Wire Size Calculator
Pick the smallest standard copper conductor whose ampacity covers your load current, with the ampacity margin, metric cross-section and the code caveats that apply.
Last updated: 2026-09-15
How the calculation works
- The tool walks the standard copper conductor list (14 AWG up to 500 kcmil) and selects the first size whose 75 °C ampacity meets or exceeds your load current.
- It reports the margin between ampacity and load, plus the approximate metric cross-section for comparison with IEC sizing.
- Above 500 kcmil the tool indicates that parallel conductors are required — that is the practical code limit for a single raceway set.
Formula
Select the smallest conductor with ampacity ≥ load current Ampacity from NEC Table 310.16, 75 °C column, ≤3 current-carrying conductors
| Symbol | Meaning | Unit |
|---|---|---|
I | Load current | A |
A | Conductor ampacity | A |
Worked example
Interpreting the result
The result is a starting point, not a code determination. Real sizing must also account for termination temperature ratings (60 °C column for circuits ≤100 A in most cases), conductor bundling deratings, ambient temperature correction and continuous-load multipliers. When any of these apply, the required size is larger than the table result.
Assumptions
- Copper conductors, 75 °C insulation rating, no more than three current-carrying conductors.
- Ambient temperature 30 °C, no bundling derating.
- Non-continuous load (continuous loads need 125%).
Limitations
- Aluminum conductors require two sizes larger on average and are not covered here.
- Does not apply the 60 °C termination column required by NEC 110.14(C) for small circuits.
- Motor circuits follow NEC Part III of Article 430, not general ampacity.
Frequently asked questions
What wire size do I need for 30 amps?
10 AWG copper is the common answer for a 30 A branch circuit. The table lists 10 AWG at 35 A (75 °C), and the 60 °C column still supports 30 A. Always verify termination ratings and derating conditions.
Why does my load need 125% sizing?
Continuous loads (running 3 hours or more) must have conductors and overcurrent devices rated at 125% of the continuous current per NEC 210.20(A) and 215.3, to prevent thermal stress at full rated ampacity.
Is AWG the same as mm²?
No. AWG is a gauge system where each size change is a fixed area ratio; mm² is a direct cross-section. 12 AWG ≈ 3.31 mm², 10 AWG ≈ 5.26 mm². IEC and NEC cables are normally specified in their native system.