PICK THE RIGHT SIZE

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Picking the right wire size isn’t just about matching a breaker – it’s about handling real-world conditions: ambient temperature, bundling, insulation rating, and termination limits. This Wire Size Calculator helps you quickly find the smallest conductor size (copper or aluminum) whose derated, termination-limited ampacity still meets your required load current. Instead of flipping through multiple ampacity tables and doing derating math by hand, you get an instant shortlist of sizes and a clear “Selected Size” based on your inputs.

Under the hood, the tool starts from simplified ampacities for each wire size at 60°C, 75°C, and 90°C, then applies two key derating factors: one for ambient temperature above 30°C, and one for the number of current-carrying conductors in a raceway or cable. After that, it enforces an optional termination temperature limit (for example, many smaller devices are limited to the 60°C column, and many larger gear terminations are 75°C). The result is a “usable ampacity” that reflects both derating and termination constraints, and that’s what the calculator compares against your required load..

Start by entering your Required Load Current (A) – this is the continuous current you need the conductor to carry. Choose the Material (copper or aluminum), then select the Insulation Temperature Rating (60°C, 75°C, or 90°C) for the conductor type you’re using. If your terminations are limited to a specific temperature (for example, 60°C for smaller branch-circuit devices, or 75°C for many breakers and lugs), set that in the Termination Temperature Limit field; otherwise, leave it at “No explicit limit” to simply use the derated ampacity.

Next, enter the Ambient Temperature (°C) where the conductors will run and the Number of Current-Carrying Conductors in the raceway or cable. The calculator shows you the combined derating as a pill (ambient × bundle), the base column being used, and then generates a table listing each size’s base ampacities, derating, and final usable ampacity. The Result line at the top will highlight the smallest size whose usable ampacity is greater than or equal to your required load.

This calculator is designed as a quick design check and educational tool, not a replacement for the Code. Always confirm final selections against the latest CEC/NEC tables, local bylaws, and manufacturer data, and account for any additional requirements such as motor starting, voltage drop, or special conditions called out by the AHJ.