"Andy Wade" wrote in message ...

first many thnks for yuour extremely helpful reply, it's made me feel
a bit saner...

snip

Can only guess as to the incoming supply
cable size, but the cable from the cutout is 16.3mm diameter overall
(including the brown insulation/sheath), which suggests something
around 70mmsq to 100mmsq depending on the sheath/insulation
thickness.

When you say "from the cutout" do you mean the individual 'tails' to the
meter, or the incoming multi-core cable from the street?

err....that should read 'across the fields', the old cable which was
replaced came in from other fields on a 180deg bearing!

it is a 3phase swa supply - maybe overall swa dia c.50mm (from memory)
but only 2 phases are used - 1 for normal elec supply the other for
(ancient) off-peak heaters. The swa disappears into a sealed box of
(presumably) board fuses, out of that emerge the 2 line cables as
described which immediately go through a current transformer. The 2
meters (1 on each phase used) are indirect and actually monitor the
CTs.


If the wires or
cable are recent theyr'e likely to have the size embossed on the sheath
somewhe look carefully.

Can anyone identify this size more exactly please?

A photograph would be useful/interesting, if you can put one up somewhere.
Is this single phase or three? A single-phase supply of more than 100A
would be very unusual, IME. What's the rating of the meter, and what type
of main fuse(s)?

A pic is tricky within the next week as I've come home (the
installation is at my parents). However I think your next comments re
the earth clear up most of the issues - I was only poking about trying
to size the incoming supply because of the top line of table 10A in
OSG & to get as much info together as poss.




[...]
The yellow/green insulated multi-strand earth-to-water bonding cable
appears to be at least 25mmsq.

snip

Please, can anyone identify the
exact size/spec of this cable?

Size Stranding
---- ---------
25 7/2.13
35 19/1.53
50 19/1.83
70 19/2.17
95 19/2.52

- does that help?

immensely. 19/1.83 seems to fit the overall dia best allowing 1.2mm
for the sheath - OTOH 50mmsq is not in the BS7671 table (below), so
maybe 70mmsq?? 35mmsq would make the insulation/sheath much thicker
(1.7mm) than it looks.

for future ref what is the source please?


This is way over the 'typical' sizes shown for PME on page 28 of the
IEE On-Site Guide.

You're beyond the scope of the OSG at 100 A.

Another key point I missed, thanks

So what size should the new earth bonding cable from the supply earth
point to the new Consumer Unit be? Any opinions/advice will be most
gratefully received.

BS 7671 requires main earthing conductors (i.e. from PME terminal to
installation main earth terminal) as follows:

Supply Earth Main
size size bonding
------ ----- -------
25 16 10
35 16 10
50 25 16
70 35 25
95 70 25

NB: the earthing conductor size is based on the phase conductor of the
supply, whereas the bonding size is based on the supply neutral size. Local
supply network conditions may require a larger bonding size.

Bear in mind that oversized earths may have been used.

that must be the key, possibly they only carried 1 earth cable size &
used it everywhere.


Also bear in mind that the earth to your new consumer unit (it's actually a
dis-bord in this case) is a CPC and not a main earthing conductor.

thanks, just wish the IEE & OSG gave clearer definitions (ie decent
illustrative pics/diagrams) of these fundamnental terms.

If the
fuse in the switch-fuse which feeds this board is 100A or less then a
'standard' 16 mm^2 earth will be OK.

that's the answer I want to hear, 16mmsq it is, many thanks.

OTOH the main bonding for the house
will go to the main earth terminal and should follow the table above (but
you're not changing that, are you?

not on your life!!!

So all cleared up enough to do the job, many thanks, but one last
niggling query:

As stated elec board put in this massive bond to the house cold water
supply at the point where the incoming water mains changes to copper.
What they missed is that there is a second (Cu/galv iron(?) water pipe
going underground into the garden to an outside tap. There is a long
(c. 3m) section of plastic pipe between the main house water Cu and
this outgoing pipe.

Presumably it needs earthing/ bonding back to where? - just bridge the
plastic insert, link back to the main bond join to the house water, or
link back to the elec board earth terminal? & what size? No doubt
there'll be some 16mmsq spare, but if not is 10mmsq OK?

snip
[BTW appologies if this posted twice - I think the first attempt
failed, but you never know)

Well I'm following up to the first of two copies of it on Zen's news server.

Thanks, I thought it had posted first time but right at the end my
computer went into a deep contemplation of its navel


cheers