Ampacity Derating
Ampacity Derating : The process where maximum current capacity of power cables is decreased by the insulating properties of all that immediately surrounds the conductors, including firestops. Thus, if a power cable were to be derated by 50%, one would have to run two such cables, or simply use a much larger cable, in order to conduct the same amount of electrical power. Also, the better a thermal insulator surrounds the conductor, the greater the derating. This applies to firestops, as well as fireproofing around the cables. Thus, a dense firestop mortar is expected to have a lesser derating effect (which is GOOD!) upon cables than a rockwool and rubber seal. Cables heat up while they are conducting power, just like the power cord attached to one's domestic vacuum cleaner, coffee machine, or indeed most any electrical appliance will heat up when in use. The more one insulates against it, thus containing that heat, the less electrical power can be sent through that cable. Cable manufacturers can at times mandate derating for certain ways of running the cables, such as inside of cable tray, fire protective wraps or even conduit. Derating is not required on control, data or communication cables. Ampacity derating and the effect of firestops or fire protective wraps (emergency circuit protection to keep conductors from shorting out during a fire) upon derating can now be quantified by testing in accordance with IEEE Standard 848 - 1996 Standard Procedure for the Determination of the Ampacity Derating of Fire-Protected Cables. Typically, this is of concern primarily in large power distribution centres, such as power generating stations
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