Electrical conductors generate heat due to internal resistance ( I2Rcap I squared cap R
[ 200 , A \times 0.446 = 89.2 , A ]
: Base tabulated ampacity under standard reference conditions. Cacap C sub a : Ambient temperature correction factor. Cgcap C sub g : Grouping or bundling correction factor. Cscap C sub s derating factor for cables
: Soil thermal resistivity correction factor (for underground installations). Cdcap C sub d
When multiple active electrical circuits run adjacent to each other in a shared conduit, cable tray, or trench, their collective thermal fields overlap. This mutual heating traps thermal energy, driving up core conductor temperatures. Cscap C sub s : Soil thermal resistivity
), the target ampacity must be divided by the total derating factor to find the minimum required baseline cable rating:
Iz=It×Ca×Cg×Cs×Cdcap I sub z equals cap I sub t cross cap C sub a cross cap C sub g cross cap C sub s cross cap C sub d Izcap I sub z : De-rated safe continuous ampacity (Amperes). Itcap I sub t ), the target ampacity must be divided by
Deeper burial reduces heat dissipation (soil is cooler but more insulating).
| | Derating (approx.) | |------------|------------------------| | 0.5 m | 1.02 | | 0.8 m | 1.00 (reference) | | 1.0 m | 0.98 | | 1.5 m | 0.94 |