Why is there a derating factor for ampacity on nickel-plated conductors? Once a base ampacity is determined from the Tables in the National Electric Code Handbook, additional factors must be considered. The base ampacity may be defined as the value for a standard size (cross-sectional area) of a particular conductor material at normal ambient temperature. The ability of the conductor to pass this amount of current along its length can be greatly affected by the buildup of heat. Heat can be introduced into the conductor by resistance to the passage of current through the metal, by exposure of the cable to radiant heat or by exposure to high ambient temperatures. Sometimes the cable may be exposed to all three of these concurrently or in sequence. With this exposure, heat is built up within the metallic conductor and this alters the conductor's ability to transfer electrical energy. Depending upon the method of installation (i.e., conduit, cable tray or directly buried), the cable may be able to dissipate this heat to a greater or lesser degree. To account for all these external factors that may impact the conductor's function, a series of de-rating factors have been postulated. Depending upon the ambient temperature, for example, the capacity of the conductor to pass the maximum current is diminished by a percentage of the original (base) value. Where multiple conductors are installed together in a confined space - which diminishes their ability to transfer heat - another de-rating factor is applied again, diminishing the allowable ampacity by a percentage of the base value. Details of these factors together with an explanation of their application may be found in Article 310 of the National Electric Code Handbook.