Tools · Voltage Drop
Voltage Drop Calculator
Estimate voltage drop and percentage drop for copper or aluminum conductors from 14 AWG to 1000 kcmil — single-phase or three-phase. Results update as you type.
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, 1 for a single (1-wire) conductor / one-way run, and
1.732 (√3) for three-phase. L is one-way feet and CM is circular mils.
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.
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