Voltage Drop Calculator
Branch-circuit and feeder voltage drop on AC conductor runs. Single-phase and three-phase, copper and aluminum, with NEC Chapter 9 Table 9 AC resistance and inductive reactance values for typical PVC conduit installations. Compares to NEC Article 210/215 informational note (3% branch / 5% combined).
Defaults: 240V single-phase, 50 A load, 200 ft #6 Cu, unity PF — typical residential 50A subpanel feeder. R and X values are for PVC conduit per NEC Table 9; steel conduit increases X by ~30% but slightly reduces effective Z for typical PFs.
NEC voltage drop limits
NEC voltage drop is informational, not mandatory (with exceptions for sensitive equipment). Article 210.19(A) and 215.2(A) recommend:
- ≤ 3% on a branch circuit
- ≤ 5% on combined feeder + branch
For sensitive loads (motors, electronics), tighter design targets apply: 2% branch, 4% combined. NEC 695.7 mandates ≤ 5% for fire pump feeders. For solar PV, NEC 690.31(D) allows up to 3% for DC strings and 1.5% for AC inverter outputs.
Why power factor matters
For purely resistive loads (heaters, incandescent, UPF), V_d = 2IRL × 0.001 (single phase) — only resistance matters. For inductive loads (motors at PF = 0.85 lagging), the reactance X also drops voltage. Worse, X depends on conduit material (steel vs PVC) and conductor type (single vs multi-conductor cable) — see NEC Table 9.
For typical 480V three-phase motor circuits at PF = 0.85, X contributes 20–40% of total voltage drop on long runs. On short runs or low-current branches, R dominates and X is negligible.
NEC Table 9 — what's coded here
This calculator uses NEC Chapter 9 Table 9 values for typical conditions: PVC conduit, three-conductor cable, 60 Hz, 75°C operating temperature. R and X values:
- Copper (R, Ω/1000 ft): 14 AWG: 3.07; 12: 1.93; 10: 1.21; 8: 0.78; 6: 0.49; 4: 0.31; 2: 0.20; 1/0: 0.13; 4/0: 0.063; 500 kcmil: 0.029
- Aluminum (R, Ω/1000 ft): 12: 3.18; 10: 2.0; 8: 1.27; 6: 0.81; 4: 0.51; 2: 0.32; 1/0: 0.20; 4/0: 0.10; 500 kcmil: 0.046
- X (PVC conduit, all sizes): 0.05 Ω/1000 ft (all standard conductors)
For steel conduit, X roughly doubles. For multi-conductor cables in trays or messenger-supported, X is similar to PVC. For DC, X = 0 (no reactance), and only R matters.
Bigger conductor or paralleled?
If voltage drop is too high, options:
- Increase conductor size (lower R/1000 ft).
- Parallel runs (two #4/0 = effective #4/0 × 2 = ~250 kcmil).
- Reduce length (shorter feeder, intermediate distribution).
- Increase voltage (480V instead of 208V drops by V² ratio in current).
Going from 240V to 480V quadruples the apparent voltage-drop margin: same I → V_d in volts is the same, but the % drops to 1/4. This is why long industrial feeders run at 4160V or 13.2 kV.
Reference: National Electrical Code (NFPA 70-2023), Chapter 9 Table 9. Articles 210.19, 215.2, 695.7. IEEE Std 141 (Red Book): Recommended Practice for Electric Power Distribution for Industrial Plants.