Ampacity of cables in solid bottom trays with and without covers

Nuclear Power Generating Stations utilize cable trays to route cables that are part of safety related systems. While these trays are predominantly ladder type (ventilated), there are several instances where solid bottom trays, with or without covers, are also applied. The guidance, to determine the ampacity of cables installed in ventilated or solid bottom trays without covers is given in Section 318-11 of the National Electric Code. Guidance to determine the ampacity of cables installed in ventilated or solid bottom trays with covers is also given under “Provision 2” of this same article which states, “a derating of not over 95% of the allowable ampacities for uncovered trays recommended”. However, adequate justification to this derating factor is not presented. Due to lack of any industry guidance to determine ampacity of cables in solid bottom trays (with or without covers) the authors performed the required analyses. A summary of the analytical techniques used and the derating factors applicable to the solid bottom trays are presented