How it works
Every wire has resistance, so current flowing through it loses voltage before reaching the load. Too much drop makes motors run hot, lights dim, and electronics misbehave.
This uses the standard circular-mil formula with a constant K for the conductor's resistivity (12.9 ohm-cmil/ft for copper and 21.2 for aluminum at about 75 °C). Length is one-way distance from source to load; the formula doubles it for single-phase because current flows out and back.
Worked example
A 240 V single-phase load drawing 40 A, 150 feet away, on 6 AWG copper:
- VD = 2 × K × I × L ÷ CM = 2 × 12.9 × 40 × 150 ÷ 26,240 = 5.9 V
- Percent drop = 5.9 ÷ 240 × 100 = 2.46%
- Voltage at load = 240 − 5.9 = 234.1 V
| Input | Value |
|---|---|
| Source voltage | 240 V |
| Load current | 40 A |
| One-way length | 150 ft |
| Wire size | 6 AWG |
| Conductor material | Copper |
| Phase | Single-phase |
| Result | Value |
|---|---|
| Voltage drop | 5.9 V |
| Percent drop | 2.46 % |
| Voltage at the load | 234.1 V |
Wire sizes and circular mils
| Wire size | Circular mils |
|---|---|
| 14 AWG | 4110 |
| 12 AWG | 6530 |
| 10 AWG | 10380 |
| 8 AWG | 16510 |
| 6 AWG | 26240 |
| 4 AWG | 41740 |
| 3 AWG | 52620 |
| 2 AWG | 66360 |
| 1 AWG | 83690 |
| 1/0 AWG | 105600 |
| 2/0 AWG | 133100 |
| 3/0 AWG | 167800 |
| 4/0 AWG | 211600 |
| 250 kcmil | 250000 |
| 350 kcmil | 350000 |
| 500 kcmil | 500000 |
Show the wire sizes and circular mils table
| Wire size | Circular mils |
|---|---|
| 14 AWG | 4110 |
| 12 AWG | 6530 |
| 10 AWG | 10380 |
| 8 AWG | 16510 |
| 6 AWG | 26240 |
| 4 AWG | 41740 |
| 3 AWG | 52620 |
| 2 AWG | 66360 |
| 1 AWG | 83690 |
| 1/0 AWG | 105600 |
| 2/0 AWG | 133100 |
| 3/0 AWG | 167800 |
| 4/0 AWG | 211600 |
| 250 kcmil | 250000 |
| 350 kcmil | 350000 |
| 500 kcmil | 500000 |
Circular-mil areas from NEC Chapter 9, Table 8.
Assumptions and limits
- Resistive DC-style approximation. It ignores reactance and power factor, which matters on large conductors, long runs, and inductive loads.
- Resistance is taken at about 75 °C. Cooler conductors drop slightly less.
- The NEC recommends (not requires) at most 3% drop on a branch circuit and 5% total for feeder plus branch.
Common questions
How much voltage drop is acceptable?
The NEC informational notes recommend a maximum of 3% on a branch circuit and 5% combined for feeder and branch. Some equipment specifies stricter limits.
How do I reduce voltage drop?
Use a larger wire, shorten the run, raise the system voltage (240 V instead of 120 V), or use copper instead of aluminum.
Do I use one-way or round-trip length?
Enter one-way length. The formula already accounts for the return path.
Sources
- Circular-mil voltage drop formula with K = 12.9 (copper) and 21.2 (aluminum) ohm-cmil/ft at about 75 °C, consistent with the conductor resistances in NEC Chapter 9, Table 8.
- Circular mil areas: NEC Chapter 9, Table 8.
- 3% branch-circuit and 5% total recommendations: NEC informational notes to 210.19(A) and 215.2(A)(2). These are recommendations, not requirements.
Updated 2026-09-30