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