Derating Cables in Trays Traversing Firestops or Wrapped in Fireproofing

Using ICEA Standard P-54-440 as the basis, this paper derives simple equations to calculate the maximum permissible load losses for cables in trays passing through firestops or covered with fireproofing material. From these loss values are deduced ampacity multiplying factors which when applied to the ICEA standard ampacities, provide the permissible ampere loading for the cables. A basic factor in the derivation is the use of previously published experimental data relating to the heating of cables in firestops to calculate the thermal conductance of the firestops.