There are 16 mmsq tails from supply to meter - from meter to Henley
block - from block to fuse boxes.

What is the rating of the main supply fuse? What is the total length of
tails from the meter to the consumer unit, via the block? What sort of
earthing do you have? What size are the earthing conductors?

Christian.

Peter Ramm wrote:

I'm replacing two fuse boxes by one CU.

There are 16 mmsq tails from supply to meter - from meter to Henley
block - from block to fuse boxes.

Do I really have to use 25mmsq tails from Henley block to my new CU?

It depends (a) on the assessed maximum demand via your new CU and (b) on
the size of the distributors main fuse.

(a) For 6181Y single-sheathed tails in free air the rating is 87 A for
16 mm^2 and 114 A for 25 mm^2.

(b) If you use 16 mm^2 tails and the main fuse is 100 A you will need to
show by calculation (using the adiabatic equation) that the tails are
protected by the fuse in the event of a s/c fault in your CU. To avoid
the need for calculation use 25 mm^2 tails. If the main fuse is 60 or
80 A this does not apply.

HTH
--
Andy

100Amp
About 18 inches
Earth from incoming cable shield
16mmsq

Personally, I would replace the tails with 25mm. It would take 2 minutes. I
did so with mine and I only have a 60A fuse.

Christian.

Peter Ramm wrote:

Yes - thanks - but my point is that any current which passes from my
new CU to Henley block in a failure is already passing through 16mmsq
so anything I use of greater area and less resistance in series cannot
improve the situation. What am I still missing here?

Sorry, I read your first post in a hurry and didn't take that in - just
assumed is was 25mm^2 from the meter to the service connector block.
Nevertheless, new work should comply with current regulations and that
means using 25mm^2 tails throughout unless the assessed MD is 87 A or
less and there's no diversity applied at the CU (i.e. sum of MCB ratings
in the CU is also 87 A, which is unlikely).

In all probability you might well get away with 16 mm^2, but if the main
fuse really is 100 A then the installation does not comply with BS 7671
and there is a risk of the wiring overheating under overload conditions.
Why not install 25 tails now and then get the distributor or meter
operator company round to upgrade the bit twixt meter and block at a
later date?

What final circuits and loads are connected to the CU?
--
Andy

In message , Andy Wade
writes
Peter Ramm wrote:

Yes - thanks - but my point is that any current which passes from my
new CU to Henley block in a failure is already passing through 16mmsq
so anything I use of greater area and less resistance in series cannot
improve the situation. What am I still missing here?

Sorry, I read your first post in a hurry and didn't take that in - just
assumed is was 25mm^2 from the meter to the service connector block.
Nevertheless, new work should comply with current regulations and that
means using 25mm^2 tails throughout unless the assessed MD is 87 A or
less and there's no diversity applied at the CU (i.e. sum of MCB
ratings in the CU is also 87 A, which is unlikely).

In all probability you might well get away with 16 mm^2, but if the
main fuse really is 100 A then the installation does not comply with BS
7671 and there is a risk of the wiring overheating under overload
conditions. Why not install 25 tails now and then get the distributor
or meter operator company round to upgrade the bit twixt meter and
block at a later date?


Yep that's basically the approach I've taken, I could seen no reason to
not replace 'my' tails with 25mm^2 when I replaced the CU, I may try and
get the Dist co. to replace theirs as well.
--
Chris French, Leeds

"Andy Wade" wrote in message
...
Peter Ramm wrote:

Yes - thanks - but my point is that any current which passes from my
new CU to Henley block in a failure is already passing through 16mmsq
so anything I use of greater area and less resistance in series cannot
improve the situation. What am I still missing here?

Sorry, I read your first post in a hurry and didn't take that in - just
assumed is was 25mm^2 from the meter to the service connector block.
Nevertheless, new work should comply with current regulations and that
means using 25mm^2 tails throughout unless the assessed MD is 87 A or
less and there's no diversity applied at the CU (i.e. sum of MCB ratings
in the CU is also 87 A, which is unlikely).

In all probability you might well get away with 16 mm^2, but if the main
fuse really is 100 A then the installation does not comply with BS 7671
and there is a risk of the wiring overheating under overload conditions.
Why not install 25 tails now and then get the distributor or meter
operator company round to upgrade the bit twixt meter and block at a
later date?

What final circuits and loads are connected to the CU?

Andy


I'd upgrade the tails to the new CU and call the Leccy Co. to come and
upgrade their bit as well. With the new requirements now, they shouldn't
have a problem doing that for you, and for nowt'.

Peter Ramm wrote:


Yes - thanks - but my point is that any current which passes from my
new CU to Henley block in a failure is already passing through 16mmsq
so anything I use of greater area and less resistance in series cannot
improve the situation. What am I still missing here?

Well, by having a length of 25mmsq at the end rather than the existing
16mmsq you'll be reducing the total impedance of the path from supply
mainfuse to a hypothetical short between the tails near your CU and back
to the supply point, thereby increasing the fault current and
(hopefully) reducing the time for the supply fuse to blow. Yes, it's a
marginal effect, but vaguely beneficial - equivalent to shortening the
16mmsq tails, as it were.

That's in addition to the general "25mmsq is the worry-free size to use,
and might even encourage the supply co. to upgrade next time one of
their finest is Doing Stuff by your meter"...

Stefek

Peter Ramm wrote:
On Tue, 16 Nov 2004 08:04:29 +0000, Peter Ramm
wrote:


I'm replacing two fuse boxes by one CU.

There are 16 mmsq tails from supply to meter - from meter to Henley
block - from block to fuse boxes.


I'm going to go ahead with 25mmsq and will then get supply company to
upgrade their bits when I've finished everything.


Many thanks to all for explanations/suggestions much appreciated.

If you were to pull the seals and run the tails all the way back to the
meter yourself you probably wouldn't be the first person to do so.

--
Dave S
(The return email address is a dummy)

I'm going to go ahead with 25mmsq and will then get supply company to
upgrade their bits when I've finished everything.

The bit between the meter and the Henley block is all yours, too. You don't
need to call the electricity company in to replace that part.

Christian.

In message , Christian
McArdle writes
I'm going to go ahead with 25mmsq and will then get supply company to
upgrade their bits when I've finished everything.

The bit between the meter and the Henley block is all yours, too. You don't
need to call the electricity company in to replace that part.

Yes,so I wonder why they seal the cover on that?

I replaced mine all the way to the meter, the ones twixt Henely block
and meter were much older than all the rest, a bit of economising here
at some point. Thee insulation was sound but seemed to be some sort of
rubber, covered with a cloth/fabric cover of some sort.
--
Chris French, Leeds

In article ,
"Christian McArdle" writes:
I'm going to go ahead with 25mmsq and will then get supply company to
upgrade their bits when I've finished everything.

The bit between the meter and the Henley block is all yours, too. You don't
need to call the electricity company in to replace that part.

Yes it's yours, but you can only replace it if your meter
has a separate cover over those terminals, and older meters
don't. (Breaking any seals that gain access to the meter
inlet and voltage coil terminals could get you into serious
trouble.)

--
Andrew Gabriel

Yes it's yours, but you can only replace it if your meter
has a separate cover over those terminals, and older meters
don't.

Are such meters still in service, though? I'd have thought they'd all been
replaced by now.

Christian.

In article ,
"Christian McArdle" writes:
Yes it's yours, but you can only replace it if your meter
has a separate cover over those terminals, and older meters
don't.

Are such meters still in service, though? I'd have thought they'd all been
replaced by now.

Of the various premises I have anything to do with, or any of
my relations live in, they're all this type. I very rarely
see the split terminal cover type. Actually, two relatively
new houses up the road from me (couple of years old) had
the meter type fitted with only a single connection cover,
so use of the split type is not universal in new meters.

--
Andrew Gabriel

Are such meters still in service, though? I'd have thought they'd all
been
replaced by now.

Of the various premises I have anything to do with, or any of
my relations live in, they're all this type. I very rarely
see the split terminal cover type.

Actually, I misread. I thought you meant meters without a separate terminal
cover at all (so you have to break the main meter seal to make connections),
not ones without separate seals for inlet and outlet.

Christian.

Yes it's yours, but you can only replace it if your meter
has a separate cover over those terminals, and older meters
don't.

Are such meters still in service, though? I'd have thought they'd all been
replaced by now.

Christian.


Our meter still has one cover over all the terminals. I broke that seal
anyway as the elec company had already said to break the fuse seals when
changing the CU, and you can just as easily steal electricity from there as
at the meter so I dont see it matters too much ;o) (apparently our meter
was changed 20 years ago and isnt due for replacement for another 15 years I
think they said!?)

On Wed, 17 Nov 2004 12:19:09 UTC, "Christian McArdle"
wrote:

Yes it's yours, but you can only replace it if your meter
has a separate cover over those terminals, and older meters
don't.

Are such meters still in service, though? I'd have thought they'd all been
replaced by now.

I just had a brand new meter (electronic) fitted, and there's a single
(sealed) cover over both sets of terminals.

But at least the man resealed the main fuse for me, after I took it out
to fit a 100A isolator...

--
Bob Eager
begin a new life...dump Windows!