Wire Gauge (AWG) Calculator
AWG Wire Sizing — Ampacity, Voltage Drop, and Safety
Selecting the wrong wire gauge is one of the most common electrical wiring mistakes — and one of the most dangerous. Undersized wire overheats under load, which can melt insulation and start fires without tripping the breaker. This calculator shows ampacity, voltage drop, and safety status for any AWG copper wire size at any current and circuit length, so you can confirm your wiring before you close the walls.
How to Use This Calculator
Select the wire gauge (AWG), enter the circuit length (one-way, in feet), your current draw, and source voltage. Hit Calculate to see max ampacity, load percentage, voltage drop, and safety status. A green "OK" means both ampacity and voltage drop are within acceptable limits.
The Voltage Drop Formula
The factor of 2 accounts for both the hot and neutral conductors (current travels the full circuit length twice). For 12 AWG (0.00193 Ω/ft) at 15 A over 50 ft: Vdrop = 2 × 50 × 15 × 0.00193 = 2.9 V = 2.4% of 120 V — just inside the NEC 3% recommendation.
AWG Ampacity Quick Reference
- 14 AWG — 15 A: Standard lighting circuits and general outlets (15A breaker required)
- 12 AWG — 20 A: Kitchen outlets, bathrooms, laundry (NEC required)
- 10 AWG — 30 A: Electric dryers, water heaters, EV Level 1 chargers
- 8 AWG — 40 A: Electric ranges, large AC units, 32A EV Level 2 charger
- 6 AWG — 55 A: Large HVAC, 48A EV chargers, sub-panel feeders
- 4 AWG — 70 A: Sub-panels, heavy commercial equipment
- 2/0 AWG — 145 A: 125A service, large commercial loads
- 4/0 AWG — 195 A: 200A residential service entry
The NEC 80% Continuous Load Rule
Continuous loads — those running for 3+ hours — must not exceed 80% of rated ampacity. A 20A circuit carries no more than 16A continuously; a 15A circuit, 12A. This headroom prevents heat accumulation in conductors and protects the breaker mechanism from sustained high-temperature operation. HVAC, water heaters, and EV chargers are all considered continuous loads.
Frequently Asked Questions
Can I use 14 AWG wire on a 20A circuit?
No — this is a code violation and a fire hazard. Wire gauge must match or exceed the breaker protecting it. A 20A breaker allows 20A to flow before tripping. 14 AWG is rated for 15A and will overheat at 20A without the breaker tripping to protect it. The rule: 15A breaker → 14 AWG minimum; 20A breaker → 12 AWG minimum; 30A breaker → 10 AWG minimum.
When should I upsize wire gauge to reduce voltage drop?
When your circuit run exceeds 50–75 feet, voltage drop becomes significant. For sensitive equipment (motors, electronics, battery chargers), keep drop under 3%. For long runs to outbuildings, subpanels, or outdoor equipment, calculate drop for your actual current and length. Upgrading one gauge (e.g., 12 AWG to 10 AWG) roughly halves voltage drop for the same run length.
Does wire gauge affect EV charger performance?
Significantly. An EV charger running at 48A continuous on a long or undersized run can lose 5+ volts to resistance — reducing actual charging power and creating heat in the wire. Size EV charger wiring generously: use 6 AWG or heavier for 40–50A chargers and calculate voltage drop for your specific run length. Many electricians recommend upsizing one gauge for future-proofing as charger capacities increase.
What's the difference between copper and aluminum wire ampacity?
Aluminum has roughly 60% of copper's conductivity — you need a larger gauge to carry the same current. Aluminum 2/0 AWG ≈ copper 1/0 AWG in ampacity. Aluminum is cheaper and lighter, so it's common in service entrance cables and large feeders. In residential branch circuits, modern aluminum wiring with CO/ALR-rated devices is safe, but most electricians use copper for branch circuits due to simpler termination requirements.
How does ambient temperature affect wire ampacity?
Higher ambient temperature reduces wire ampacity because the wire can't shed heat as effectively. NEC tables are based on 30°C (86°F) ambient. In attic spaces reaching 50°C+ in summer, wiring must be derated — often to 58% of rated capacity. This is why electricians use 90°C-rated wire (THHN) in conduit through hot spaces. Always check the specific derating table for your installation's ambient temperature and conduit fill conditions.