When to use this: at design time, to confirm the wire size you picked for ampacity also keeps voltage drop inside the recommended 3% branch / 5% total NEC targets. For an ampacity-first sizing pass, start with the NEC Wire Size Calculator and bring the result here to verify drop on long runs.

Voltage Drop Calculator

Compute drop in volts and percent for copper or aluminum conductors. Flags results above 3% (branch) and 5% (total) per NEC 210.19(A) Informational Note 4.

Results

Voltage drop—
Drop percent—
Voltage at load—
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Targets are NEC Informational Note recommendations (3% branch, 5% total) and not code-enforceable. Sensitive electronics, motors, and long EV-charger feeders may need tighter limits. Always also verify ampacity per NEC 310.16.

Last reviewed: April 2026

Tools we used or recommend for this work: a Klein clamp meter for measuring actual current under load, a Fluke voltage tester at the far end of the run, and Southwire THHN in the upsized gauge if you needed to bump up to keep drop under 3%. Links may be affiliate-tagged where the brand has a program.

Related tools: NEC Wire Size Calculator · Electrical Load Calculator (NEC 220) · Conduit Fill Calculator · All calculators