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Wire Size Calculator

Find the minimum copper THHN wire gauge for a given ampacity per NEC Table 310.16.

Introduction

Wire Size Calculator — Find the minimum copper THHN wire gauge for a given ampacity per NEC Table 310.16. Enter Desired ampacity (A) to get an instant, accurate result.

Formula

Looks up the smallest standard copper THHN/THWN-2 conductor whose 75°C ampacity (NEC Table 310.16) meets or exceeds your required current.

Step-by-step

  1. Enter the Desired ampacity (A).
  2. Click Calculate to see your result instantly.

Real-world example

Example: With Desired ampacity (A) = 30, the Wire Size Calculator gives Awg: 10, Ampacity Amps: 35, Basis: NEC Table 310.16, copper THHN 75°C.

Frequently Asked Questions

Should I size for the load or for the breaker?
Wire ampacity must be at least equal to the breaker/fuse rating protecting it (with limited exceptions), and continuous loads should already be increased by 125% per NEC 210.19(A) before you check wire size.
Does this account for temperature or bundling derating?
No — it uses base 75°C copper ampacity from Table 310.16. High ambient temperatures, more than 3 current-carrying conductors bundled together, or aluminum conductors all require further derating per NEC 310.15, which isn't applied here.
Why 75°C and not 60°C or 90°C?
The 75°C column matches most common THHN/THWN-2 conductor terminations and equipment ratings; some devices (especially at 100A or less) are only rated for 60°C terminations, which would require using the lower 60°C ampacity column instead.
What inputs does the Wire Size Calculator need?
You'll need to provide: Desired ampacity (A). All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Wire Size Calculator?
The Wire Size Calculator applies the formula exactly as calculated — Looks up the smallest standard copper THHN/THWN-2 conductor whose 75°C ampacity (NEC Table 310.16) meets or exceeds your required current. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Wire Size Calculator free to use?
Yes, the Wire Size Calculator is completely free, requires no signup, and runs instantly in your browser.

About the Wire Size Calculator

The Wire Size Calculator uses a real, verifiable formula — Looks up the smallest standard copper THHN/THWN-2 conductor whose 75°C ampacity (NEC Table 310.16) meets or exceeds your required current. — so results are accurate every time, not an approximation.