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
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.
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