Percent drop = 7.94 ÷ 120 × 100 = 6.61%; end voltage = 112.06 V.
About 7.94 V dropped (6.61%), leaving 112.06 V — aluminium drops more for the same gauge.
Frequently asked questions
- What is an acceptable voltage drop?
- The NEC recommends keeping voltage drop to 3% or less on a branch circuit and 5% or less for the combined feeder and branch. Excessive drop causes dim lights, weak motors, and wasted energy as heat.
- Why does the calculation multiply by two conductors?
- Current travels out on one conductor and back on the other, so the resistance of the full round-trip path — twice the one-way length — is what causes the drop.
- Why does aluminium drop more voltage than copper?
- Aluminium has higher resistivity than copper, so for the same gauge it offers more resistance. This calculator approximates that with a 1.6× multiplier on the copper resistance per foot.
- Which wire gauges are supported?
- It covers common branch-circuit copper gauges: 14, 12, 10, 8, and 6 AWG. Larger or smaller conductors are outside the built-in resistance table.