Voltage Drop Calculator
Voltage drop calculator: find the volts lost and percent drop on a copper or aluminum wire run by length, size and amps.
Editorial review: Farrow (pseudonym; identity and professional credentials are not disclosed). This page is for educational purposes, not professional advice.
Updated
Your inputs
Results update as you type. Nothing you enter leaves your device.
Results
- Percent drop
- 4.14%
- Voltage at load
- 115 V
- Against 3% guideline
- Over limit: use thicker wire
Example with the starting values
These results use the calculator's initial inputs. Change the values above to calculate your own estimate.
Inputs
- Conductor
- Copper
- Phase
- Single-phase
- Wire size
- 10 AWG
- Current
- 20 A
- One-way length
- 100 ft
- Source voltage
- 120 V
Results
- Voltage drop
- 4.97 V
- Percent drop
- 4.14%
- Voltage at load
- 115 V
- Against 3% guideline
- Over limit: use thicker wire
How does the Voltage Drop Calculator work?
Voltage drop = multiplier x K x current x one-way length / circular mils. K is 12.9 for copper and 21.2 for aluminum. The multiplier is 2 for single-phase and 1.732 for three-phase.
The National Electrical Code recommends keeping branch-circuit drop to about 3% and the total to 5%.
Frequently asked questions
What is an acceptable voltage drop?
Under 3% on a branch circuit is the usual guideline, and 5% for feeder plus branch together.
How do I reduce voltage drop?
Use a thicker wire, shorten the run or raise the voltage.
Do I use one-way or round-trip length?
One-way. The formula's multiplier already counts the return path.
Why does 3-phase use 1.732?
It is the square root of 3, which reflects the phase relationship in a balanced three-phase circuit.