Home › Electrical Engineering Forum › General Discussion › Which mm armoured cable required
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2011/03/20 at 8:26 am #10426BritTanzParticipant
I am installing 4 machines in my factory in rural Tanzania and I am finding it very difficult to get a definitie answer here with regards to the size and type of armoured cable I need to install to carry the load required.
I, myself do not know anything about electrics so the following is the information I think you will need to help but if you require anything else please let me know:
Motors to be served by cable: 4 x 3 phase motors: 3 x 40hp + 1 x 25hp. Total 145hp
Cable will need to be approximately 30m long – I have been told that if the current is to be carried more than 25m the cable needs to be bigger, is this correct?
Cable will be buried
Voltage in Tanzania is 220v
Please could someone advise the size of cable I will need? So far I have received answers here of 50mm, 70mm and 16mm!
Thank you in advance
Stuart2011/03/20 at 5:05 pm #11910jatin333Participant
as looking your load 50,70sq. mm cable is not much enough if you are optimising total load than current will be approx. 400amp since your supply voltage is 220v3-ph.
so main cable you need 300sq. mm aluminium as incomming and then distributo via distribution box to all four machines your 40HP load machines need 70Sq. mm aluminium and 25HP Machine need 35sq.mm Aluminium Cable
conductor dia. increase b'caouse of your 220v 3-phase power2011/03/31 at 11:13 pm #11924adminKeymaster
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.
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