Ring vs Radial - Sockets

[ldoodle]

Hey,

For those that read my 1960's House thread, you will know I will shortly be having my lighting circuits rewired.

I've taken a couple of sockets off to replace them with newer ones, and the cables look a bit worse for wear. I'm thinking if I'm having the lighting redone, I may as well have the socket circuits redone as well (as we're about to start a major refurb downstairs).

So, which is the preferred circuit type - why are the UK the only ones who seem to love rings? I will obviously engage the sparky doing the lights, but it's more for my own understanding really. I've read a bit about testing rings vs radials, and that a broken ring on a 30/32a breaker using 2.5mm cable is not a good thing.

TLC do 100m 2.5mm for £41 + VAT or 100m 4mm for £71 + VAT, so I'm not sure cost is such a major issue, in the grand scheme of things.

As I have a blank canvas, what would you go for in my position? 3x 4mm radials (upstairs, downstairs and kitchen)?

 

[echoes]

An advantage of radials is that they can be extended/branched/spurred without having to fuse down.
In a kitchen, the potential problem of an unbalanced ring goes away, but this issue depends on the layout of the kitchen and it's position in relation to the consumer unit.
Ensure the length of the circuit will not result in excessive voltage drop.

As an example, I chose to do a sockets rewire as follows:
Kitchen 32A/4 mm² radial (clipped direct)
Then each other room on its own 20A/2.5 mm² radial (oval conduit)

You might like to consider sockets that can take 4 x 4mm² cables in the terminals if you/your electrician choose to branch a 4 mm² radial. And 35 mm back boxes.

 

[viewer]

Radials every time. Use 4mm for everything - 100m will go a long way

 

[ivixor]

What do you mean 'worse for wear' ?

If they are modern PVC insulated ones, there's almost certainly no need to replace them. It's a good idea to have a seperate ring (or radial) for the kitchen though if you don't have one already. Dishwasher + toaster + Kettle + 13A electric oven..

 

[ldoodle]

Thanks echoes.

One thing; according to TLC, 4mm has a current rating of 32 amps. On a 32A breaker is that not pushing it to the limit, even without taking volt drop and installtion type into account? Whereas they say 2.5mm is 24 amps so a 20A breaker seems 'safer'.

My CU is relatively central to the house - it's certainly right next to the kitchen. So take a radial, going round the room for sockets/FCUs, I'm looking at probably 25m end-to-end, and that's burying the cable in conduit in the wall, running in the joist bays going down > up > down > up.

Downstairs would be similar but longer length. Upstairs would probably be shortest because they come up from the floor, so about 300mm up to the socket.

 

[ldoodle]

What do you mean 'worse for wear' ?

I believe they're original to the house, so 50+ years of use. I'll be here for 10 years plus and as ww're making mess may as well get it done.

 

[davelx]

Thanks echoes.

One thing; according to TLC, 4mm has a current rating of 32 amps. On a 32A breaker is that not pushing it to the limit, even without taking volt drop and installtion type into account? Whereas they say 2.5mm is 24 amps so a 20A breaker seems 'safer'.

The current rating of the cable depends (amongst other things) on the installation method. So for 4mm², clipped direct to supporting structure (not covered by insulation) or chased into plaster, it's 37A. On the other hand, enclosed in conduit in an insulated wall, it's only 26A and surrounded by insulation in a stud wall it's only 17.5A. There are other methods that give different ratings. Similarly for other sizes of cable (table 4D5 BGB)

So, you need to decide/tell us how the cables will be run in order to determine what size cable you need for a given MCB rating.

 

[ldoodle]

Radials every time. Use 4mm for everything - 100m will go a long way

My breakers are currently 32A. With 4mm would I need to change them for smaller ones bearing in mind cable distances?

 

[PrenticeBoyofDerry]

One thing; according to TLC, 4mm has a current rating of 32 amps. On a 32A breaker is that not pushing it to the limit, even without taking volt drop and installtion type into account? Whereas they say 2.5mm is 24 amps so a 20A breaker seems 'safer'.

The current carrying capacity of circuits need to be calculation, as there are often other de-rating factors that reduce the capacity of the cable, most common are:
*Ambient temperature
*If installed within containment, such as trunking and conduit.
*The grouping of cables
*If installed within thermal installation

Not sure where TLC get the current ratings from but in generally the MAXIMUM current rating of 4.00mm T&E would be 37A and 2.5mm T&E 27A.
In a radial circuit with a 32A device providing de-rating factors allow, 4.0mm cable is the normal, but you could have 20A or even 16A radial socket circuits and again allowing for de-rating it would possible to have this circuits installed using 2.5mm T&E.
The ring is a little different as the current is shared (20A) and this allows for a 32A to protect a 2.5mm T&E cable (again providing de-ratings are satisfactory.

 

[ldoodle]

So, you need to decide/tell us how the cables will be run in order to determine what size cable you need for a given MCB rating.

Inside plastic conduit chased into breeze, then plastered over. They'll be a small portion of it in a stud wall, though that won't be insulated. Horizontal runs will be in the ceiling void.

 

[davelx]

Radials every time. Use 4mm for everything - 100m will go a long way

My breakers are currently 32A. With 4mm would I need to change them for smaller ones bearing in mind cable distances?

WRT voltage drop, for a 25m run , 2.5mm²@20A or 4mm²@32A will both be within the 5% limit for power circuits.

 

[ldoodle]

In conduit chased in breeze/brick falls under which installation method?

In conduit in an insulated wall seems closest.

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

 

[ldoodle]

WRT voltage drop, for a 25m run , 2.5mm²@20A or 4mm²@32A will both be within the 5% limit for power circuits.

What calculation did you use?

 

[davelx]

So, you need to decide/tell us how the cables will be run in order to determine what size cable you need for a given MCB rating.

Inside plastic conduit chased into breeze, then plastered over. They'll be a small portion of it in a stud wall, though that won't be insulated.

OK so that's ref method B from table 4A2 line 60 (assuming the breeze blocks are not the insulating type) so 2.5mm² is rated at 23A and 4mm² is rated at 30A so technically you can't used a 32A MCB with 4mm². In which case you go for 20A radials and use 2.5mm², and install 2 radials if 20A is not sufficient. (You won't easily get 2x6mm² cores into the terminals of the accessories)

 

[davelx]

WRT voltage drop, for a 25m run , 2.5mm²@20A or 4mm²@32A will both be within the 5% limit for power circuits.

What calculation did you use?

VD for 2.5mm² T&E is 0.018V/A/m 0.018*20*25 = 9
9/230=0.039, i.e. 3.9%

similarly for 4mm² at 0.011V/A/m