Published June 13, 2026
If you've spent any time around the trade, you've heard someone say "keep it under 3%" when talking about voltage drop. But where does that number actually come from, is it a hard rule, and what happens if you go over it? Here's the breakdown.
As current travels down a conductor, some voltage is lost to the resistance of the wire itself. The longer the run and the higher the current, the more voltage is lost by the time it reaches the load. This is voltage drop. A small amount is normal and unavoidable — the question is how much is too much.
The 3% and 5% figures most electricians reference come from Informational Notes in NEC 210.19(A) for branch circuits and NEC 215.2(A) for feeders. These notes recommend limiting voltage drop on a branch circuit to about 3%, and limiting the combined drop across the feeder and branch circuit to about 5%.
For most installations, no. Because these limits appear as Informational Notes rather than enforceable code sections, the NEC itself treats them as recommended design practice, not a violation if exceeded. The one major exception is Article 647 (Sensitive Electronic Equipment), where voltage drop limits are mandatory.
That said, don't take this as a green light to ignore voltage drop. Many local jurisdictions and inspectors do enforce the 3%/5% guidance as a practical standard, and excessive voltage drop can cause real problems even where it isn't technically a code violation — see below.
Voltage drop depends on the conductor material (copper vs. aluminum), wire size, circuit length, and the amperage of the load. The basic idea: longer runs and higher currents need larger wire to keep the percentage drop within the 3%/5% guidance.
The 3%/5% voltage drop guidelines aren't a hard NEC mandate for most circuits, but they represent solid, widely accepted design practice for a reason. When in doubt, size up — and always check with your local jurisdiction, since some AHJs do enforce these numbers as a practical requirement.
← Back to Blog