Related tools: Box Fill Calculator | Conduit Fill Calculator | Electrical Load Calculator. If this helped on a job, the blog has more field-tested notes on layout tools and coordination workflows.

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.

Load current (A)

One-way distance (ft)

System voltage (V)

Phase

Conductor material

Conductor size

Results

Voltage drop—

Drop percent—

Voltage at load—

Status—

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.

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