Is a 3 percent voltage drop mandatory under the NEC?

No, not as a general rule. The familiar 3 percent for branch circuits and 5 percent for branch circuit plus feeder combined figures come from Informational Notes under 210.19(A) and 215.2(A), and informational notes are explicitly non-mandatory text in the NEC. They describe a design practice that keeps equipment operating efficiently, they are not an enforceable limit an inspector can cite you for exceeding on a typical dwelling circuit.

Where voltage drop actually becomes mandatory

Certain specific applications do turn voltage drop into an enforceable requirement, fire pump conductors under 695.7 have a hard voltage drop limit at motor starting, and some healthcare and other specialized occupancies reference voltage drop limits directly in their governing article. Outside of those callouts, voltage drop calculations are a best practice tool, not a blanket code mandate, and local jurisdictions can adopt amendments that change this.

Why you should still calculate it

Motors that run hot and fail early, dimming lights on long runs, and electronic equipment that faults out on undervoltage are all real consequences of ignoring voltage drop even where it is not required. A long garage or shop feeder, an EV charger run to a detached structure, or a well pump circuit are classic cases where skipping the calculation causes a callback even though nothing on the print technically failed inspection.

Field takeaway

Treat the 3 and 5 percent figures as sound design guidance, not a code violation trigger, except on fire pumps and applications that reference voltage drop directly in their own article. Run the calculation anyway on long runs, because the customer feels the voltage drop even when the inspector cannot cite it.