About the Voltage Drop Calculator
This voltage drop calculator shows how much voltage is lost in a length of cable before it reaches the load. Enter the wire gauge, conductor material, one-way run length, load current and supply voltage, and it returns the drop in volts and as a percentage, the voltage left at the far end, and the power wasted as heat in the wire.
Electricians and DIYers use it to check long runs to a garage, shed, well pump, EV charger or outdoor lighting; solar and RV installers use it on low-voltage DC runs where a small drop is a big percentage. A chart compares the drop across neighbouring wire sizes so you can see how much upsizing helps.
The US National Electrical Code recommends (in informational notes) keeping drop to about 3% on a branch circuit and 5% total from service to load. The calculation uses NEC Chapter 9 conductor areas and resistivity at 75 °C, which is conservative for most installations.
With the default inputs, the voltage drop is 2.96 V. Change any value above to recalculate instantly.
How to use the voltage drop calculator
- 1Choose DC, single-phase or three-phase and enter the supply voltage.
- 2Enter the load current in amps.
- 3Enter the one-way distance from panel (or battery) to the load.
- 4Select copper or aluminum and the wire gauge.
- 5Check the percent drop — aim for 3% or less — and compare sizes in the chart.
Formula and method
The K method multiplies the conductor resistivity constant K by the current and the one-way length, then divides by the wire’s cross-sectional area in circular mils. Single-phase and DC circuits multiply by 2 because current flows out and back; balanced three-phase circuits use √3 instead.
K is 12.9 Ω·cmil/ft for copper and 21.2 Ω·cmil/ft for aluminum at a 75 °C conductor temperature, and circular-mil areas come from NEC Chapter 9 Table 8. Power lost in the wire is I² × R summed over the current-carrying conductors. The method ignores conductor reactance, which only matters for very large cables or low power factor loads.
- VD
- Voltage drop in volts
- K
- Resistivity constant: 12.9 (copper) or 21.2 (aluminum)
- I
- Load current in amps
- L
- One-way length in feet
- CM
- Conductor area in circular mils
Worked examples
15 A on 12 AWG copper, 50 ft at 120 V
VD = 2 × 12.9 × 15 × 50 ÷ 6,530 ≈ 2.96 V, or 2.47% of 120 V — inside the 3% guideline. About 44 W is dissipated as heat in the cable.
30 A on 10 AWG copper, 150 ft at 240 V
VD = 2 × 12.9 × 30 × 150 ÷ 10,380 ≈ 11.18 V, or 4.66% of 240 V. That is over 3% for a branch circuit, so 8 AWG would be the better choice for this distance.
60 A three-phase on 4 AWG aluminum, 200 ft at 480 V
VD = 1.732 × 21.2 × 60 × 200 ÷ 41,740 ≈ 10.56 V, only 2.2% of 480 V. Higher voltage keeps the percentage drop low.
20 A DC on 8 AWG copper, 10 m at 12 V
10 m is 32.8 ft. VD = 2 × 12.9 × 20 × 32.8 ÷ 16,510 ≈ 1.03 V — an 8.5% loss on a 12 V system, which is why low-voltage DC runs need much heavier cable.
Frequently asked questions
What is an acceptable voltage drop?+
The NEC informational notes recommend no more than 3% on a branch circuit and 5% total including the feeder. Sensitive electronics and low-voltage DC systems often target 2–3% or less.
Do I use one-way or round-trip length?+
Enter the one-way distance from the source to the load. The formula already multiplies by 2 for single-phase and DC circuits to account for the return conductor.
How do I reduce voltage drop?+
Use a larger wire gauge, shorten the run, switch from aluminum to copper, or use a higher supply voltage (for example 240 V instead of 120 V), which halves the current for the same power.
Why is voltage drop worse on 12 V systems?+
Voltage drop in volts depends only on current, length and wire size. The same 1 V drop is under 1% of 120 V but over 8% of 12 V, and low-voltage systems also draw more current for the same power.
Does voltage drop waste electricity?+
Yes. The lost voltage times the current is power turned into heat in the wire. On long, heavily loaded runs this can be tens of watts continuously, which a larger conductor reduces.