A method is presented for calculation of ampacity of cable in fire protection wrapped cable trays. A brief description of the model is presented. The iterative technique used for solution of the model's simultaneous transcendental equations is presented with a description of the computer program. Cable derating for several typical installations is given and a graph is developed for use with ICEA P-55-440. To use the graph, the only data needed are the wrap thickness-to-conductivity ratio and emissivity. Ampacity derating derived from the model is also compared with published test results for two different fire wrap systems. >