Motor Power Cables and a 3 Phase Motor

Yes - you can use a 3 core + Earth(ground) cable to supply the motor direct from the starting unit/contactor, but check the coil voltage and controls of the associated starter equipment to ensure that a neutral is not required there.

Ypu have to use 3 or 4 core 06 sq mm copper cable flxible if laid on cable trays or armoured cable if laid in under ground.

Dear All

Will omision of neutral cable cause any problem for operation of the pump? because by using 3 core one instead of 4 core will help to save lot on cable cost (this is suppose to be a lenghty cable). I wont to know the recommended practise in this regard.

Also could you please let me the methord of calculating % voltage drop for 3 phase cable and what are the acceptable V.D limits especially for motors?

At work I use about 20 pumps and all are supplied with 3 core and a seperate earth.

I use tables which you can get on the net for seeing what the voltage drop is and over here in Scotland the max permissable voltage drop is 4%.

You motor does not use a neutral, there is no point in running it. 3 phase conductors and ground is all you need.

The volt drop of 4% in Scotland is the total from the incoming supply point to the current consuming equipment not just for the final circuit to the motor. In this case you need to calculate initial volt drop to the distribution board and subtract that from the 4%. Whatever you are left with is the motor volt drop.

Volt drop is calculated by - Full load current(A) x Length x Vd/m for the selected cable (available from manufacturer or local regulations)

The allowed volt drop will depend upon which country you are operating

Thank you. I should have made myself more clear. When we are calculating cable size we start with the voltage drop to the dist board and make sure this is well under the 4% if possible. Then size the cable to the pump(or whatever) to fall within the 4%. I'm not in heavy industry so the max current or pumps draw is at about 35 amps(running speed not start up current)and they are never far from the dist boards so cable selection in my case is relatively simple.

Depending on the size of the 3-phase motor, across the line starting current can be 600% of FLA. Find out from the manufacturer what the starting current is though it probably can be derived from a Letter code on the motor nameplate for locked rotor current.

If you are planning on protecting your cable by running it in a conduit or to bury it, it will have to be derated (meaning you cannot use the ampacity of the conductors in the cable). Cable ampacity is usually based on conductors in free air unless otherwise noted.

i think 3CX4sqmm cable is quite enough for 8KW motor if started with DOL starter, if the motor is far distant from the power source, then you can go for 3Cx6sqmm after calculating the VD on the cable.

You still need to run an earth wire seperately if you plan to use 3- core cable.

Hope you are satisfied,,

That is what I use most of the time. SWA 3 core with seperate earth.

no not possible.for 3phase motor load 4core power cable required.for this load neutral is necessary.otherwise separate earthing need for that motor and that can utilized as neutral connection

I don't think the neutral is required in the power circuit of a three phase motor, rather, you need only the earthing wire for earth fault protection purposes. Moreover if you don't have earth fault protection utilized in the supply system, you can simply installed an earthing system near the pump location by simply digging the required copper rods( qty depends on the soil resistivty) and bond the earth wire from the rod to the motor outer shell.You can also measure the earthing resistance which should not be more than 5.00 ohm.

You certainly don't need 4 core cable for running a three phase pump motor. If you do then our pumps must be running by magic.

If the motor is not "double insulated", then a separate earthing conductor will be required in addition to the three current-carrying conductors. Sometimes the armour of a steel-wire-armoured cable can be used for the earthing conductor, depending on application and local Regulations.

3 core means there are sepration betwin phases. yes u can do so if ur motor has a delta connection

It will also work in "star". What is not possible with the suggested cable is star-delta starting.

If this is a 3phase well pump or other submersable metel clad pump, then the well it is in or the water deposit it is in is probably the best earth you can get! every pump(well) I have installed has been 3phase with 3core cable! (except of course the single phase ones!!) All the circuits can be changed to 3phase including the coil in the contacts thus eliminating the need for the neutral! Off the top of my head, 8000W, about 10hp (don´t use watts any more, just the Hp), need to know length of run, to acomadate the 4% VD! SWA is abit OTT if your cable is in a protective tube or the like! If it is a well, there is not much likelyhood of sticking a spade through it is there!

if it is not submersable, it is well advised to ground out the case with a local earth or even use a 4core if it make life easier! You can get cable with for example 3*4+2.5 or 3*6+2.5mm ! whatever you do, if you are not sure, get a qualified sparkie to check it out for you! you don´t want to find out the hard way! (sorry for any spelling errors but the spell checker is not working!)

The earth cable is needed because during a power cut or any time the pump looses power the NRV on the suction line has been known to stick open and the pump has lost it's prime. Because in our facility it is not common practice to call an engineer out there have been times when the pump has been running dry. I know this is a bad situation but I can't get through to management to change the protocols here. In this event there is no earth and so a seperate earth cable has to be used. I can hear you saying that isn't likely to happen much and your right but you have to account for all events.

Most of my pumps are in remote areas so the use of SWA is justified over the cost and time of running ducting for each pump.

Even though I'm not a qualified sparky, my boss is and this is the way he does it. I've learned from him.

If the <...we...> has wired the <...3 phase motor (8kW)...> in delta, then yes. Otherwise no.

However the nature of the question suggests that the <...we...> does not have the capability to do this. Consult a qualified local Electrician instead, who will calculate/design, install, test and certify the installation as safe on completion.