What Is Voltage Drop? The 3% and 5% Targets Explained
Every conductor resists current, and resistance over length steals voltage before it reaches the load. That theft is voltage drop — invisible on a breaker, obvious in a dimming garage light or a compressor that trips on hot afternoons.
Quick answer: Voltage drop is the voltage lost to conductor resistance: VD = 2×K×I×L ÷ circular mils (single phase), K = 12.9 copper / 21.2 aluminum. The NEC's informational notes target 3% for a branch circuit and 5% total from service to load. Fix a failing run by stepping up wire size, shortening the run, reducing load current, or moving to a higher supply voltage.
The physics in one paragraph
Current flowing through resistance produces a voltage loss proportional to both (V = I×R). Longer runs mean more resistance; more amps mean more loss, linearly. A 12 AWG conductor is about 1.6 ohms per thousand feet — 100 feet of it at 20 A burns roughly 3.2 V going out and back. On a 120 V circuit that's 2.7%, on the same current at 240 V it's only 1.3% — which is why long runs belong on higher voltages.
Why the NEC only 'suggests' 3%
- The 3%/5% figures live in informational notes (210.19, 215.2) — performance guidance, not enforceable text.
- What IS enforceable: equipment that works. Under-voltage stalls motors, trips electronics, and cooks compressors.
- Utilities are typically held to a separate service-voltage standard (often ±5%); the NEC notes cover your wiring after the meter.
- Inspections may cite drop indirectly — via a failed appliance or a load calc that ignores distance.
Symptoms of excessive drop
- Lights dim noticeably when a big appliance starts (and stay dim while it runs)
- Motors run hot, hum, or trip thermal overloads on long afternoons
- Electronics reboot during compressor start; UPS units flag low voltage
- Electric heat takes visibly longer; EV charging reports reduced rate
The four fixes, in order of preference
- Step up conductor size — the 12 AWG that fails at 100 ft becomes 8 AWG; cost scales but so does capacity.
- Raise the voltage — a 240 V circuit halves the current (and the drop) versus 120 V for the same watts.
- Shorten or reroute the run — sometimes a different panel path or subpanel beats bigger copper.
- Cut the load — a heat-pump water heater instead of resistance, or staged motors, reduces current at the worst point.
Worked example
A 40 A, 240 V EV charger 150 feet from the panel on 8 AWG copper: VD = 2 × 12.9 × 40 × 150 ÷ 16,510 = 9.4 V = 3.9% — fails the 3% target. Step to 6 AWG (26,240 cmil): 5.9 V = 2.5% — passes with margin. The voltage drop calculator on this site runs exactly this comparison across every size in one pass.