Voltage Drop on Long Runs — When 12 AWG Isn't Enough
A breaker trips and you find a roasted aluminum conductor at the panel. Or an outdoor light flickers on a long backyard run. Or a motor takes forever to spool up and finally bricks the start capacitor. All three are voltage-drop stories. The wire was big enough by the ampacity table — it just wasn't big enough for the distance.
The formula in one line
Single-phase: VD = (2 × K × I × L) / cm
Three-phase: VD = (√3 × K × I × L) / cm
K is a resistivity constant — 12.9 for copper, 21.2 for aluminum. I is the load current in amps. L is the one-way length in feet. cm is the wire's circular-mil area, which you read off the NEC chapter 9 table.
You then divide VD by the source voltage to get a percentage. NEC informational notes recommend keeping branch circuits under 3% drop and feeders under 2%, with the combined drop not exceeding 5%. Past 5%, equipment starts misbehaving — motors heat up because they pull more current to make the same torque at lower voltage, electronics see brownout, and the wire itself runs hotter than it should.
Why aluminum hurts more
Aluminum's K is 21.2, which is 64% higher than copper's 12.9. Same wire size, same current, same length — aluminum drops 64% more voltage. That's why aluminum service entrance is one or two sizes bigger than copper for the same ampacity, and why you cannot directly substitute one for the other.
Why three-phase helps
The factor changes from 2 (return path through neutral) to √3 (about 1.732). On the same circuit, three-phase delivers the same power with about 13% less voltage drop than single-phase. That's part of why long industrial feeders go three-phase.
A reference table you can almost memorise
Drop in volts per 100 ft of one-way run, copper, single-phase, at 20 A:
- 14 AWG: 12.5 V (5.2% on 240 V)
- 12 AWG: 7.9 V (3.3% on 240 V)
- 10 AWG: 5.0 V (2.1% on 240 V)
- 8 AWG: 3.1 V (1.3% on 240 V)
- 6 AWG: 2.0 V (0.8% on 240 V)
- 4 AWG: 1.2 V (0.5% on 240 V)
Numbers scale linearly with current and with length. So 12 AWG at 20 A over 200 ft drops twice 7.9 V = 15.8 V. At 30 A, drop scales by 30/20 = 1.5x. And so on.
When you need to actually worry
For a typical residential branch circuit at 120 V or 240 V, runs under 100 ft on appropriately-sized copper rarely exceed the 3% target. The math gets serious when you have:
- Long runs to a detached garage, well pump, barn, or RV pedestal
- High-current loads like EV chargers, electric ranges, water heaters
- Aluminum conductors rather than copper
- Low source voltage (120 V circuits drop noticeably more in percent than 240 V circuits)
- Three or more flexible loads at the end of a chain — drop accumulates
When you hit any of those, run the math.
A worked case
EV charger pulling 40 A continuous at 240 V single-phase. Garage is 150 ft from the panel. Copper.
By ampacity, 8 AWG at 50 A (75°C copper THHN) is enough for the 50 A breaker that protects a 40 A continuous load (NEC 625.41 + 80% rule). But voltage drop on 8 AWG over 150 ft at 40 A is about 2 × 12.9 × 40 × 150 / 16510 = 9.4 V, or 3.9% on 240 V. Marginal — over the 3% target.
Upsize to 6 AWG: voltage drop drops to about 5.9 V, or 2.5%. That's the right pick. Now the charger sees 234 V at the far end instead of 230, charging is faster, and the wire runs cooler.
Skip the math
The Voltage Drop Calculator does this in one shot. Pick a wire size, enter load + length + voltage + phase + material, and it returns the drop in volts and percent, plus the end-of-run voltage and a green/amber/red verdict against your target. When the answer's amber or red, it points you at the Wire Size Calculator to find the smallest size that fixes it.
The short version
The chart on the wall covers ampacity. Voltage drop is the second answer that nobody told you about. Long runs and high currents are where it dominates — and the fix is almost always one wire size up.