Big ring circuit

[regsmyth]

The new ring circuit for my extension is rather large. I know that a ring final circuit can cover up to 100m2, but is there a limit to the length of cabling. Owing to the nature of the extension, there's nearly 100m of 2.5 in the circuit, and that's before it heads off to the cu....
I am assuming the area limit covers the area bounded by the sockets, and doesn't need to include the 15m run back to the cu.
Also, with lighting circuits, where 1.5mm is ok for up to 1500W of lights, does this assume a 100W bulb in every holder, or does diversity apply. Three of my lighting circuits have 10-15 lights on each of them. Can 1mm T+E only be used where there are definately less than 10 lights on the circuit?
Thoughts and comments appreciated.

 

[Adam_151]

A ring final circuit is limited in both the area it can serve and the length of cabling used.

1) 100m² is the absolute limit for space served by the outlets on a ring final circuit, its based on a diversified heating load, if kitchens and utility rooms are to be served then there is clearly more than small portable appliances and space heating to consider, also you need to consider that high loads are unlikely to end up clustered to one end of the RFC (so a rfc for downstairs with the kitchen outlets all on one leg of the ring final is unlikely to be a good design!)

2) Cable length is limited by Zs and Volt drop, the OSg puts the max length as 88m I believe and IIRC its volt drop that bites first (and they have allowed for a load thats spread around the rfc...a fag packet calculation that puts all the load bang in the centre would suggest a much shorter max length

Split your ring circuit into two such circuits

Lights, assume a 100w lamp in each standard fitting and then multiply the whole lot by 0.66 (assuming a house), they won't all have 100w GLS lamps in, and they won't all be on at once!

 

[stargazer]

Owing to the nature of the extension, there's nearly 100m of 2.5 in the circuit..

 

[regsmyth]

Well, probably about 110m with the runs to the cu included. It's not that big, just a complicated shape. I might rethink the layout based on your comments. Have to admit that the first 40m of one leg will take virtually all the power. Will try to put the other lot on a different circuit I think. Thanks.

 

[ban-all-sheds]

Have to admit that the first 40m of one leg will take virtually all the power.

What about making that a 4mm² radial?

And/or run a 10mm² sub-main to a small CU in a more central point in the extension and run a few radials from that?

 

[regsmyth]

Brilliant.
Anyone want to buy 100m of 2.5T+E. Various lengths.

 

[RF Lighting]

I'll give you £4 for it.

 

[ban-all-sheds]

Brilliant.
Anyone want to buy 100m of 2.5T+E. Various lengths.

Err - didn't realise you'd actually installed it.

It's customary to do the design part first.

What did you tell LABC you'd be doing to ensure compliance with Part P, 'cos installing 100m long ring circuits isn't going to do.....

 

[regsmyth]

I've put in the bits I'm sure about.
I'm still in the design phase.
BCO wasn't interested in electrics as planning application was before 2005.
I am paying an electrician as "consultant" to help me get things right, but don't expect him to work out circuit lengths or areas of coverage. I have produced drawings showing all circuits, and consumer unit setup, showing areas of coverage, size of cables and mcb, and specific routing of circuits. This is what has shown up the big ring main, so now I'm rethinking it, with the help of the experts on this fantastic forum.
The other problem is that alterations to the house are going to continue over the next few years and are going to involve further minor mods to the electrics, so all these need to be part of the plan now.
The electrician will test everything when the major mods have been completed.
Surely I'm not the only one....

 

[Pensdown]

In offices the same problem often exists. DB is miles away from the ring final. As Adam has pointed out, the killer is the volt drop, and with T&E Zs. To get over this we install a 6mm or 10mm radial to a 45amp DP switch and then wire the ring from there.

Measure twice - cut once

Design 20+ times - install once

 

[ban-all-sheds]

BCO wasn't interested in electrics as planning application was before 2005.

Did you actually start work on the extension before 2005?

If not, then let's hope that when you come to sell in the future your buyers and their solicitors are also mistakenly not interested in the compliance of the electrics...

I am paying an electrician as "consultant" to help me get things right, but don't expect him to work out circuit lengths or areas of coverage.

I'd stop paying him then, if I were you. Circuit lengths are very important from a safety point of view - in theory you could be killed if you get it wrong.