Voltage Drop Limits Cheat Sheet
NEC recommended limits, allowable volts per system voltage, single and three-phase formulas, and a worked example. Print it or save as PDF for the truck or the workshop wall.
The two limits to remember
| Segment | Recommended limit | Reference |
|---|
| Branch circuit (panel to outlet) | 3 % | NEC 210.19(A) Informational Note |
| Feeder (service to sub-panel) | 2 % | NEC 215.2(A) Informational Note |
| Feeder + branch combined | 5 % | Combined total to the final outlet |
In the NEC these percentages are Informational Notes, so they are recommended practice rather than a hard rule for general circuits. Many local codes, and most engineering specifications, adopt the 3 percent and 5 percent figures as binding. The IEC 60364 limit is also 3 percent for final circuits and 5 percent total.
Allowable voltage drop in volts
| System voltage | 2 % (feeder) | 3 % (branch) | 5 % (total) |
|---|
| 120 V (1-phase) | 2.4 V | 3.6 V | 6.0 V |
| 208 V (3-phase) | 4.16 V | 6.24 V | 10.4 V |
| 230 V (1-phase) | 4.6 V | 6.9 V | 11.5 V |
| 240 V (1-phase) | 4.8 V | 7.2 V | 12.0 V |
| 277 V (1-phase) | 5.54 V | 8.31 V | 13.85 V |
| 480 V (3-phase) | 9.6 V | 14.4 V | 24.0 V |
Voltage drop formulas
L is the one-way circuit length. Multiply by 2 for single-phase (out and back) or by 1.732 for three-phase.
Imperial (cmil):
1-phase: VD = (2 × K × I × L) / CM
3-phase: VD = (1.732 × K × I × L) / CM
K = 12.9 (copper) or 21.2 (aluminium), in ohm-cmil/ft · L in feet · CM = wire area in circular mils
Metric (mm²):
1-phase: VD = (2 × ρ × L × I) / A
3-phase: VD = (1.732 × ρ × L × I) / A
ρ = 0.0172 (copper) or 0.0282 (aluminium), in ohm-mm²/m · L in metres · A in mm²
Percentage drop = (VD / source voltage) × 100. Circular mils by size: 14 AWG = 4110, 12 AWG = 6530, 10 AWG = 10380, 8 AWG = 16510, 6 AWG = 26240, 4 AWG = 41740, 2 AWG = 66360, 1/0 = 105600.
Worked example
20 A at 120 V, 100 ft one-way run, copper.
12 AWG (6530 cmil): VD = (2 × 12.9 × 20 × 100) / 6530 = 7.9 V = 6.6 %. Too high.
10 AWG (10380 cmil): VD = 51600 / 10380 = 4.97 V = 4.1 %. Still over 3 percent.
8 AWG (16510 cmil): VD = 51600 / 16510 = 3.13 V = 2.6 %. Within the 3 percent branch limit. Use 8 AWG for this run.
Practical rules of thumb
- Voltage drop only becomes a sizing factor on longer runs. Below roughly 50 ft (15 m) at normal current, ampacity almost always governs the wire size instead.
- Doubling the run length doubles the drop. Doubling the wire area (going about three AWG sizes larger) roughly halves it.
- Low-voltage DC systems (12 V and 24 V solar, RV, automotive) hit the percentage limit far sooner, because the same volts lost is a much larger share of the supply. Always check drop on those circuits.
- Size the conductor for ampacity first, then check voltage drop. Use whichever result calls for the larger wire.