Wire Size Calculator
Undersized Wire Is a Fire Hazard, Not Just an Inefficiency
Every gauge of copper wire has a maximum current it can carry continuously without overheating its insulation. Pick a gauge too thin for the load and the wire itself becomes the weak point in the circuit — not the breaker, not the appliance. This calculator matches a load current against a simplified version of the NEC's standard copper ampacity table to recommend a minimum wire size.
The Reference Table
Based on a simplified reading of NEC Table 310.16 for copper THHN conductors (75°C column):
| Wire Gauge (AWG) | Ampacity |
|---|---|
| 14 | 20 A |
| 12 | 25 A |
| 10 | 35 A |
| 8 | 50 A |
| 6 | 65 A |
| 4 | 85 A |
| 3 | 100 A |
| 2 | 115 A |
| 1 | 130 A |
| 1/0 | 150 A |
| 2/0 | 175 A |
| 3/0 | 200 A |
| 4/0 | 230 A |
The Continuous Load Rule
Design Current = Load Current (if non-continuous)
NEC 210.19(A)(1) requires conductors supplying a continuous load to be sized at 125% of that load's current. A 16 A continuous load, for example, is treated as a 20 A design load when selecting a wire gauge — the calculator then picks the smallest gauge in the table whose ampacity meets or exceeds that design current.
Where This Calculation Matters
- New circuit installation — sizing a branch circuit before running wire, so the gauge matches both the load and the breaker.
- EV chargers and large appliances — these are almost always continuous loads, making the 125% factor especially relevant.
- Panel upgrades — confirming existing wiring is adequate before adding new circuits to a panel.
- DIY and contractor cost estimates — knowing the minimum gauge needed before pricing out materials.
How to Use This Calculator
- Enter the Load Current in amps.
- Select whether the load is a Continuous Load (running 3 or more hours) or Non-Continuous.
- Select Calculate to see the recommended minimum wire gauge and its rated ampacity.
Related Calculations
Confirm the chosen gauge won't lose too much voltage over distance with the Voltage Drop Calculator, or total up the load first with the Electrical Load Calculator.