# Re: Which mm armoured cable required

Home Electrical Engineering Forum General Discussion Which mm armoured cable required Re: Which mm armoured cable required

#11924
Keymaster

I am hoping each motor will have its own circuit from a distribution board, otherwise all the following will not be valid and the installation is questionable at best.  ie Power supply feed to four circuit breakers, one for each motor.

I can only provide australian details.  Firstly, is the 220 volts single phase voltage, or 3 phase?  This will determine amps per phase and thus cable sizing.  If you're only a little bit lucky the motor should have a nameplate amp rating on it, this will be the amps per phase and will mean you won't need to know the phase voltage.

If 220 volts is 3 phase then you're looking at approx 98 Amps for the 40HP/30kW motors, and 59 Amps for 25HP/18kW motors.

You have to consider voltage drop, typically allow 3% (6.6 volts) drop to motors, and total 5% (11 volts) over entire circuit.  25mm copper cable over 30 metres will be ok for this as only approx 4.5 volts drop.  7.1 volts drop with 16mm copper.  (4.3 volts drop using 16mm copper for the 25HP motor).

Next check current carrying capacity – for 98 Amps buried direct (not in conduit) 25mm is 162 Amps, so again OK.  16mm current carrying capacity is 125 Amps.  If in conduit and buried current capacity drops to 123 Amps for 25mm, and 95 Amps for 16mm.

Now check to see if the cable will hold up in case of short circuit.  Earth cable on standard 16mm is 4mm, and 25mm has 6mm earth.  Check sum allows circuit length of 34metres for 16mm cable.  However the 25mm will only just (less than 1 metre) allow this circuit at 98 Amps over 30 metres.  This means that over only 30 metres the resistance of the cable is low enough to ensure sufficient current to trip the circuit breaker in the event of a short circuit within a specified time.  If the motors are not running at full load you could use a smaller circuit breaker (ie 80 Amps) or you should go up to 35mm cable.

Are the motors in a hazardous area?  If not, why use armoured cable?  If so, make sure you use the appropriate glands.  In Australia & New Zealand, the cable must be insulated and sheathed (whether or not it is armoured) to be buried without conduit and must also have 50mm of sand/friable soil surrounding the cable, be 500mm deep, with a plastic cover (polymeric) providing mechanical protection (from the likes of shovels) less than 75mm above the cable and marker tape at approximately half depth.

Good luck