When distance picks the wire
What you'll learn
Ampacity ignores distance; the motor doesn't. Check voltage drop with the √3 formula and upsize a 350 ft run past what the table asked for.
Ampacity has no idea how long your run is. A 2 HP motor at 240 V draws 6.8 A, and 14 AWG carries that with room to spare — whether the motor is 9 feet away or 900. But the voltage arriving at the motor cares about every foot.
The rooftop fan
The roof exhaust fan sits 350 ft from the panel. Wire has resistance, and current through resistance costs voltage:
drop = √3 × 12.9 × amps × feet ÷ circular mils
(12.9 is copper's constant; circular mils is the wire's cross-section from Chapter 9, Table 8; the √3 is the three-phase factor.) The target is 3% or less — an informational note in the code, but a motor fed thin runs hot, torques weak, and trips overloads on hard starts.
At 350 ft, 14 AWG loses 5.4%. Not a code violation — just a motor quietly being starved. The fix is the only lever the formula offers: more circular mils.
| wire | drop @ 350 ft | verdict |
|---|---|---|
| 14 AWG | 5.4% | starved |
| 12 AWG | 3.4% | still over |
| 10 AWG | 2.1% | healthy |
Distance picked the wire
So the run gets pulled with 10 AWG — two sizes above what ampacity asked for. The breaker doesn't move (it keys off FLA, not wire), the overload doesn't move. Only the copper grows.
The habit to build: any long run, check the drop. The designer does it on every calculation — tap the conductors chip on the fan circuit and you'll see the honest note: upsized from 14 AWG for the run. Drag the run length shorter and watch the wire relax back down.
Check your understanding
Question 1 of 2
The rooftop fan's 350 ft run reads 5.4% drop on 14 AWG — ampacity-legal. What's the real-world consequence of leaving it?