Voltage Drop Calculator

How it’s calculated. NEC approximate (circular-mil) method: VD = (phase × K × I × L) ÷ (CM × sets). The phase factor is 2 for single-phase 2-wire and 1.732 (√3) for three-phase. L is one-way feet — do not double it, because the phase factor already accounts for the return path. CM is circular mils. The third option, per-conductor only, applies a factor of 1 and reports the drop in a single conductor; it is an advanced check, not a system type. Use 2-wire or three-phase for a real circuit.

The insulation / jacket sets the conductor’s temperature rating, which changes its resistance. We adjust K from the 75°C base (12.9 copper / 21.2 aluminum, Ω·cmil/ft) to the selected rating with K·(T∞ + T) ÷ (T∞ + 75) (T∞ = 234.5 copper, 228.1 aluminum) — e.g., copper at 90°C ≈ 13.5, at 60°C ≈ 12.3. Using the rated temperature is the conservative choice.

NEC recommends (informational) ≤3% branch and ≤5% total. Resistance-only — ignores power factor and reactance. For design/permit work rely on a stamped calculation; our engineering team performs full studies.

Worked example. A 20 A, 120 V circuit feeding a receptacle 150 ft from the panel on 12 AWG copper THWN (75°C, K = 12.9, 6,530 circular mils): VD = 2 × 12.9 × 20 × 150 ÷ 6,530 = 11.85 V, or 9.9% — more than three times the 3% branch-circuit guideline, and the receptacle would see about 108 V under full load. Moving to 10 AWG (10,380 cmil) brings it to 7.46 V (6.2%); 8 AWG (16,510 cmil) to 4.69 V (3.9%); and 6 AWG (26,240 cmil) to 2.95 V, or 2.5% — the first size inside 3%, and the calculator’s “min size” answer. That is why a long 120 V run to a parking-lot pole or a remote outbuilding so often lands several sizes above what its ampacity alone requires. Higher voltage is the cheaper fix: 100 A three-phase at 480 V on 1/0 copper (105,600 cmil) over 250 ft drops 1.732 × 12.9 × 100 × 250 ÷ 105,600 = 5.29 V, only 1.1%.

Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.