The short answer
The NEC recommends, in informational notes rather than requirements, that voltage drop stay at or below 3% on a branch circuit or a feeder and 5% for the two combined (NEC 210.19 and 215.2). Voltage drop is 2 × K × I × L ÷ circular mils on a single-phase circuit and 1.732 × K × I × L ÷ circular mils on a three-phase circuit, with K = 12.9 for copper and 21.2 for aluminum.
Step by step
- Note the system voltage, whether the circuit is single-phase or three-phase, the conductor material and its size.
- Measure the one-way length of the run in feet and take the load current in amperes.
- Multiply 2 (single-phase) or 1.732 (three-phase) by K (12.9 copper, 21.2 aluminum), the current and the one-way length.
- Divide by the conductor's circular-mil area from Chapter 9, Table 8 to get the drop in volts.
- Divide the drop by the system voltage and multiply by 100 for the percentage; compare it with 3% for the circuit and 5% overall.
Worked example
A 16 A load at the end of a 100 ft run of 12 AWG copper on a 120 V single-phase circuit drops 6.32 V, which is 5.27%. The smallest copper size that keeps this run at or under 3% is 8 AWG.
| Conductor | Drop | Percent | Within 3% |
|---|---|---|---|
| 12 AWG | 6.32 V | 5.27% | No |
| 10 AWG | 3.98 V | 3.31% | No |
| 8 AWG | 2.5 V | 2.08% | Yes |
| 6 AWG | 1.57 V | 1.31% | Yes |
Circular-mil areas (Chapter 9, Table 8)
| Conductor | Circular mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 2 AWG | 66,360 |
| 1/0 AWG | 105,600 |
Common mistakes
- Using the round-trip length: the formula's 2 (or 1.732) already accounts for the return conductor, so enter the one-way length.
- Treating 3% as a requirement: the NEC states it in informational notes; an energy code, a specification or the AHJ can make it one.
- Sizing for the drop and forgetting ampacity: the conductor still needs the ampacity for the load and its overcurrent device.
Code numbers and editions
Section numbers here are NEC 2023's, which most states enforce. In states on NEC 2026, 215.2 is 215.4.
Code adoption cycles change. Always confirm the edition and amendments in force with your Authority Having Jurisdiction (AHJ) before permitting work.