Calculating maximum load on ring/radial


Postby reefflex » Thu Apr 24, 2008 12:26 am

Hi

Can anyone help me to calculate the max load I can carry on a ring or radial main.

My flat has one ring 2.5mm t&e running behind the skirting supplying 15 sockets. Length about 50m. I want to add three electric heaters connected on spurs into the ring on fused swithches. The heaters are 1500w each.

The CU has a 32mcb for the ring.

Can i safely connect all three heaters and not have the ring overload, if for example the kettle and the washing machine ar on at the sme time?

would it make any difference if the circuit is a radial instead?

Thanks for your help guys.
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Postby ericmark » Thu Apr 24, 2008 11:51 am

Amps = Watts/Volts so
1500W = 6.5 Amp
Kettle = 2400W = 10.5 Amp
So three heater and kettle = 30 amp
As to safe well ring mains are protected by 30 or 32 amp overload of some sort so all it would do is operate the overload and fail safe.
These sort of things need weighing up with local knowledge like will the heaters be running 24/7 or are they thermostatically controlled and what are the alternatives? Can you run independent supply to heaters normally you would run independent supplies to high usage items like immersion heater and the like. But where the item is not used much like a bathroom heater than sometimes one will connect to a ring main. I would say your sailing close to the wind and you should consider the alternatives.
ericmark

Postby chris_on_tour2002 » Mon Apr 28, 2008 6:32 pm

[quote]I would say your sailing close to the wind [/quote]

and thats if you are just running all 3 heaters and a kettle. here's a scenario:

breakfast time in the household, cold winter's morning. all three heaters on full pelt, kettle boiling for a morning cuppa, tv is on watching the breakfast news. close to the load limit. go to make yourself some toast or a hairdryer goes on and pop! mcb trips. thats without plug in lamps, computer, fridge, washing machine, dishwasher etc etc.

it may be safe enough but you would get tired of the power cuts during high demand? unless you are happy to go without a cuppa until the house is warm! and you cook your toast under the grill...

best to run an independent supply.
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Postby chris_on_tour2002 » Mon Apr 28, 2008 6:37 pm

a secondary thought, the kitchen is often on a separate ring due to the demand. if thats the case with you and you do not intend to run a heater from that circuit then it might not be so bad. nonetheless it is stil close to the wind.
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Postby ericmark » Tue Apr 29, 2008 1:51 am

I noticed Sparx on another post says how your not allowed to run fixed equipment from a ring main. He was talking about an immersion heater but he's normally spot on. I expect it's in one of the guides? Regulation 314 talks about splitting into circuits and is still open to some interpretation. It has now been made clear it includes RCD's and nothing should be connected to a circuit which if it caused it to fail would cause danger which is why lights and sockets should not be connected to same RCD some people read into this same area but the regulations are designed for all not just domestic so often not a straight forward read. There are also building regulations for example Part M and Part P giving socket heights and requiring certain work to be notified and even Part L and what energy efficient measures need to be taken. With so many regulations it is easy to say you can't but not very easy to say you can as you may have missed one being in another book. Rummer rules where some one says you can't do something and you believe it for years even though it's not true. For years I thought the minimum height for sockets was 450mm max 1200mm but when I read Part M I then saw it was 400mm to 1000mm and it's not only word of mouth that gets it wrong that was from guide to 16th Edition. But after all this I would not connect those heaters to the same supply as sockets. If anyone reading does know the regulation number that says immersion heaters can't be run off a ring main I would like to know it. I am sure it's fact as it makes sense would nice to be able to quote a reg when someone wants to do it.
ericmark

Postby reefflex » Wed Apr 30, 2008 10:07 pm

thanks for all the advice....after also checking with some electricians ive decided only to connect two of the heaters up, to reduce the max draw to a more reasonable limit, even with other appliances etc. A separate ring would be ideal, but in my flat is nigh on impossible as means removing all the skirtings to get to the cable/feed it around the walls.

As for the issue of connecting fixed appliances to a ring, ive seen quoted elsewhere that the reg specifies space heaters, immersion heaters and water heaters for more than 15l of water.

So yes that does cover the panel heaters if you view them as space heaters. However does this mean that if the heaters have a 13a plug on instead, and are plugged into a single socket, more or less permanently, that this is then okay. Seems a bit funny. None of the leccys have brought up the fixed equipment issue, only the load issue. Not to say they are right though!

Also if you are thinking watts, a 1.5kw heater draws less than a water heater for 15 l (im guessing about 2kw like a kettle) although may be running for longer.

DIY is so much more complicated nowadays!
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