Voltage Drop: The 3% / 5% Guideline and the Calculation

Wire has resistance, so voltage "drops" as current travels down a long run. Too much drop means dim lights, sluggish motors, and wasted energy as heat. Here's what the code says and how to compute it.

1. The 3% / 5% guideline — and its legal status

The NEC recommends:

• ≤ 3% voltage drop on a branch circuit, and
• ≤ 5% for the combined feeder + branch-circuit path.

(CEC 210.19(A) Informational Note; CEC 215.2(A) Informational Note)

Know this distinction for the exam: these figures live in Informational Notes, which per CEC 90.5(C) are advisory, not mandatory — best-practice guidance, not an enforceable rule unless a jurisdiction specifically adopts them.

California caveat: the CA Energy Code (Title 24, Part 6) does impose mandatory voltage-drop limits for certain buildings/feeders. So in California the 3%/5% Electrical-Code figures are advisory, but a separate Energy-Code limit may still apply to a given project.