I am instaling a piece of equipment that has a total loading of 4.5kW, but am constraint on space to run cable from the starter to the equipment. I intend to use a 7 core cable, of which only three cores are for the 3phase power the remainder are for start / stop and indicator light. Power is supplied via an MCCB and 2.5mm cable is large enough to run the cable even with the derating factor of multicore cable. My problem is the impedance, (Zc) value is adjusted by the 7 cores meaning a 4mm cable is needed as opposed to 2.5. Is it acceptable to calculate using 3 cores as these are the only cables carrying power or is 4mm cable required.