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.
Open calculatorLast 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.
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.
Sizing is a chain, not a single lookup
A real wire-size decision runs four checks in sequence: ampacity (the table lookup this calculator performs), voltage drop on the actual run length, overcurrent protection matching the conductor, and the terminal temperature rating that caps the ampacity column you may use. Stopping after the first check is the classic undersizing error: a conductor that passes ampacity can still fail the 3% drop rule on a long run, and the fix is a size up, not a bigger breaker.
The 125% rule and continuous loads
- Continuous loads (3+ hours) require conductors and OCPD sized at 125% of the load — 16 A continuous means a 20 A circuit, not 15.
- Derating for ambient temperature above 30 °C and for bundled conductors stacks multiplicatively with the 125% rule — check both before finalizing.
- The calculator uses the 75 °C column for typical installations; 60 °C terminations (older devices, some equipment lugs) cap the table lower regardless of conductor rating.
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.