A 10 mm² copper cable has a base ampacity of 60 A in free air at 30°C. If installed in a 45°C ambient (factor 0.79) with three other circuits (factor 0.70), the final ampacity is: 60 A × 0.79 × 0.70 = 33.18 A . Thus, the load current must not exceed 33 A, not the nominal 60 A.
: Ensure your circuit breaker and load do not exceed this adjusted value. Common Standards
A is a vital reference tool in electrical engineering used to adjust a cable's current-carrying capacity (ampacity) based on its real-world installation environment. Standard ampacity tables are typically based on ideal conditions—such as a single cable in free air at 30°C (86°F)—but actual projects often involve bundled cables, high temperatures, or underground burials that trap heat.
Note: Factors apply to adjacent cables. If spacing between cables is greater than the cable diameter, factors may be improved.
Most tables break down derating into two primary categories: 1. Ambient Temperature Derating