Enter circuit details
Use one-way length; results are estimates
Quick example: The default 12 V, 5 A, 25 ft copper AWG 14 circuit shows why longer runs and smaller conductors lose more voltage.
Free electrical calculator
Voltage Drop Calculator
A voltage drop calculator estimates how much voltage a wire loses between a source and a load. Enter your circuit details to see the drop, percentage loss, and voltage available at the load.
How to use this calculator
- 1
Enter source and load values
Provide the circuit voltage, load current, and one-way distance.
- 2
Choose wire details
Select feet or meters, material, AWG size, and circuit type.
- 3
Review the estimate
Compare voltage loss and load voltage with the 3% rule-of-thumb guideline.
Voltage drop formula
V drop = current × resistance
For DC and single-phase circuits, resistance uses twice the one-way length. Three-phase circuits use √3 × length. Wire resistance varies with temperature, installation, and conductor construction, so treat this as an estimate.
The 3% value is a common design guideline, not a substitute for code, ampacity, or professional electrical review.
Frequently asked questions
What is voltage drop?
Voltage drop is the voltage lost as current travels through a wire's resistance. The load receives the source voltage minus that loss.
What is the 3% voltage-drop rule?
Three percent is a common design guideline for branch circuits. It is a rule of thumb, not a universal code requirement; always check the applicable electrical standard.
Should I use one-way or round-trip length?
Enter the one-way distance from the source to the load. The calculator accounts for the return conductor for DC and single-phase circuits.
Does this work for three-phase power?
Yes. Select three-phase AC to use the square-root-of-three relationship for a balanced three-phase circuit.
Are these results an installation approval?
No. This is an estimate using nominal conductor data. A qualified electrician must verify ampacity, temperature, conduit fill, protection, and local requirements.
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