Glossary: Ampacity
The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Factors Influencing Conductor Ampacity
Determining wire sizes is not just about measuring electrical loads; insulation heat tolerances, surrounding air temperatures, and raceway crowd factors limit the safe current load. Under the National Electrical Code (NEC), calculations must factor in:
- Conductor Material: Copper offers higher conductivity and ampacity ratings than aluminum for equivalent circular mil (AWG) sizes.
- Insulation Temperature Ratings: Common wire types like THHN, THWN-2, and XHHW carry distinct ratings (60°C, 75°C, or 90°C), which establish their baseline ampacities in NEC Table 310.16.
- Ambient Temperature Adjustment: If ambient temperatures exceed 30°C (86°F), baseline ampacity must be derated using correction factors to avoid melting conductor insulation.
- Conduit Conductor Derating: When more than three current-carrying conductors are bundled in a single raceway, their capacity is reduced (derated) to account for mutual heating limits.