MCC Cable Selection NEC and BS

NEC is very much a "rule" based standard. British Standard, maybe partly because of move to common European Standards ,is much more "Principle" based. A set of rules which covers all cases must allow for the most severe. In most cases, plant and cables will be over-sized.

British Standard, working on principles, requires that cables should deliver adequate voltage at the point of use, not be overheated by normal loads or short circuits and in the case of motors, have sufficient voltage at starting currents. This allows the designer to ensure that motors are started one after another, to keep the peak current down. Also he could select motors with low starting current relative to running current or inverter drives which could reduce the peak demand or the normal load by operating the load at reduced speed. Of course, this means the designer must keep a design file which proves the standards are met in unusual circumstances. An example is that years ago "IEE Regulations" required that if you branched off with a smaller cable, the fuse had to be at the place where the smaller cable began. This is no longer so - IF you can prove that the small cable is short-circuit protected adequately by a fuse up-stream which has a bigger rating than the cable, THEN the OVERLOAD protection fuse can be put at the LOAD end of the small cable. The end result is that the branch point does not have to be at a place easy to reach to change a fuse. This can reduce the cost to the customer, over a housing project, for example, despite the design cost increase. In summary, if you are a design engineer, you can do more than look up standard sizes in a table -you should know how to work out currents and voltage drops and their effect on equipment and how to keep sizes to minimum.