Ring and Radial

[JohnW2]

If installing HI earthing, which is what you are doing, then the ring CPC should be connected to separate terminals otherwise the occurrence against which you are protecting may still happen.

True - but only at one socket/accessory - all other sockets/accessories would still have an earth connection even if the CPC ring was broken somewhere.

If one does as chivers suggested and the sockets/accessories all have two earth terminals, then it obviously would make sense to use them. However, if some (or all) of the sockets/accessories have only one earth terminal, then having the extra earth wire going to the end of the circuit is still "much better than nothing", redundancy-wise.

Kind Regards, John

 

[chivers67]

Ring finals: separate CPC from back box earth terminal to adjacent/nearest accessory terminal and other two CPCs into other side.

If installing HI earthing, which is what you are doing, then the ring CPC should be connected to separate terminals otherwise the occurrence against which you are protecting may still happen.

I do this due to not liking three cables into one terminal

 

[JohnW2]

Single load circuits? No - should I

As EFLI says, yes, if you want protection against the primary CPC becoming disconnected. However, in this case it would be pretty essential to use separate earth terminals since, with only one, if one CPC came out, it's quite likely that the other would as well.

Kind Regards, John

 

[EFLImpudence]

Ring finals: separate CPC from back box earth terminal to adjacent/nearest accessory terminal and other two CPCs into other side.

If installing HI earthing, which is what you are doing, then the ring CPC should be connected to separate terminals otherwise the occurrence against which you are protecting may still happen.

I do this due to not liking three cables into one terminal

Yes but you do not want both Ring CPCs to come loose otherwise it defeats what you are doing.

That socket would then be left with no earthing.

 

[chivers67]

So do you suggest one CPC with the back box CPC the other double on it's own? - might do that from now on...

 

[EFLImpudence]

Yes, it would be better.

 

[ericmark]

As far as original question the radial is fine.

However in general a ring final (32A) can have 106 meters of cable, where a radial (20A) can have 32 meters of cable, so in general you need 3 radials to supply the same area as a single ring final, if using RCBO's this means it is far more expensive to run radials than ring finals.

 

[JohnW2]

However in general a ring final (32A) can have 106 meters of cable, where a radial (20A) can have 32 meters of cable, so in general you need 3 radials to supply the same area as a single ring final ...

Isn't there a bit of 'chalk and cheese' there, in that you are comparing a 32A ring with 20A radials? Even if it weren't for the voltage drop considerations which lead to the figures you quote, one would need two 20A radials to be 'at least equivalent' (current-supply-wise) to a 32A ring.

A fairer comparison would probably be between a 32A ring (2.5mm² cable) and a 32A radial (wired in 4mm² cable). In terms of voltage drop, that would allow you around 40m of cable for the radial (if one assumed the same load distribution as per your ring calculations) - and, provided one was not having to supply all the sockets with individual cable drops from above the ceiling, that would probably be plenty in any 'normal' house - so you would probably only need one 32A radial as an alternative to one 32A ring.

Also, the 106m/32m comparison makes things sound worse than they actually are in some situations. Particularly in the case of an upstairs circuit, a radial may require a lot less cable length than a ring.

Kind Regards, John

 

[ericmark]

You are correct just over 40 meters using 4 mm² cable, using same figures i.e. design current 26A against 106 meters for a ring final with 2.5 mm² cable. So in general the loop impedance will be better with a ring final than with a radial.
OK with 6 mm² it goes up to 64 meters however the 13A socket will take if made to spec either three 2.5 mm² or two 6 mm² so with a radial spurs can only be done with certain makes of socket which have larger terminals like the MK.
So in real terms to move to 6 mm² radials means a redesign of the socket so it can take three 6 mm² cables to allow the circuit to be expanded. So what ever the pros and cons over ring final or radial unless the sockets are redesigned we are stuck with the ring final if we want to cover the 100² meters floor space with a single circuit.

 

[EFLImpudence]

unless the sockets are redesigned we are stuck with the ring final if we want to cover the 100² meters floor space with a single circuit.

No we aren't.
Anyway, of what relevance is the area? 10 x 10, 20 x 5 or 33 x 3.

I frequently wonder about the validity of these 'discussions' about
a) the length of circuits
b) the oft stated need that all sockets should be on the ring and spurs are bad, and
c) the location of all these long thin houses.

A 100m. ring in the first floor ceiling void can have umpteem 3m. dropped and 1m. rising spurs.
A 40m. radial can have several 10m. branches. It could be a 20m. cross with umpteen branches (spurs) or any combination of these. It could easily be more than 100m. in total

It is up to the designer to satisfy all the requirements with suitable circuits.

No need for larger cables or larger socket terminals.

 

[ericmark]

You are correct it has to be designed, I remember a warehouse where they wanted sockets to run the cleaners, we ran a 6 mm² ring around the top of the warehouse with 2.5 mm² drops to each socket so there was not a single socket on the ring, every one was a spur from the ring the 6 mm² reduced the volt drop it would have been crazy to have taken the supply down 15 meters to each socket and back up again. And the warehouse was well over the 100 meters² area.

In a house you have access problems, I know we now have maintenance free JB's but with a house the idea of doing as done with that warehouse does not make sense. However there are some things we can do with houses to reduce the volt drop or the loop impedance. Wiring side to side rather than upper and lower floor often reduces the volt drop, also cheaper as less cable used.

With domestic there is a balance between convention and design. People expect a house to be wired upper and lower not side to side, so wiring side to side can mean some one in the future makes a mistake. But that is really down to the guy in the future not testing and inspecting first and should we really plan for some idiot in the future not doing his job correctly?

The same goes for 4 mm² radial circuits, I have found where the circuit was extended half way using 2.5 mm² clearly who ever did it thought it was a ring. But that's not a good reason for not using 4 mm² as a radial.

The whole idea of the ring final was to save copper after the war, but rules like 1/3 along the beam to drill the hole have resulted in not saving that much copper any more, so there is a case for using radials. However it's all down to design. And circuits should be designed, not done a set way because we have always done it that way. We are after all electricians not plumbers.

 

[Blimey]

Thanks for the answers and advice. Another question, the original consumer unit is fitted to wooden panels in an under-stairs cupboard, is there any reason that the electrician can't fit the new one in the same place?

 

[ericmark]

It has need a traditional location, however there is a risk of fire if started under the stairs from preventing egress from the building. As a result I intended to move my mothers consumer unit to a visible location, still technically under stairs, but where nothing can be placed around the consumer unit to prevent the heat escaping. However all the people I got to give estimates all wanted to keep the consumer unit under the stairs. So it seems electricians still fit them under stairs.

What has to be remembered is any fuse or MCB makes heat, that's how they work, so that heat must escape, so 13A sockets for example should be in free air so the heat can escape from the plug. For a domestic consumer unit that heat is very small, so in the main not a problem, but under the stairs is where many people stuff things out of the way and the area can end up full of junk which stops cooling and could in extreme circumstances lead to a fire.

So it has to be left to the electrician to do his risk assessment and decide if he feels it is a safe location. Can't really do it for him. He signs the paperwork, so it is up to him.

 

[deadshort]

Thanks for the answers and advice. Another question, the original consumer unit is fitted to wooden panels in an under-stairs cupboard, is there any reason that the electrician can't fit the new one in the same place? View attachment 103428

No reason why not if the replacement CU will be fully compliant with amendment 3 , i.e. all metal.

Regards,

DS

 

[Blimey]

@deadshort Had a look at the regulations and am amazed that loads of consumer units being sold (plastic) would need to be fitted inside a metal box to conform http://electrical.theiet.org/wiring-matters/55/consumer-units/index.cfm ?