Wire Ampacity Calculator

Calculate the maximum safe ampacity for copper and aluminum wires. Includes AWG size charts, NEC temperature ratings, and wire gauge power capacity.

How Wire Ampacity is Determined

Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. This is not a fixed number for a wire; it changes based on how well the wire can dissipate heat.

The Ampacity Factors

While the calculator uses the base NEC Table 310.15(B)(16) values, actual safe ampacity is determined by:

• Conductor Material: Copper has lower resistance and higher ampacity than Aluminum for the same size.
• Insulation Rating: Wires like THHN (90°C) can handle more heat than TW (60°C).
• Ambient Temperature: If the air around the wire is hotter than 30°C (86°F), the ampacity must be “derated” (reduced).
• Conduit Fill: More wires in one pipe mean more trapped heat, requiring lower current per wire.

AWG Ampacity Reference Table (Copper)

This table summarizes the most common residential and commercial wire sizes for Copper conductors.

*Note: NEC 240.4(D) limits overcurrent protection for small conductors (14, 12, and 10 AWG) to these specific values regardless of insulation temp.

Copper vs. Aluminum Ampacity

Aluminum is a popular choice for large service entrance wires because it is lighter and cheaper than copper. However, aluminum has higher electrical resistance. As a general rule of thumb, you often need an aluminum wire that is two sizes larger (lower AWG number) than a copper wire to carry the same current safely.

The 80% Rule (Continuous Load)

For safety, if a load is expected to run for 3 hours or more (like an EV charger or a heater), it is considered a continuous load. In these cases, you should only load the wire to 80% of its rated ampacity. For example, a 20A circuit should only carry a 16A continuous load.