Voltage Drop Calculator

Q: What is an acceptable voltage drop?

The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% total (feeder + branch). Voltage drops above 5% can cause equipment malfunction and overheating.

Q: How do I reduce voltage drop?

Use a larger wire gauge (lower AWG number), shorten the wire run, reduce the load, increase the voltage, or use copper instead of aluminum wire.

Q: Does aluminum wire have more voltage drop than copper?

Yes. Aluminum has about 60% the conductivity of copper, so it produces roughly 65% more voltage drop at the same gauge and current. You typically need to go one or two AWG sizes up when using aluminum.

Calculate the voltage drop across your wire run for copper or aluminum conductors. Includes drop percentage and end voltage.

Calculate Voltage Drop

Current (Amps)

One-Way Wire Length (ft)

System Voltage (V)

Wire Material

Wire Size (AWG)

Result

—

Voltage Drop

—

Drop Percentage

—

End Voltage

Formula

Voltage Drop = Current (A) × Wire Resistance (Ω)
Wire Resistance = (Resistance per 1000ft ÷ 1000) × 2 × Length
Drop % = (Voltage Drop ÷ System Voltage) × 100

The factor of 2 accounts for both the hot and neutral/return conductors in the circuit.

AWG Resistance Reference Table

Resistance in ohms per 1,000 feet at 75°C:

AWG	Copper (Ω/1000ft)	Aluminum (Ω/1000ft)
AWG 14	3.14	5.17
AWG 12	1.98	3.25
AWG 10	1.24	2.04
AWG 8	0.778	1.28
AWG 6	0.491	0.808
AWG 4	0.308	0.508
AWG 3	0.245	0.403
AWG 2	0.194	0.319
AWG 1	0.154	0.253
AWG 1/0	0.122	0.201
AWG 2/0	0.0967	0.159
AWG 3/0	0.0766	0.126
AWG 4/0	0.0608	0.100

Example Calculation

AWG 12 copper wire, 20A load, 75ft one-way run on a 120V circuit:

Total resistance = (1.98 ÷ 1000) × 2 × 75 = 0.297 Ω
Voltage drop = 20A × 0.297Ω = 5.94V
Drop % = (5.94 ÷ 120) × 100 = 4.95%
End voltage = 120 − 5.94 = 114.06V

This exceeds the recommended 3% — consider upgrading to AWG 10.

Frequently Asked Questions

What is an acceptable voltage drop?

The NEC (National Electrical Code) recommends a maximum 3% voltage drop for branch circuits and 5% combined (feeder + branch). Voltage drops above 5% can cause equipment to malfunction, overheat motors, and shorten appliance lifespan.

How do I reduce voltage drop?

Your main options are: use a larger wire (lower AWG number), shorten the run, reduce the load, increase the system voltage, or switch from aluminum to copper wire. Increasing wire size is the most practical solution for most installations.

Does aluminum wire have more voltage drop than copper?

Yes. Aluminum has about 60% the conductivity of copper. This means aluminum produces roughly 65% more voltage drop at the same gauge and current. When using aluminum, you typically need to go one or two AWG sizes larger than copper.