Wire size calculator
Enter the load and the conditions in the raceway to get the conductor size, the breaker, and every derating step that led to it.
Inputs
Result
Every size under these conditions
Base ampacity from your chosen column, then corrected and adjusted, then capped by the termination column.
| Size | Base, A | After derating | Termination limit | Usable, A |
|---|
The three limits that decide a wire size
A conductor has to pass all three, and the smallest answer wins.
- Ampacity after derating. Start in the column for the conductor's insulation rating in Table 310.16, then multiply by the ambient correction from Table 310.15(B)(1) and the adjustment factor from Table 310.15(C)(1).
- Termination temperature, 110.14(C). Equipment terminations are usually rated 60 °C up to 100 A and 75 °C above that, unless the equipment is marked otherwise. You may start from the 90 °C column for derating, but the final ampacity cannot exceed the termination column's value.
- Overcurrent protection, 240.4. The device protects the conductor. 240.4(D) caps small conductors regardless of ampacity: 15 A for 14 AWG copper, 20 A for 12 AWG, 30 A for 10 AWG.
What counts as a current-carrying conductor
Ungrounded conductors always count. A neutral counts only when it carries current in normal operation, so the neutral of a balanced 3-wire circuit does not, while the neutral of a circuit supplying nonlinear loads does. Equipment grounding conductors never count. The adjustment factor only applies when more than three current-carrying conductors run together for more than 24 inches.
Questions
What size wire do I need for 100 amps?
With 75 °C terminations and no derating, 3 AWG copper or 1/0 aluminum carries 100 A. Many jurisdictions see 4 AWG copper on a 100 A dwelling feeder, which comes from the separate 83% allowance in NEC 310.12 rather than from this general calculation.
Can I use the 90 °C column?
For derating, yes, if the conductor is rated 90 °C such as THHN. The final ampacity still cannot exceed the 60 °C or 75 °C termination column, because the connection points are the weak link, not the insulation.
When does the 125% continuous factor apply?
When the load runs for three hours or more without interruption. Tick the box and the calculator sizes the conductor and device for 125% of the load.
Do ground wires count toward the conductor count?
No. Equipment grounding conductors are not current-carrying under normal conditions, so they are excluded from the adjustment factor, though they still count toward conduit fill.
Why does my 12 AWG only get a 20 amp breaker?
NEC 240.4(D) limits overcurrent protection on small conductors regardless of their ampacity: 15 A on 14 AWG copper, 20 A on 12 AWG and 30 A on 10 AWG.
More Electrical calculators
Voltage drop calculator
Drop in volts and percent for copper or aluminum, single-phase, three-phase or DC.
Conduit fill calculator
Fill percentage and the smallest EMT, rigid or PVC conduit for a mix of THHN conductors.
Electrical load calculator
Dwelling service size by the NEC 220.82 optional method, with every demand factor shown.