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.

K ≈ 21.2 (vs 12.9 copper). Aluminum has more drop per gauge — typically use 1 size larger.

Voltage Drop Calculator

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

Result

Voltage drop

7.90 V (6.58%)

Exceeds 3% — upsize wire.

Current20 A
One-way length100 ft
Gauge12 AWG (CM 6530)
Drop (V)7.90
Drop (%)6.58%
NEC OK?✗

Step-by-step

Vd = (2 × K × I × L) / CM = (2 × 12.9 × 20 × 100) / 6530 = 7.90 V.
% drop = 7.90 / 120 × 100 = 6.58%.

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.