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Free Tool

Check your voltage drop in seconds

Enter the conductor, wire size, load, distance, and voltage to get the percent voltage drop — checked against the 3% and 5% NEC guidelines.

A

Amps the circuit carries.

ft

Length of the run, one way.

Voltage drop

2.1%

Under 3% — within the recommended limit.

5.0V

volts dropped

235V

volts at load

Sizing a job with long runs? Put the wire, breakers, and labor into a free estimate your customer can approve.

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Cómo se calcula

Single-phase: drop = (2 × K × amps × one-way distance) ÷ circular mils. Three-phase uses 1.732 in place of 2.

K is the resistivity constant — 12.9 for copper, 21.2 for aluminum. Circular mils come from NEC Chapter 9, Table 8. Percent drop is the voltage drop divided by system voltage.

The 3% / 5% thresholds are the NEC's recommended maximums (informational notes, not hard rules) — local code and the spec govern.

Es una estimación de planeación: conviene confirmarla siempre con la normativa, las especificaciones y tu proveedor antes de pedir o instalar.

Voltage Drop Calculator: FAQ

What is an acceptable voltage drop?

The NEC recommends (but does not mandate) a maximum of 3% voltage drop on a branch circuit, and 5% total across feeder plus branch. Under 3% is the target; over 5% and you should go up a wire size.

How do you calculate voltage drop?

For a single-phase circuit: voltage drop = (2 × K × amps × one-way distance) ÷ circular mils, where K is 12.9 for copper and 21.2 for aluminum. Three-phase uses 1.732 instead of 2. Divide the result by the system voltage for percent drop. This calculator does it from your inputs.

How do I fix too much voltage drop?

Increase the conductor size (more circular mils lowers the drop), shorten the run, or reduce the load. Going up one or two AWG sizes is the usual fix on a long run.

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