Voltage drop calculator
Voltage lost along a copper or aluminium cable, from wire size, length and current.
About this calculator
This voltage drop calculator shows how much voltage is lost along a cable, and the voltage left at the load.
Choose copper or aluminium, the wire size in AWG or mm², the circuit type and the length. It checks the result against the usual 3% and 5% guidelines.
How it works
Wire resistance is resistivity × length ÷ cross-section. For DC and single-phase circuits the current goes out and back, so the drop is 2 × length × current × resistance per metre; for three-phase circuits the factor is √3 instead of 2.
Last reviewed:
Worked example
- Conductor: Copper
- Wire size in: AWG
- Wire gauge: 12 AWG
- Circuit: Single phase
- Supply voltage: 120 V
- Current: 15 A
- One-way length in feet: 100
Voltage drop: 5.79 V (over 3%: acceptable only as a total for feeder and branch circuit)
Common questions
What voltage drop is acceptable?
A common guideline in the US National Electrical Code is up to 3% on a branch circuit and 5% in total. Many other countries use similar limits.
How do I reduce voltage drop?
Use a thicker wire, shorten the run or use a higher supply voltage. Copper also drops less than aluminium of the same size.
Why does voltage drop matter?
Motors run hotter, lights dim and electronics can misbehave when the voltage at the load is too low, and the lost voltage is wasted as heat.
Sources and further reading
The story behind it
Where the idea came from: The AWG wire sizes used in North America date from 1857, when J. R. Brown created the Brown & Sharpe gauge.
Every 3 gauge steps roughly halve a wire's cross-section and double its resistance. That is why a longer run often needs a thicker wire to keep the voltage drop down.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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