cable/mcb for swimming pool motor


Postby danny001 » Mon Aug 04, 2008 10:22 pm

HI,
i have a few questions about the swimming pool motor/heater
which i was told was a pump so starter this new topic for help

in the house is the main consumer unit and off this is a seperate breaker which is marked shed and is a 20amp wylex swa 2.5mm runs off this

this runs to the shed and to a small consumer unit,
in the unit is a main 32amp breaker and off this is 2 x 20amp mcbs 1 x 10amp mcb and 1 x 6amp mcb all type c and it says series e on the mcbs

may main problem is a new swimming pool motor is to be installed and it has a current of 30amp ( this is on the spec )
i understand a 4mm cable must be used
but can i have a 32amp mcb put in the shed mcb ( as there is a space )and run the 4mm cable off this to the motor
also will i need to replace the main 32amp in the shed consumer unit to a larger breaker 40 or 50amp

any help would be great
thanks
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Postby sparx » Tue Aug 05, 2008 8:25 pm

Danny your problem is the 2.5mm main feed to shed which has a max current carrying capacity [assuming SWA buried cable] which is no where near big enough for load required. End of story!! needs at least 6/10mm2 swa fed by a 45/50A mcb.
Your existing swa 2.5mm2 has a max when buried of 29A so that's why there is a 20A mcb protecting it from overload. There is just a small possibility if cable fairly new that it's XLPE insulated rather than PVC in which case the supply end mcb could be changed to a 32A one as that 90Deg. cable is rated for 33A max but that still won't be enough.
You CAN put whatever size main mcb you like in shed board, even a 63A one, but it won't help as max load will still be 20A.
To run/pump/heater unit proposed will mean running a new feed from house, sorry but no choice.
regards SPARX
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Postby danny001 » Wed Aug 06, 2008 12:10 pm

Hi
thanks for the reply
i was told from a electrician that this is the best option

replace the house fuse to the shed with a 45amp
10mm cable off this to the shed

and the a new consumer unit in the shed with a 63amp amp breaker and the 32amp off this for the heater pump and this would cover the rest of the mcbs which are 2 x 20 1 x 10 and 1 x 6

if this is correct i will have him do the work
the problem is i keep getting different versions and need to know if this is the best oprtion or at least a start and work from there
thanks danny
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Postby sparx » Thu Aug 07, 2008 9:00 pm

Hi Danny, sounds fine except not sure why you have a main mcb in garage when a main sw. rated at anything over 45A would suffice, and would be normal set up on a small consumer unit. No harm though to have extra mcb it's just pointless as a total load over 45A will still trip in house first.
Don't forget there is a diversity factor which means that although your pump will run a lot the heater part will cut in/out and the other circuits won't be all drawing full load at once,
regards SPARX
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Postby danny001 » Fri Aug 08, 2008 10:04 am

[quote="sparx"]Hi Danny, sounds fine except not sure why you have a main mcb in garage when a main sw. rated at anything over 45A would suffice, and would be normal set up on a small consumer unit. No harm though to have extra mcb it's just pointless as a total load over 45A will still trip in house first.
Don't forget there is a diversity factor which means that although your pump will run a lot the heater part will cut in/out and the other circuits won't be all drawing full load at once,
regards SPARX[/quote]

hi,
the unit in the shed at the moment is
32amp main and off this is
2x 20amp
1 x 10amp
1 x 6amp

we got told this would not do as we needed to add a 32amp for the pump/heater
and this would over load the consumer unit in the shed
thanks
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Postby ericmark » Sat Aug 09, 2008 9:28 am

Between what you have been told on this post and what was said on "cable for pump help" I am sure you realize you need heavier cables than what you have and it will be a Part P job.
As such either you will have a registered electrician on site or a council building inspector who will be responsible for site safety so should advise on cable sizes.
I think you will find both what I said before and what Sparx says now match up and you must realize 2.5 cable is not good enough I did try to explain size of cable is not selected purely on amps but also on length of run I would think you are looking at 6mm cable and most likely three core not that you need third core but the steel wire armoring of 3 core 6mm is big enough for earth system i.e. equivalent to over 10mm copper where 2 core is not. I did direct you to the write up on how to wire out buildings which explains it all better than I can.
All best Eric
ericmark

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