Voltage Drop Calculator
Estimate the voltage drop in a wire run. Choose the wire size and material, enter the one-way length, the current, and the voltage, and see the drop in volts and percent and the voltage left at the load.
For planning only
Voltage drop
5.91 V
Check the highlighted fields to see your result.
- Drop
- 4.93%
- Voltage at the load
- 114.09 V
- Recommended max 3%
- Above 3%: consider a larger wire
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
The calculator multiplies the wire's resistance by the current flowing through it. Resistance depends on the material, the cross-section, and the length. It uses resistivity values at 75 °C, the same basis as the NEC's standard K factors (12.9 for copper and 21.2 for aluminum, in ohm-circular mils per foot).
The formulas
Single-phase: VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM
K is the resistivity constant, I is the current in amps, L is the one-way length in feet, and CM is the wire's area in circular mils.
Worked example
A 120 V single-phase circuit uses 12 AWG copper wire (6,530 circular mils), runs 100 ft, and carries 15 A.
- VD = 2 × 12.9 × 15 × 100 ÷ 6,530 ≈ 5.93 V.
- Percent: 5.93 ÷ 120 ≈ 4.9%, above the recommended 3%.
- A 10 AWG wire would bring it down to about 3.1%.
Frequently asked questions
How much voltage drop is acceptable?
The National Electrical Code (NEC) suggests, in an informational note, no more than 3% on a branch circuit and 5% in total from the service to the farthest outlet. It is a recommendation for efficiency and good performance, not a strict rule, but local codes may be stricter.
Why does a longer wire lose more voltage?
Every wire has some resistance, and it adds up with length. The current has to travel to the load and back, so a single-phase circuit counts the length twice. A thicker wire has less resistance and less drop.
Does this tell me what wire size to use?
Not on its own. A wire must also be rated for the current it carries (its ampacity), which depends on the insulation, the temperature, and how it's installed. Voltage drop is one check among several, and a licensed electrician should choose the final size.