Voltage Drop Explained: NEC Limits and Why They Exist

The physics, in short

Every conductor has resistance, and Ohm's law says current flowing through resistance produces a voltage loss (V = I × R). Wire resistance is small per foot, but it adds up over distance — which is why a 100-foot run drops noticeably more voltage than a 10-foot run carrying the identical current on identical wire. The load at the far end of the wire receives the source voltage minus whatever was lost to that resistance along the way.

The 3%/5% numbers aren't a hard code violation — usually

This surprises people: the NEC's voltage-drop guidance (in Informational Notes under sections like 210.19(A) and 215.2(A)) recommends 3% maximum for a branch circuit alone and 5% for a branch circuit plus its feeder combined — but as written in the base NEC, these are informational, not mandatory. That said, plenty of local jurisdictions adopt amendments that make some form of voltage-drop compliance mandatory for permitted work, and inspectors in many areas will flag an obviously excessive drop even without a local amendment forcing the issue. Treat 3%/5% as the safe design target regardless of whether your specific jurisdiction enforces it as code — the consequences below happen either way.

What actually happens when you exceed it

• Motors run hotter, less efficiently, and with reduced starting torque — shortened lifespan on compressors, pumps, and HVAC equipment is a common real-world symptom.
• Lighting visibly dims, most noticeably under simultaneous load (multiple fixtures or appliances drawing current at once).
• Electronics and control equipment can behave unpredictably — anything with sensitive power-supply tolerances is more exposed than simple resistive loads.
• Nuisance issues compound over the life of the circuit rather than showing up immediately, which is exactly why it's easy to under-size a long run and not notice the problem until well after the work is done and inspected.

Why longer runs are where this matters most

Voltage drop scales directly with distance, but ampacity (the safety rating covered in the NEC wire size chart) doesn't change with distance at all — a given AWG size is rated for the same current whether the run is 10 feet or 200 feet. That gap is exactly why a wire that's completely code-legal on ampacity can still fail a voltage-drop check on a long run, and it's why this site's voltage drop calculator checks the two requirements separately rather than assuming ampacity alone tells the whole story.