Wire Ampacity.

What Is a Wire Ampacity Calculator?

A wire ampacity calculator is an essential safety tool used in electrical engineering to determine the absolute maximum amount of current that a specific wire can carry before its insulation melts or catches fire.

According to the National Electrical Code (NEC), pushing too much current through a wire creates immense heat due to the metal's natural resistance. By matching the wire's physical thickness (AWG), metal type (Copper or Aluminum), and the thermal rating of its plastic insulation, this tool pulls exact safe limits directly from NEC Table 310.16.

How to Use This Wire Ampacity Calculator

1. Select Conductor Material: Copper is the standard for household branch circuits. Aluminum is typically used for heavy service feeders to save cost.
2. Select Temperature Rating: This is printed on the wire's jacket. For standard NM-B (Romex®) inside a wall, you must legally use the 60°C rating.
3. Select Wire Size: Choose your AWG size. Lower numbers mean thicker wire, which can handle higher amperages safely.

The 60°C / 75°C / 90°C Rule

The biggest mistake beginners make is always selecting the 90°C column just because their wire says "THHN 90°C" on the jacket. The NEC dictates that your entire circuit is only as strong as its weakest link.

• Most household breakers and device terminals are only rated for 60°C or 75°C.
• Therefore, even if you run 90°C wire, you usually have to limit your ampacity to the 75°C column to prevent the ends of the wire from transferring too much heat into the circuit breaker itself.

Frequently Asked Questions