When to use this: at design time or in the field, 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. Last reviewed June 2026 against NEC 2023/2026 Informational Notes.
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—
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.
