length of ring main

[davetherave1979]

Hi all what is the maximun length of a ring main in 2.5 te can you install as doing an extension in a bungaloe and the board is the other side of the building we going to be protecting the circuit with a 20amp mcb no larage loads as it feeds bedroom and lounges there will be no insulation on the cable we are looking at about 150m including all the drops and back to the board is this ok to do or should we install a 16amp mcb thanks

 

[telectrix]

i would be inclined to run 2 separate radials, 4mm cable, 32A or 20A MCB for each. separate ring or 4mm radial for kitchen

 

[davetherave1979]

if we were to run from board in 4mm to each end of ring then the rest in 2.5 do you recon that would be fine all it will be feeding will be tv and lamp no big loads just wont to no what the maximum mcb would be best to fit

 

[Jimmyb]

off top of my head thinks it is around 100 meters or just over so @ 150 it too long.

 

[telectrix]

running 4mm then 2.5 seems sloppy to me. run from board as you say in 4mm and leave as 2 radials. load is then split to 2 circuits. more convenient, no hassle with end to end , broken ring, etc. i hate the mindset that thinks rings are the dog's nuts, when radials are so much more convenient for adding, extending, none of the " only one socket on an unfused spur etc., etc.

 

[telectrix]

off top of my head thinks it is around 100 meters or just over so @ 150 it too long.

is that the end/end length? if so then 2 radials would be 50 - 70m.each. check volt drop for 4mm at whatever your design current is. think you'll find it's well within tolerance.

 

[davetherave1979]

decided to fit a sub board in the garage which is by the extension as it will be easyer to run a swa to other side of the building and then feed sub board then feed the ring that we going to install as there is also a load of lighting ciruits to install as well by the labour in volved clipping loads of cable back to the other side of the building makes sens to install a sep board for the new bit would you agree. The maximum lentg of a ring main you say is 100m do you mean 100m in length of cable running up and down to each socket or 100m floor space will run a 10mm 3core swa to feed board what you recon thanks

 

[Jimmyb]

i meant ring main max lenght is around 100

 

[telectrix]

i meant ring main max lenght is around 100

surely the permissible length is dependent on volt drop, which itself is dependent on load current. each installation is different. it's recommended that 1 ring main does not cover a floor area of more than 100 square meters

 

[Sintra]

Volt drop is key to the maximum length and to work out the maximum length of the ring we need the design current. The op stated 20A so using that figure the maximum length is.

4 x 11.5 x 1000 / 20 x 18

127.8M is the maximum length.


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[davetherave1979]

thanks for your help im just going to instal a small sub board will be easyer

 

[i=p/u]

if i was going to install another say 2 way board in shed out back garden, where would i connect the cable in the db. its own mcb or in with the tails. i have a million question for ya's ta.

 

[Paul.M]

I would fit a mains isolation switch from the incomer and split from there but the other thing to think about is the location of the 2nd db. OP says bungalow, just powering tv and light. This is an older persons house. Wouldn't like to put a db in an out building for the livingroom incase mcb trips and so can the house holder getting to the garage. As per previous post I would run 2 raidials 4mm. No need for new db or swa, just a bit more work running your cables.

 

[i=p/u]

so more less keeping all isolation switches together....

 

[Paul.M]

so more less keeping all isolation switches together....

It comes down to practical design from us the electricians. You have to take into account like the OP has the use, demand, lay out of the building. In this case it sounds better to have a second db. This can be fed from an isolation switch at the incomer, from the main db on its own mcb (swa being used, rcd not needed). You have to approach things like this logically and weigh up things like time, costs, practical, usability, regs, safety etc.