Voltage Drop Calculator
Calculate voltage drop in electrical wiring based on wire gauge, length, and load current.
A voltage drop calculator determines the voltage loss across electrical wiring based on wire gauge, length, and current draw to ensure safe and efficient installations.
Examples
20A load on 100 ft of 12 AWG copper
Frequently Asked Questions
What is an acceptable voltage drop?
Why does wire length double in the formula?
Quick Tips
- •NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder + branch.
- •Use a larger wire gauge (lower AWG number) to reduce voltage drop on long runs.
- •Always verify calculations against local electrical codes before installation.
A voltage drop calculator determines the voltage loss across electrical wiring based on wire gauge, length, and current draw to ensure safe and efficient installations.
How to Use This Calculator
Enter the source voltage, load current in amps, one-way wire length, wire gauge, and material. The calculator determines the voltage drop across the wire run and whether it falls within acceptable limits.
Understanding the Formula
Voltage Drop = Current x (Resistance per 1000 ft) x (2 x Length) / 1000. Percent Drop = (Voltage Drop / Source Voltage) x 100. NEC recommends no more than 3% drop for branch circuits and 5% total.
Examples
20A load on 100 ft of 12 AWG copper
Voltage drop = 20 x 1.98 x 200 / 1000 = 7.92 V, or 6.6% on a 120 V circuit. This exceeds the 3% recommendation; consider upsizing to 10 AWG.
Frequently Asked Questions
What is an acceptable voltage drop?
The NEC recommends a maximum 3% voltage drop for branch circuits and 5% for the entire system (feeder + branch). Excessive drop leads to dimming lights, overheating motors, and wasted energy.
Why does wire length double in the formula?
Current flows out to the load and back to the source, so the total conductor length is twice the one-way distance.
Assumptions & Limitations
- Uses NEC standard copper wire resistance values at 75°C conductor temperature.
- Assumes a single-phase circuit; three-phase circuits use a different multiplier.
- Does not account for connection resistance, temperature derating, or conduit fill effects.