Voltage drop
2.1%
Under 3% — within the recommended limit.
5.0V
volts dropped
235V
volts at load
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How this is calculated
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.
This is a planning estimate — always confirm against code, spec, and your supplier before ordering or installing.
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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