radial or ring?

[JJ]

About to wire up the new kitchen (yup, all under BCO and needing part P signoff) and all the sockets are mounted and conduit in place.

I now have a choice

4mm T&E radial on 32A RCBO

or

2.5mm T&E ring on 32A RCBO

both seem equally valid according to regs and my calculations.

I can see the argument both ways, the ring is more traditional for UK but entails longer checks and radial in simpler but entails getting hold of some 4mm T&E (need to order it I think, nobody seems to stock it round here).

Whats the feeling on this here?

Ta :)

[TheScullster]

"JJ" wrote

From a DIY rather than electro-technical perspective, 2.5 will be easier to work with.

Phil

[george [dicegeorge]]

radial (without RCD?) for the fridge/freezer so it doesnt trip out.

[g]

[JJ]

A good point, one that I thought about.

The old kitchen was on an RCBO# and never tripped once in >2 years.

My reading of the regs is that if there are sockets on the ring (and there will be) then I *must* have RCD protection hence the RCBO.

A separate non RCD circuit for fridge freezer *is* allowed on the regs as far as I can tell - but I thought with modern equipment that is not really a likely issue anymore...... is it? I mean I thought it was only the older fridges where the compressors had a habit of tripping the newer ones do not..?

# I actually have a bank of RCBOs instead of split box, my choice though I feel sorry for the sparky who will need to individually remove them for insulation tests (but he'll charge me for time taken anyhow). If only my BCO would accept that they are actually responsible for paying for the checks...

[george [dicegeorge]]

no, its so that if something trips an rcd when youre away your fridge doesnt melt all your food... i think an armoured cable to it may be needed, not sure though..

[g]

[george [dicegeorge]]

if you know where the cooker, microwave etc are going to be then perhaps radial it,

if youre selling the house in a few years and people might move the microwave then maybe a ring so the loads are distributed better.

i am not an expert.

am going on a Part P course in a few weeks!

[g]

[Tim Watts]

On Wed, 20 Jan 2010 02:47:45 -0800, JJ wibbled:

Table 4D2A, ref method B (conduit on wall, and my understanding is this is the correct ref method for conduit in a masonry wall) states 30A for

4mm2.

OK, being pedantic - what do others think?

Cheers

Tim

[TheScullster]

"JJ" wrote

AIUI if you put anything on a non-RCD circuit (according to 17th edition) you need it to be buried 50mm or more, or have mechanical protection - e.g. use armoured cable - anywhere it could be drilled through in wall chases etc.. Or you surface run it so it's visible.

If you are talking about a separate radial for the fridge freezer with its own breaker, then using an individual RCBO in this case shouldn't cause too many issues. Problems have occurred in the past where one RCD covered a number of power circuits (including freezer) and a fault with any socket/appliance anywhere in the house could take out all power. This is not what you are considering here IIUC.

Phil

[Dave Osborne]

See 17th OSG Appendix 8

"Standard Circuit Arrangements for Households and Similar Installations"

Sec 8.2 "Final Circuits Using Socket Outlets complying with BS1363..."

(Table 8A):

A2 - Radial Circuit with 30A or 32A Overcurrent protective device 4mm2 conductor + 2.5mm2 CPC servicing a maximum floor area of 75m2.

(Sec 8.2.2):

Table 8A is applicable for circuits protected by: fuses to BS3036, BS1361, BS88 and MCBs type B, C, 1, 2, 3 or BS EN 60947-2.

(Sec 8.2.3):

The minimum size of conductor CSA in the circuit and non-fused spurs is given in table 8A. However, the actual size of the cable is determined by the current carrying capacity for the particular method of installation after applying appropriate rating factors from Appendix 6.

The as-installed current carrying capacity for Radial Circuit A2 is 30A or 32A (i.e. the rating of the over-current protective device).

There is also a bit on needing a minimum current-carrying capacity of

41A for an A2 radial circuit protected by a BS3036 semi-enclosed fuse (i.e. 30A replaceable fusewire).

So, technically, you are right. Reference method B for 4mm2 cable without any other derating would be suitable only for a 30A cartridge fuse or MCB.

However, the rating for reference method C (clipped direct) is 36A and reference method D (free air) is 40A therefore, provided you don't have a voltage drop issue, if you can argue that for a substantial part of its length, reference method C or D (or a combination thereof) applies, then you could split the difference with impunity and use a 32A MCB.

Cheers, DaveyOz

[JJ]

Many thanks for all the detailed advice.

It seems that either option is still valid.

But I'm thinking I'll probably stick with the more conventional 2.5mm ring for a number of reasons.

Probably more straightforward in the long run, easier to get the T&E and certainly much easier to work with.

I'll have to extend it when I bring the utility room behind the kitchen online, but thats ok.

Once again, many thanks for your great advice.

JJ.

[js.b1]

Circuits without RCD:

- BS8436 cable is the only practical way of no RCD in domestic, it is supposedly available in cut lengths from

(Vicki Thompson was my email contact, ask for Prysmian Earthshield

2.5mm about 1.10/metre plus carriage plus VAT).

- Note protection must be a Type-B device of rating no higher than

32A, Earthshield quoted is PVC sheath so will be limited to 20A as a radial (wiring accessories are rated to PVC BS6004 cable anyway).

SWA is out because terminations must be accessible. Pyro is very expensive to buy (few hundred a reel now) and probably more trouble than it is worth now. Steel conduit gets expensive and impractical unless you are lucky with a "corridor strip kitchen" and appliances on one side, with the CU very close.

4.0mm FTE:

- Radials with 4.0mm FTE can be easier in some circumstances, or where someone wants several power circuits in a house but is limited on space re number of cables (routing around 150mm zone or having to create their own zone re sloping ceilings, doorways, chimneys etc).

- 4.0mm FTE is stranded, not solid like 2.5mm FTE, so is easier than you may think certainly in PVC (not in Prysmian BS7211 which will be pig rigid!)

Ring v Radial:

- Ring gives two routes for CPC, whereas radial has only one

- Ring can be split to produce two 20A radials, can be very handy for "resiliency" with elderly so they don't lose either everything in a house or everything in a room (taking 314.1 to its logical conclusion)

- Ring has one downside in that a double socket is rated to 19.5A continuous yet fusing will tolerate 26A and fuse 30/32A, this one is upsetting europe even now as a known defect with a potential counter being "phasing out PVC and using XLPE insulation" which would require wiring accessories to be certified to 90oC from 70oC

- Radial has one benefit in that it is fused to 20A thereby matching a double socket 19.5A continuous limitation

One issue with a ring is ensuring the legs are reasonably balanced, this combined with 314.1 means a "whole house ring" is now out of favour with a kitchen now ideally on its own ring separate from the house. Some councils require this, however its more to prevent penny pinching and a resulting useless installation (just removed part of my mother's original kitchen ring and the PVC cable sheath shattered lengthways when snipped, totally unbalanced design and ring broken at an undercounter socket).

[Tim Watts]

On Wed, 20 Jan 2010 16:18:26 +0000, Dave Osborne wibbled:

Yes - I took my figures straight from the big red book.

I'm not sure about the OP - but my conduit runs are a greater portion of the circuit length - the rest being clipped direct or on open form basket (very good for cooling, 40A IIRC for 4mm2).

It is a shame the IEE didn't tweak the numbers very slightly and say 4mm2 ref B was 32A. Me being me wouldn't feel happy to sign off under those conditions (being very critical and by the book of my own work), although their is almost certainly sufficient safety margin engineered in those numbers.

I have one kitchen circuit that has a high local concentration of load and a 32A radial would be a better design. As it is, for a ring, I will have to ensure that this load is as close to the mid point of the ring to ensure balanced loading of the two legs.

[John Rumm]

It does not have to be armoured as such, an earth shield cable would be ok, as would MICC, or ordinary singles in earthed metal conduit.

Indeed, often the simplest solution these days. RCBOs are cheaper than they were.

[John Rumm]

One thing to watch, much 4,00mm^2 FTE has the same 1.5mm^2 CPC as 2.5mm^2.

[NT]

Ring circuit safety advantages

[<me9]

I've got a separate radial surface mounted in the garage feeding the burglar alarm and the freezer (on the other side of the wall in the kitchen) plus a couple of IT items in the garage,not on an RCD/RCBO. However the boiler (also in the garage) is on an RCD, so that any leak trips it before major electrical damage.