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Thread: Pump cable size

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    Pump cable size

    Good morning, just wanted some advice on correct size cable for a single phase 1,5kw borehole pump.The distance from the sub station to the pump room is 90m and is supplied by a 10mm 4 core three phase supplying a RO system.
    The borehole pump is 260m from the pump room.will use a 10 amp circuit breaker.
    Thanks in advance.

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    Diamond Member Justloadit's Avatar
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    I am assuming that it is a three phase motor with a maximum of 3.9A per phase
    minimum of 4mm for up to 280m
    minimum of 6mm for up to 305m

    You must also take the depth into consideration in your calculations.

    Don't forget about a good earth and surge arresters!
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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    Quote Originally Posted by shaneo View Post
    Good morning, just wanted some advice on correct size cable for a single phase 1,5kw borehole pump.The distance from the sub station to the pump room is 90m and is supplied by a 10mm 4 core three phase supplying a RO system.
    The borehole pump is 260m from the pump room.will use a 10 amp circuit breaker..
    For single phase 10 Amp C/Breaker at 260m you would need to use 6sqmm according to SANS 10142-1 Table D2(b)

    You don't mention the size of the RO plant and if I took your distance of 260m + 90m = 350m then your cable would need to be 10sqmm for the borehole pump only -
    The SANS table works pretty well when going the distances you are talking especially for tripping and short circuit.

    I have seen cables installed longer than stipulated by SANS and the CB does not trip on a dead short due to impedance .
    I have seen the burn marks left on a guys hand and not a CB tripped - The piece of 2,5sqmm wire operated like a welding rod. The 20Amp CB even though it was only 10m was fed from a 250mp supply 300m away on a 95sqmm cable.

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    The voltage = 230 VAC

    Power = 1500 watts

    Current = 6.5 amps

    The distance is 260 + 90 + cable to pump = xyz m ( I would check the cable supply from the sub to the pump room is designed correctly or include it in the calculation)

    I would assume most of the cable is buried in the ground ?

    A 10 amp D or curve 1 breaker would be required for start up current a standard 10 amp breaker will trip especially if a 3 ka domestic type is used.

    Suitable borehole pump protection device (prevent the pump running dry etc.) would be mounted as close to the pump as possible with lightning protection ( not that it help ... we find that when the system is hit by lightning it is blown to bits) installed just to give the customer a warm fuzzy feeling about being protected.

    6.2.7 Voltage drop from page 114 has all the details you require to do the calculations and derating.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    Quote Originally Posted by ians View Post
    The voltage = 230 VAC



    Suitable borehole pump protection device (prevent the pump running dry etc.) would be mounted as close to the pump as possible with lightning protection ( not that it help ... we find that when the system is hit by lightning it is blown to bits) installed just to give the customer a warm fuzzy feeling about being protected.
    .

    Don't run an earth wire down from the top to the borehole pump and you may not attract the lightening.
    An earth wire to a borehole pump is the best earth you could ever possibly imagine and the problem is that generally every other slight earth fault close by will use that piece of earth wire to rid the fault.
    If there is a slight elevation on ground voltage due to lightening nearby it could end up flowing down to the pump
    It is the reason that a bare copper earth wire to a borehole pump is generally black in color , not from chemicals in the water , but from current flow and overheating.

    I am assuming that is why the regulations where adjusted from Ed2 to Ed3 6.16.5.2.1 c .

    6.16.5.2 Submersible motors and motors liable to flooding
    NOTE See 7.2.4.4 for pump motors used in pools, spas and fountains.
    6.16.5.2.1 A submersible motor shall
    a) be supplied by suitable marine type flexible cable firmly attached to the
    motor,
    b) use a cable gland that has an efficient water seal, and
    c) be bonded to the earthing system of the installation, unless installed out of
    arm's reach and in contact with earth mass, like a borehole.

    NOTE A portable water pump can have a submersible motor.

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    The last time I built a water treatment plant was in 1998(prior to that I spent literally 10 year just designing and building them) and then I did a plant in Namibia about 5 years ago ... I have had a few calls to do some installs and repairs ... I rather stay away from borehole pumps especially the ones out in the middle of nowhere ... many years a go when they started installing cellphone towers I did a course of lightning protection and earthing for the towers and borehole pumps.

    These past fews days have been quite interesting catching up with the regs ... the last time I took the time to read a SANS 10142 was changed from the green to the black book ... since then just been winging it.

    Looking into Solar/inverter and batteries has made me realize it is about time I took the time to catch up.
    Comments are based on opinion...not always facts....that's why people use an alias.

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    Diamond Member Justloadit's Avatar
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    Quote Originally Posted by GCE View Post
    Don't run an earth wire down from the top to the borehole pump and you may not attract the lightening.
    An earth wire to a borehole pump is the best earth you could ever possibly imagine and the problem is that generally every other slight earth fault close by will use that piece of earth wire to rid the fault.
    If there is a slight elevation on ground voltage due to lightening nearby it could end up flowing down to the pump
    It is the reason that a bare copper earth wire to a borehole pump is generally black in color , not from chemicals in the water , but from current flow and overheating.

    I am assuming that is why the regulations where adjusted from Ed2 to Ed3 6.16.5.2.1 c .

    6.16.5.2 Submersible motors and motors liable to flooding
    NOTE See 7.2.4.4 for pump motors used in pools, spas and fountains.
    6.16.5.2.1 A submersible motor shall
    a) be supplied by suitable marine type flexible cable firmly attached to the
    motor,
    b) use a cable gland that has an efficient water seal, and
    c) be bonded to the earthing system of the installation, unless installed out of
    arm's reach and in contact with earth mass, like a borehole.

    NOTE A portable water pump can have a submersible motor.
    Interesting you say this, but the water fir is also a conductor, which means if there is a lightning strike even from another point, the potential difference between the motor windings and the casing will cause the motor windings to fail, and the motor then destroys itself due to the winding resistance changing and the inductance being changed. Note that borehole motors only have a 1KV insulation resistance so they are vulnerable. Standard surface motors have a 4KV insulation resistance.

    Every bore hole installation should have a good earth, which is usually part of the connection cable. The failure lies in the poor earth that these installations are connected too. In rural areas, he transformers are usually on poles which is a high point, and overhead wires act as conductors to lightning strikes.

    So from my experience, earthing the motor is one of the most important parts of any installation.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    The kilowatt rating of a pump or any motor for that matter refers to the mechanical output, not the electrical power consumption. You can't use I=P/V to work out the current of a motor, the value you get from calculating it that way will always be considerably lower than the actual max run current.

    The run current can be calculated but you need to know the power factor and the efficiency rating of the motor as well as the voltage and kilowatt rating.

    As a general rule always get the max run current from the manufacturers specs sheet or the info plate on the motor itself.
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    Quote Originally Posted by Justloadit View Post
    Interesting you say this, but the water fir is also a conductor, which means if there is a lightning strike even from another point, the potential difference between the motor windings and the casing will cause the motor windings to fail, and the motor then destroys itself due to the winding resistance changing and the inductance being changed. Note that borehole motors only have a 1KV insulation resistance so they are vulnerable. Standard surface motors have a 4KV insulation resistance.

    Every bore hole installation should have a good earth, which is usually part of the connection cable. The failure lies in the poor earth that these installations are connected too. In rural areas, he transformers are usually on poles which is a high point, and overhead wires act as conductors to lightning strikes.

    So from my experience, earthing the motor is one of the most important parts of any installation.
    The best earth is to place a piece of metal in water 40 to 100m under the ground, which will be the metal casing of the motor and pump.

    Why run an inferior earth 40 to 100m down to the pump ?

    To me you are now earthing the installation that you are taking the earth wire from to a perfect " earth spike" 40 to 100m down and always wet .

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    Diamond Member Justloadit's Avatar
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    Quote Originally Posted by GCE View Post
    It is the reason that a bare copper earth wire to a borehole pump is generally black in color , not from chemicals in the water , but from current flow and overheating.
    The discoloration is due to Oxidation a result of copper's exposure to air, though water, and may also be due to other minerals in the water causing electrolysis due to current flowing in dissimilar materials, very much like the Sacrificial anode or Galvanic anode
    A galvanic anode, or sacrificial anode, is the main component of a galvanic cathodic protection system used to protect buried or submerged metal structures from corrosion. They are made from a metal alloy with a more "active" voltage than the metal of the structure.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar pumping, Solar Geyser & Solar Security lighting solutions - www.microsolve.co.za

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