Voltage Drop Calculator
Calculate voltage drop and percentage drop for electrical wire runs based on gauge, length, and current.
Voltage Drop
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% Drop
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Recommendation
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What is Voltage Drop Calculator?
The Voltage Drop Calculator determines how much voltage is lost along a wire run due to the wire's resistance. It helps electricians and DIYers select the correct wire gauge to keep voltage drop within acceptable limits (typically under 3% for branch circuits).
How to use
- 1 Select the wire gauge from the dropdown (AWG sizes #14 through #1/0).
- 2 Enter the one-way length of the wire run in feet.
- 3 Enter the load current in amps.
- 4 Select the system voltage (120 V or 240 V).
- 5 Click Calculate to see the voltage drop and percentage. If drop exceeds 3%, a larger gauge recommendation is shown.
Formula
Example calculation
240 V circuit, #12 AWG (1.98 Ω/1000 ft), 50 ft run, 15 A: V_drop = (2 × 50 × 15 × 1.98) / 1000 = 2.97 V, which is 1.24% — well within the 3% limit.
Frequently asked questions
What is an acceptable voltage drop?
The NEC recommends no more than 3% for branch circuits and 5% total for feeders plus branch circuits combined.
Why does the formula multiply by 2?
Electricity must travel from the panel to the load and back, so the total wire length is twice the one-way distance.
What happens if voltage drop is too high?
Excessive voltage drop causes motors to run hot, lights to dim, and sensitive electronics to malfunction or fail prematurely.
Does wire temperature affect resistance?
Yes — resistance increases with temperature. The values used here are for copper wire at 75°C, which is standard for NEC calculations.
Should I upsize the wire gauge?
If voltage drop exceeds 3%, the calculator recommends a larger gauge. Going up one or two sizes is often the easiest solution for long runs.