The 3% and 5% figures everyone quotes are recommendations, not requirements — here is exactly where they come from and when they become enforceable.
NEC 210.19(A), Informational Note No. 4 recommends that branch-circuit conductors be sized so the voltage drop does not exceed 3% at the farthest outlet for power, heating and lighting loads — and that the combined drop on feeder and branch circuits not exceed 5%. NEC 215.2(A), Informational Note No. 2 repeats the same guidance from the feeder side.
Per NEC 90.5(C), informational notes are explanatory and not enforceable. However, the limits become mandatory in several common situations: energy codes (IECC C405.9 has required ≤2% feeder / ≤3% branch drop in commercial work in many adoptions), fire pumps (NEC 695.7 caps starting drop at 15% and running at 5% — a hard requirement), sensitive equipment specs, and any jurisdiction or project specification that adopts the notes as binding. When in doubt, treat 3%/5% as the design target and confirm with the AHJ.
Excessive drop dims lighting, overheats motors (a motor compensates for low voltage by drawing more current), trips electronics, slows charging and wastes energy as cable heat. The 5% figure roughly bounds equipment to its nameplate ±5% tolerance after utility variation.
Check any circuit against these limits instantly with the voltage drop calculator — the verdict badge cites the exact note that applies.
Under the base NEC, no — the 3%/5% figures are informational. But it may violate an adopted energy code, a project spec, or simply make equipment misbehave. Treat it as a defect to fix, not a code citation to fear.
NEC Article 690 (solar) and low-voltage articles carry the same philosophy. Industry practice for 12–48 V systems is ≤3% for critical circuits precisely because the absolute volts are so scarce.