Free tool · NEC Chapter 9, Table 8

Voltage Drop Calculator

By Mason Garry, Master Electrician · October 5, 2026

Long run, worried about voltage drop? Enter the circuit details — the calculator runs the circular-mils formula with copper's K constant and tells you the drop in volts and percent.

How the Math Works

Single-phase voltage drop, straight from the exam formula:

VD = (2 × K × I × L) ÷ CM

• K = 12.9 for copper (the constant — memorize it)
• I = load current in amps, L = one-way length in feet (the 2 covers the round trip)
• CM = circular mils from Chapter 9, Table 8 (12 AWG = 6,530 CM)

The Code recommends keeping drop under 3% on branch circuits and 5% total (feeder + branch) — 210.19(A) informational note. It's a recommendation, not a hard rule, but inspectors and exams both treat it as the target.

Keep Drilling

Voltage drop is one of 33 NEC topics. Sparky's 1,087 codebook-first questions drill all of them — every question a lookup rep with your book open.