Voltage Drop Calculator
Voltage drop is the voltage a long wire loses on the way to a load. Enter the current, the one-way length of the wire, its resistance and the supply voltage.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
The current travels out and back, so the length counts twice. The drop is two times the current times the resistance of the whole run. The percent is that drop divided by the supply voltage.
Wire resistance is looked up in a wire table for the size and metal you use. The default value is for copper and must be checked against your table.
Worked example
A load drawing 15 amps at the end of a 75 ft run on a 120 volt supply loses 3.58 volts, which is 3% of the supply.
Questions
Where do I find the wire resistance?
Wire tables list ohms per 1000 feet for each gauge. Use the value for your metal and size.
Is there a percent that counts as too much?
This shows the math only. Limits for drop come from local rules and from the equipment you connect, so check those.
Does this work for three-phase wiring?
No. This is for a simple two-wire run. Three-phase circuits use a different formula.
More like this
- Parallel Circuit CalculatorBranch currents and total current for resistors in parallel.
- Rigging CalculatorLoad on each sling leg at a set angle.
- Carbon Equivalent CalculatorCarbon equivalent of a steel from its chemistry.
- NPSH CalculatorNPSH available at a pump inlet, for water.
- Amps to Watts CalculatorWatts from amps and volts, for DC or AC.
- Bend Deduction CalculatorBend allowance and deduction for sheet metal.