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Derating factor:
Every equipment is manufactured to operate in certain ambient conditions. But when it is procured and operated at site, the equipment may not be operating under ambient conditions. So, in order to account for this, the equipment is derated.
For example, power cables are manufactured to operate at an ambient air temperature of 30 deg.C. But at site, the air temperature may be 45deg.C. So the current rating of the cable is derated. (i.e) it might have been designed to carry 70A at 30deg.C, but at 50 deg.C, it will carry only 60A.
Diversity factor:
In a plant, all the connected load are not operated at the same time. You may be having 4MW connected load, but you may be using only 3MW load at a time. So you can design the transformer such that it feeds 3MW instead of 4MW. Thus, you can reduce capital cost.
