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

"JJ" wrote

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

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

Phil

TheScullster wrote:
"JJ" wrote

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.



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

[g]

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


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...

JJ wrote:


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..?



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]

JJ wrote:
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
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]

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

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

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

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

"JJ" wrote


A separate non RCD circuit for fridge freezer *is* allowed on the regs
as far as I can tell

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

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

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

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?


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

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.

Circuits without RCD:
- BS8436 cable is the only practical way of no RCD in domestic, it is
supposedly available in cut lengths from www.discount-electrical.com
(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).

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

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

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

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?


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.

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

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.

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.



--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

On Jan 20, 10:47*am, JJ wrote:
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

Ring circuit safety advantages
http://wiki.diyfaq.org.uk/index.php?title=Ring_circuit


NT