Table of Contents
Wire Sizing Basics
Correct wire sizing must satisfy two criteria: ampacity (safety) and voltage drop (efficiency). The NEC provides minimum gauges for ampacity, but long distances often require larger wire. Low-voltage systems (12V, 24V) are especially sensitive since the same absolute drop is a larger percentage.
A 0.5V drop on 12V is 4.2%, but on 120V it is only 0.4%. This is why solar and automotive 12V systems need much heavier wire than household 120V circuits for the same current.
Voltage Drop Formula
Where ρ = 10.4 Ω·cmil/ft (copper), L is one-way length (ft), I is current (A), CM is wire area in circular mils. Factor of 2 is for round-trip.
NEC Ampacity (60°C copper)
| AWG | Amps | Circuit |
|---|---|---|
| 14 | 15A | Lighting |
| 12 | 20A | Outlets |
| 10 | 30A | Dryers |
| 8 | 40A | Ranges |
| 6 | 55A | Sub-panels |
Frequently Asked Questions
Why does distance matter?
Longer wire = more resistance = greater voltage drop. A 20A load on 12 AWG loses 0.32V over 10 ft but 3.2V over 100 ft. Keep total drop under 5%.
Wire for 30A EV charger 50 ft away?
10 AWG meets ampacity. At 50 ft on 240V, drop is ~1.5%. For 100+ ft, upgrade to 8 AWG.
Can I use aluminum?
Yes for large circuits. Use 2 sizes larger than copper equivalent with AL-rated connectors and anti-oxidant compound.