Voltage Drop Calculator

Vd = (2 × K × I × L) / CM for single-phase circuits. NEC recommends ≤ 3% drop.

Inputs

Result

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How to use this calculator

  • Enter current and one-way length.
  • Pick wire gauge.
  • Read drop and verify ≤ 3%.

About this calculator

Voltage drop on long runs: longer the wire, more drop. NEC recommends ≤ 3% drop on branch circuits, ≤ 5% combined feeder + branch. Standard formula uses K = 12.9 for copper at 75°C; CM = circular mils (cross-sectional area). For aluminum, K ≈ 21.2.

Frequently asked

Why 2× length?+
Current flows out through hot, back through neutral. Total wire = 2 × one-way distance.
What if drop exceeds 3%?+
Upsize to next gauge. 12 AWG to 10 AWG halves the resistance, halves the drop.
When does drop matter most?+
Long runs to outbuildings, dedicated motor circuits, sensitive electronics.
3% is recommendation, not code?+
For branch circuits, NEC 210.19 informational note recommends 3%. Not strictly required but good practice.
Aluminum K?+
K ≈ 21.2 (vs 12.9 copper). Aluminum has more drop per gauge — typically use 1 size larger.

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