Selecting the Appropriate Core Cable for Powering a Motor

For all practical purposes, a 3 core cable is sufficient to power the motor irrespective of the fact that the motor winding is in star or delta.

If a 3.5 core cable is used, then the fourth core is used for earthing of motor terminal box. It is not used as "neutral" while delivering power to the motor.

For a star connected motor, the neutral point will be located inside the motor. It may also be brought out (3.3kV and upwards) for the purpose of starting/protection. But, the fourth core is never used as neutral.

The 3 and 3 1/2 core for a motor does not matter in squirrel cage motor, UNLESS it is continuously running in star mode. Independent of any other sensor devices, 3 phase supply via 3 core will operate the motor. anand sk feedback@maintenancecircle.com

Dear Mr. Muthukrishnan,

Since your application of motors is for fire fighting, the normal method of selection of motors,cables and switchgear is not applicable for fire fighting pumps.

The electrical parameters are taken 2 or 3 steps higher for fire fighting. this is due to the reason that even is the motors, cable or switchgear gets damaged, its not important. the life and building property gets more priority. the cost of replacing life(which is invaluable) and building is much more than the electrical equipment for fire fighting. for more clarifications, mail me at shanbhag@sequentindia.com

If the motor is to be switched from star to delta remote from the motor, then a 7-core cable is required between the starter and the motor: 2 cores for each phase and 1 for the earth conductor.

If the starter is mounted on the motor and a connection made simply to the starter then a minimum 4-core cable is required and possibly a 5, depending on whether a neutral is required. A 4-core carries the three phases and earth. A 5-core carries the three phases, a neutral and an earth.