What are limitations of derating and where it is applied?
Answers
Derating factor depends on ambient temperature and how you are laying the cable i.e. in Air,Duct,Burried. Normally the current carring capacity of a perticular cable is calculated at 40*c, but if u lay the cable in air then it includes the temparture around it, if it increases more the 40*c then the conductor gets heated up by increasing the resistance in it thus the current carring capacity of a given cable decreases, so we will derate the cable.
Type of derating factor:
soil thermal resistivity
Conductor grouping factor(cable distrubtion in trays horizontial -verticals)
Tempt. derating factor
burial depth derating factor
SoilThermal resistivity : Basically the max temp in the surrounding. 40*C is the standard.
Conductor grouping : Due to arrangement of multiple conductors there is a development of an EM field which opposes the flow and thus we need to consider this factor.
Temp. Derating Factor : In this the arrangement of cables in a conduit is of prime importance. It should be in such a way that all cables have a minimum space to dissipate heat
into the surroundings. The cables can touch each other but anyone cable should not be fully surrounded by other cables.
Burial Depth : Depends on the length below ground where the conduit is laid.