How to Use the Wire Voltage Drop Calculator
As current travels down a wire, the wire's own resistance causes some voltage to be lost along the way — this is voltage drop. This calculator takes your one-way circuit distance, load current, AWG wire size, and circuit voltage, then calculates the round-trip voltage drop (in volts) and the drop as a percentage of your supply voltage, using standard copper wire resistance values in ohms per 1,000 feet.
The National Electrical Code (NEC) recommends keeping voltage drop to 3% or less on a branch circuit, and no more than 5% total when you combine the feeder and branch circuit. Excessive voltage drop can cause dim lighting, reduced motor performance, or overheating, so exceeding these limits usually means you should move up to a larger wire gauge.
For the same distance and current, a lower AWG number (a physically larger conductor) has less resistance and produces a smaller voltage drop. Longer circuit runs to a garage, shed, or outdoor outlet, or higher-current appliances, generally call for a larger wire size to stay within the recommended limit. These results are a standard estimate — always verify sizing against the NEC and local code, or consult a licensed electrician for the final installation.
Frequently Asked Questions
The NEC recommends keeping voltage drop to 3% or less on a branch circuit and 5% or less for the combined feeder and branch circuit. Going above that suggests you should use a larger wire gauge.
Yes. A lower AWG number means a larger conductor with less resistance, which reduces voltage drop for the same distance and current. Longer runs or higher-current loads need a larger wire size.