Voltage drop on a long wire run is one of those electrical problems that doesn't trip a breaker — the circuit just runs warm, motors hum harder, and LED lights flicker. The NEC's 3% / 5% recommendation is informative (not enforced), but every electrician follows it because the alternatives are equipment failures and unhappy customers. Here is the math.

The 3% / 5% rule

The National Electrical Code recommends:

  • ≤ 3% drop on the branch circuit alone — the wire from the panel to the outlet
  • ≤ 5% drop combined on feeder + branch — the wire from the service to the panel, plus the wire from the panel to the outlet

Per NEC Article 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2. "Informational" means they aren't code-violations on their own — but every jurisdiction inspector reads them as the standard, and energy-code adoption (IECC) is gradually making them enforceable.

The math — 2KIL ÷ CM

Voltage drop on a single-phase circuit:

VD = 2 × K × I × L ÷ CM

  • K = 12.9 for copper at 75°C; 21.2 for aluminum
  • I = current in amps
  • L = one-way length in feet (formula doubles it for round-trip)
  • CM = wire circular mils from NEC Chapter 9 Table 8

Run your numbers on the voltage drop calculator.

When voltage drop matters most

The cases that bite electricians:

  • Long runs to detached structures. A garage 150 ft from the house on a 20A circuit can lose 6% on 12 AWG — switch to 10 AWG or 8 AWG.
  • Pool pumps and well pumps. Inductive loads draw 3-5× the running current at startup. A pump that runs at 12A but starts at 50A on a borderline-sized wire trips overload protection.
  • HVAC condensers. Compressor short- cycling under low voltage costs the compressor in 3-5 years vs the rated 15-20.
  • EV chargers. A 40A Level 2 charger on a 100 ft run needs 8 AWG, not 10, despite both being ampacity-legal.

Aluminum runs higher

Aluminum has 60% the conductivity of copper, so you need one to two AWG sizes larger to hit the same drop. Service entrance feeders are commonly aluminum (cheaper for the large gauges), but 10 AWG and smaller is almost always copper. The calculator's K = 12.9 is for copper — multiply its result by 1.6 for aluminum.

Common errors

Forgetting to double the length. The formula uses one-way distance, then multiplies by 2 inside to account for the return path on the neutral. Many online calculators confuse this — be sure you're entering one-way distance.

Using ampacity instead of actual load. A 20A breaker doesn't mean a 20A continuous load. Use the design load (motor running current, EVSE charging current, total fixture wattage), not the breaker rating.

Solving the drop with longer runs. If a 100 ft run on 12 AWG is borderline, going to 12 AWG with a 150 ft run isn't fixing anything — drop scales linearly with length. Upsize wire, not the run.

Quick FAQ

What's the maximum length for 12 AWG on a 20A circuit? About 60-70 ft for 3% drop at full 16A continuous load (120V single-phase). Less if the circuit runs near full capacity.

Does voltage drop apply to DC circuits? The same physics, different formula — drop on a DC circuit is 2 × R × I where R is wire resistance per foot. Solar PV systems on long roof-to-inverter runs use the same NEC 3% target.

Why does my LED light flicker on a long run? LED drivers are sensitive to low voltage. A 6-8% drop on a 120V circuit pushes the driver below the input voltage threshold and it cycles on/off. Upsize the wire or add a voltage stabilizer.

Estimate only. The voltage drop calculator uses K = 12.9 for copper at 75°C single-phase. Three-phase circuits use the same formula with √3 in the denominator; derate aluminum by 1.6. Verify designs for sizing critical circuits (pumps, HVAC, EVSE) with a licensed electrician.