SpecCalcsReference calculators for the shop floor

Voltage Drop Calculator

20 A through 100 ft of #12 AWG copper at 120 V drops 7.92 V (6.6%) — exceeds the NEC 3% branch-circuit recommendation.

Voltage drop for a wire run
Voltage drop7.92 V
Drop percentage6.6%

The formulas

Single-phase
Vdrop=2×I×R×L1000V_{\text{drop}} = \frac{2 \times I \times R \times L}{1000}
Three-phase
Vdrop=3×I×R×L1000V_{\text{drop}} = \frac{\sqrt{3} \times I \times R \times L}{1000}
I current in amps · R wire resistance (Ω/1000 ft from NEC Table 8) · L one-way length in feet · the factor of 2 accounts for the round-trip (supply + return conductors)

The wire resistance values come from NEC Chapter 9 Table 8 — DC resistance of uncoated conductors at 75 °C, in ohms per 1000 ft. For single-phase circuits the current travels down the hot conductor and back on the neutral, so the total conductor length is twice the one-way distance. For three-phase circuits, the factor is √3 instead of 2 because the three phases share the return path.

NEC 3% and 5% recommendation

NEC 210.19(A) Informational Note No. 4 and 215.2(A) recommend that voltage drop on a branch circuit not exceed 3%, and that the combined drop from the service entrance to the farthest outlet not exceed 5%. These are recommendations, not code requirements — but exceeding them can cause motors to overheat, lights to dim, and sensitive electronics to malfunction.

Wire sizing guidance

If the calculated drop exceeds 3%, you have three options: increase the wire gauge (lower AWG number = thicker wire = lower resistance), shorten the run, or reduce the load current. For long runs in outbuildings and barns, it is common to jump two or three wire sizes above the ampacity minimum just to keep the drop under 3%. Aluminum wire has roughly 1.6× the resistance of copper at the same gauge, so aluminum runs typically need to go one or two sizes larger.