How to calculate the voltage drop of a 3 run cable, say 3Rx3.5×185 Sq.mm cable?
A 3 run cable represent 3 resistors in parallel, so its effective resistance will be one third. Hence, the effective voltage drop also will be one third. So, when you consider the 3 run cable as a single unit, the voltage drop shall be one third of the usual value of a single run cable. (say 0.37/3mV=0.123mV for 3×3.5×185 Sq. mm cable). Otherwise, you may calculate the adequacy of a single run cable, by taking the current as one third of the proposed feeder capacity. Both methods must give you same value.
