cable for pump help


Postby danny001 » Mon Jul 07, 2008 5:27 pm

HI,
iam having a pump fitted and have a question if anybody can help,

i have a 2.5 cable swa from my home to a shed and i need to install a pump,
i have been told the pump ia a 30amp and need to know if the 2.5 cable is ok for the pump,
i have access to a new 17th ed book but dont know where to look
so any help would be great
many thanks
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm

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Postby ericmark » Wed Jul 09, 2008 10:02 am

Well Danny001 simple answer is no 2.5 cable will not take 30 amp page 280 you look at worse case and somewhere the cable will be clipped direct so 28 amp max. Having said that 30 amp seems very high for a pump and I wonder if that is run or start current? There are many ways to start a motor and also many types of motor with such a big motor to ensure you don’t get problems blowing incoming fuse and having all lights in area dipping if that’s the run current then most likely would be a three phase pump running from an inverter which can be set to give a really soft start. But pumping what? If start too slow the pressure can build up quicker than power and cause stalling. So really do need to know a lot more.
Eric
ericmark


Postby danny001 » Wed Jul 09, 2008 1:55 pm

hi,
thanks for the reply,
i have a friend who has a pool in his rear garden and wanted to install a pump and the only info he emailed me to ask was that on the pump its says 30amp ( i think he may of got this wrong )

he has 2.5mm swa from his house to his shed and that just runs a few lights and a socket or two nothing more then that,

he just needed to know if this pool pump would/could be wired in to the 2.5mm cable

i think he has a small fuse board in the shed - could he not put a mcb in there and if so what type of mcb/size

thanks again
danny001
Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm


Postby ericmark » Wed Jul 09, 2008 10:23 pm

Once your sure on facts come back 30 amp in a pond pump would give a river of water flow seems wrong.
ericmark


Postby ericmark » Thu Jul 10, 2008 10:22 am

I did a quick check to draw 30 amp we are looking at a 4” pump giving about 35 litres per second so would fill a bath in 10 seconds it would have to be some pond to use a pump that big.
If we are looking at start current the we are looking at the letter in front of the MCB feeding a B = 3 to 5 times rated value a C = 5 to 10 times rated value and a D = 10 to 20 times rated value. So to allow a 30 amp start current you need a B10 or C5 or D5 MCB pond pumps are rarely more than 5 amp run and so with the 2.5mm² cable able to run at 25 amps without any real problems for capacity the cable is OK for normal pond pump but of course we are also concerned with volt drop now 5% is allowed see page 358 and 281 so 18 mV/A/m so at 25 amp we have a maximum of 25 meters half the amps double the meters and last but not least earthing arrangements and for use in a pond it would really need to be a TT supply i.e. using earth rods not using the boards earth and as a result using a RCD would be required. There is an IET publication on shed wiring http://www2.theiet.org/Publish/WireRegs ... tdoors.pdf this may give you more insight. I would say wire a shed feeding a pond in the same way as supplying a caravan because the same problem exists in that under fault conditions you have a large area of conducting material accessible. Section 702 deals with ponds page 170.
ericmark


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