DIY Doctor

overloading a ring main

Postby crimper » Fri Nov 30, 2007 9:48 am

I have recently move into a house with a substantial outbuilding and garage and want to use the outbuilding as an office. The electrics are fairly new and are rcd protected. The ring main in the outhouse has 7 double sockets on approx 60 metres of 2.5mm2 cable and is protected by a 32 amp mcb. I want to run a heater/air condition unit that draws 12.8 amps (manifacturers handbook) ,3 computers and fax/printer, etc. As the cable is in run through insulated stud walls I believe that the cable can only support about 18 amps.

Am i in danger of overheating the cable as these items coud be running for 9-10 per day?

Am i better splitting the ring main into 2 seperate ring mains

I would be grateful for any advice
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Postby 333rocky333 » Fri Nov 30, 2007 8:12 pm

if it was a radial it would be about 18 amps , things change for a ring,if the installer worked it out right it should be able to carry the load of the mcb fitted, otherwise he should have used 4 or 6mm to allow for derating of the cable
If it is definetly a ring it should run all day at about 25 amps.
If possible get a 20 amp radial supply for the heater and that will reduce load on the ring
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Postby ericmark » Sat Dec 01, 2007 3:03 am

You are right on the edge 19mV/A/M = 19 x 60 x 32 / 4 = 9.12 Volts the maximum is 9.2 Volts the divide by 4 at one cable only 30 meters to furthest point and is doubled up. So I would run extra spur to heater/Air Con and up to 4mm as Air Cons do not like volt drop. But of course then we are looking at new circuit so would come under Part P. So you need an electrician who will advise as to best way forward. Because to the startup current on A.C.s and the problems if they stall due to volt drop in the end burning out the internal overload it would be fool hardy not to ensure low volt drop.

Yours Eric

Postby kbrownie » Sat Dec 01, 2007 8:35 am

Hi crimper,

I agree with 333rock333,
Your maxium demand would most probally exceed 8kva, so you will be okay with PCs,fax+printer on the 2.5mm T+E ring main protected by 32A MCB but heater best on Radial circuit independant to it's self. As suggested
Good Luck
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Postby crimper » Sat Dec 01, 2007 12:30 pm

Thanks everyone for your replies will get sparky in as advised.


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