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The run you have in mind
Paralleled sets carry their own code rules.
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How much voltage does this run cost you?
Conductor resistance turns part of your supply into heat before it ever reaches the load. Enter the run and see exactly how much — and, if it's too much, the smallest conductor that fixes it.
Result
Voltage drop over the run
3.11 %Over 3%
- Conductor10 AWG copper
- Circular mils in the run10,380
- Volts dropped7.46 V
- Voltage at the load232.54 V
- Drop as a percentage3.11%
Over the limit. The smallest published conductor that lands inside 3% for this run is 8 AWG copper. Sizing up for voltage drop does not change the ampacity or overcurrent requirements — check both.
Voltage-drop targets of 3% on a branch circuit and 5% overall are performance recommendations, not requirements — though some jurisdictions and specifications enforce them. This estimate uses the circular-mil method and ignores conductor reactance and power factor, which matter on large or long AC feeders.
Source: NFPA 70, National Electrical Code, 210.19(A) and 215.2(A) informational notes; NFPA 70, National Electrical Code, Chapter 9, Table 8. Published by NFPA.NEC 2023
What voltage drop actually costs you
Every conductor has resistance, and current through resistance produces a voltage difference. The load at the end of a long run therefore sees less voltage than the panel supplies. Lights dim, heaters underperform in proportion to the square of the voltage loss, and motors draw more current to make the same torque — which produces more heat in both the motor and the conductor feeding it.
The method used here
This uses the circular-mil method: the drop scales with the resistivity constant for the material, the current, and the one-way distance, divided by the conductor's circular-mil area. Single-phase runs use a factor of 2 because the current travels out and back; three-phase runs use the square root of 3.
It is an approximation. It ignores conductor reactance and load power factor, which become significant on large or long AC feeders, especially in metallic raceway. For precise work on a big feeder, use the AC impedance figures rather than this estimate.
Are the limits mandatory?
The commonly cited 3% branch-circuit and 5% combined figures appear in the 2023 NEC as informational recommendations for reasonable efficiency of operation — not as enforceable requirements in general. That said, project specifications routinely make them contractual, some jurisdictions amend them into requirements, and specific applications (sensitive equipment, fire pumps, elevators, EV supply equipment) have their own rules. Confirm what applies to your job.
Sizing from scratch instead? Use the wire size calculator, which checks ampacity and voltage drop together.