Voltage Drop & Wire Size Calculator
Calculate voltage drop over a cable run and find the wire gauge that keeps it within limits. Supports AWG and metric cross sections, copper and aluminium, single phase and three phase.
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Long runs lose voltage
Every conductor has resistance, so current flowing through it produces a voltage drop that never reaches the load. On a short run it is negligible. Over a hundred metres of thin cable at any real current it is not: motors run hot and start badly, LED strips dim toward the far end, and electronics reset when a compressor kicks in. The drop is proportional to current, to length and to the resistance per unit length, which is set by the conductor material and the cross section.
The limits people design to
Common practice keeps the drop at or below three percent on a branch circuit and five percent overall from the supply to the load. Those are design targets rather than a safety requirement in most codes, but they are the numbers that keep equipment working within its rated voltage window. Low voltage systems are far less forgiving: a three percent drop on 12 volts is only 0.36 volts, and a long twelve volt run frequently needs cable several sizes larger than the current alone would suggest.
Ampacity and drop are different problems
A cable can be thick enough to carry the current safely and still drop too much voltage over distance. Sizing for ampacity protects the cable; sizing for voltage drop protects the load. The larger of the two required sizes wins. This calculator solves the voltage drop side and reports the smallest standard gauge that meets your chosen percentage limit, along with the resulting power lost as heat in the cable.
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Frequently Asked Questions
Should I use one way or round trip length?
Enter the one way distance from source to load. The calculation doubles it internally for single phase and DC, because the current returns along the second conductor. Three phase uses a different factor, which the tool applies for you.
What drop percentage should I design to?
Three percent to the load on a branch circuit and five percent total is the usual guidance. For 12 V and 24 V systems many designers use three percent as a hard limit because the absolute voltage available is so small.
Copper or aluminium?
Aluminium has about 61 percent of the conductivity of copper, so an aluminium conductor needs roughly two AWG sizes larger for the same drop. It is lighter and cheaper for long runs, which is why utility feeders use it.
Does this replace a code sizing calculation?
No. It solves voltage drop only. Ampacity, insulation temperature rating, grouping and derating factors, and protective device coordination all still apply and are governed by your local electrical code.
Why does temperature matter?
Copper resistance rises about 0.4 percent per degree Celsius. A cable running at 70 degrees has roughly 20 percent more resistance than the 20 degree figure, so the drop is proportionally worse under load. The tool shows both cold and hot figures.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter current, run length, voltage and conductor material to see the drop and the smallest adequate gauge.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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