Wire Derating ((free)) Access

While derating focuses on heat, undersized wires often suffer from increased voltage drop , which can damage sensitive electronics.

), a correction factor must be applied. These factors vary based on the temperature rating of the wire's insulation (e.g., How to Calculate Derated Ampacity wire derating

Does not include ground wires, neutrals that carry only imbalance (in multiwire branch circuits), or control wires not carrying continuous load. While derating focuses on heat, undersized wires often

You might look at a spool of THHN wire and see a label that says it is rated for 90°C (194°F). This is the wire's . However, the equipment you are connecting to (breakers, outlets, switches) is often only rated for 60°C or 75°C. You might look at a spool of THHN

For example, a wire rated for 20 Amps at 30°C (86°F) might only be rated for 16 Amps if the ambient temperature rises to 40°C (104°F). If you are running wires through a boiler room, a hot attic, or a rooftop in direct sunlight, you must derate based on temperature.

The default current-carrying capacity () of a wire is calculated for ideal conditions—usually a single cable in open air. In reality, several factors hinder heat dissipation: