Wire Gauge Ampacity Calculator
Find the correct AWG wire gauge for any electrical circuit based on current, conductor material, insulation rating, and conduit fill. References NEC ampacity tables.
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Recommended Wire
12 AWG (copper) — 20A rated
The Formula
Wire gauge selection uses NEC Table 310.15 ampacity ratings, adjusted for temperature rating and conduit fill derating factors. The required ampacity must equal or exceed the load current divided by the derating factor.
Variable Definitions
American Wire Gauge
Smaller AWG number = thicker wire = higher ampacity. 14 AWG is the smallest common household wire. Each gauge step (e.g., 14 to 12) approximately doubles the cross-sectional area.
Conduit Fill Derating
Multiple conductors in conduit generate mutual heat, requiring derating: 80% for 4-6 conductors, 70% for 7-9, 50% for 10+. This means a 20A circuit in a conduit with 4 conductors must use wire rated for 25A (20 ÷ 0.8).
Current-Carrying Capacity
The maximum continuous current a wire can safely carry without exceeding its insulation temperature rating. Depends on gauge, material, and insulation type.
How to Use This Calculator
- 1
Enter the circuit current in amperes.
- 2
Select copper or aluminum conductor.
- 3
Select the insulation temperature rating.
- 4
Select how many conductors share the conduit.
- 5
The calculator recommends the correct AWG gauge per NEC tables.
- 6
Always consult a licensed electrician for actual installations — local codes may differ.
Quick Reference
| From | To |
|---|---|
| 14 AWG Copper (60°C) | 15 A (lighting, outlets) |
| 12 AWG Copper (60°C) | 20 A (kitchen, bathroom) |
| 10 AWG Copper (60°C) | 30 A (dryer, AC) |
| 8 AWG Copper (60°C) | 40 A (range, EV charger) |
| 6 AWG Copper (60°C) | 55 A (subpanel, large appliance) |
| Resistance of 12 AWG | ~0.0016 Ω/ft |
| Resistance of 14 AWG | ~0.0025 Ω/ft |
| Derating: 4-6 conductors | 80% of rated ampacity |
| Derating: 7-9 conductors | 70% of rated ampacity |
Common Applications
- Home electrical wiring — select the correct wire gauge for new circuits, whether running lighting in 14 AWG or installing a 50A EV charger circuit in 6 AWG copper
- Industrial panel building — size power and control wiring for motor control centers, PLC cabinets, and distribution panels per NEC code requirements
- Solar panel installation — calculate wire gauge between panels, inverter, and battery bank to minimize voltage drop over long DC runs at high current
- Automotive and marine wiring — choose wire sizes for 12V and 24V systems where voltage drop is more critical due to the lower voltage baseline
- Audio system installation — select appropriate speaker wire gauge based on run length and impedance to minimize signal loss and maintain audio quality
Wire gauge comparison — smaller AWG number means thicker wire with higher ampacity
Understanding the Concept
Selecting the correct wire gauge is critical for electrical safety. Undersized wire overheats, degrades insulation, and can cause fires. The NEC (National Electrical Code) publishes ampacity tables based on wire gauge, material, and insulation type. When multiple conductors share a conduit, they generate mutual heat and must be derated. Always consult a licensed electrician for actual installations. Real-world scenario: you are running a 20A kitchen circuit with 4 conductors in a conduit (two hots sharing a neutral, plus ground). At 80% derating, the required ampacity is 20 ÷ 0.8 equals 25A. With 75°C rated copper wire, 12 AWG is rated for 25A — just barely sufficient. If the ambient temperature is high (for example an attic), additional derating may push the requirement to 10 AWG. This is why professional electricians always verify ampacity for the specific installation conditions. Another common scenario: running a 50A EV charger circuit. At 75°C copper, 6 AWG is rated for 65A and would work. But if the run is in conduit with 4 other circuits (requiring 80% derating), the effective capacity of 6 AWG drops to 52A — still sufficient for 50A. If there are 7 conductors total, derating to 70% drops capacity to 45.5A, requiring an upgrade to 4 AWG.
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