Earthing Conductor size (massive??) & new Consumer Unit

Am looking into replacing an antique fuse box with a split load Contactum consumer unit - in part to provide a modern ring circuit. But a survey of the current installation indicates potential earthing problems, so help please!
By way of background the supply is PME - upgraded (new cable, meters etc etc) around 1995-98 from TT(?) after a fault by the successors(?) of SEEBOARD.
The incoming supply cable looks quite beefy. It supplies a largish house plus several small flats via switchgear leading off in parallel to the main house switchgear (everything goes through the same elec board meter & cutout). 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. Can anyone identify this size more exactly please?
There's quite a lot of switch gear attached, one of which is a 60A switch fuse leading to the antique fuse box in question. It is hoped to leave this switch & the cable (imperial?? - around 25mmsq) to the antique fuse box in place as it looks healthy. However the earthing is via steel conduit & looks to require upgrading to a proper 'modern' copper earth cable.
The yellow/green insulated multi-strand earth-to-water bonding cable appears to be at least 25mmsq. It has crimped lugs so it isn't easy to measure directly: the overall diameter (including the yellow/green pvc insulation) is 10.4mm and it is multi-stranded - perhaps 15+ strands. I can't get an exact fix on the size of the strands, but they are perhaps 2-2.5mm in dia. Please, can anyone identify the exact size/spec of this cable?
This is way over the 'typical' sizes shown for PME on page 28 of the IEE On-Site Guide. Then looking at the table 10a on page 161 gives a wider picture, and it all rather implies the supply must be 70mm^2 (or larger?).
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.
If the supply cable is really 70mmsq then am I reading On-Site Guide table 10a (pg 161) correctly that a 35mmsq earth cable should be specified for the run earth terminal-to-Consumer Unit? Or should it be sized according to the 25mmsq cable coming from the switch-fuse (16mmsq if I read pg 161 aright)?
BTW I enquired about 25mm^2 earth cable at a TLC branch only to be greeted with astonishment, so if a heavy gauge earth cable turns out to be needed, where can you buy it?
many thanks
[BTW appologies if this posted twice - I think the first attempt failed, but you never know)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Am looking into replacing an antique fuse box with a split load Contactum consumer unit - in part to provide a modern ring circuit. But a survey of the current installation indicates potential earthing problemss, so help please!
By way of background the supply is PME - upgraded (new supply cable, meters etc etc) around 1995-98 from TT(?) after a fault, by the successors(?) of SEEBOARD.
The incoming supply cable looks quite beefy. It supplies a largish house plus several small flats via switchgear leading off in parallel to the main house switchgear (everything goes through the same elec board meter & cutout). Can only guess as to the incoming supply cable size, but the cable from the board cutout is 16.3mm diameter overall (including the brown insulation/sheath), which suggests something around 70mmsq to 100mmsq depending on the sheath/insulation thickness. Can anyone identify this size more exactly please?
There's quite a lot of switch gear attached, one of which is a 60A switch fuse leading to the antique fuse box in question. It is hoped to leave this switch & the cable (imperial?? - around 25mmsq) to the antique fuse box in place as it looks healthy. However the earthing is via steel conduit & looks to require upgrading to a proper 'modern' copper earth cable.
The yellow/green insulated multi-strand earth-to-water bonding cable appears to be at least 25mmsq. It has crimped lugs so it isn't easy to measure directly: the overall diameter (including the yellow/green pvc insulation) is 10.4mm and it is multi-stranded - perhaps 15+ strands. I can't get an exact fix on the size of the strands, but they are perhaps 2-2.5mm in dia. Please, can anyone identify the exact size/spec of this cable?
This is way over the 'typical' sizes shown for PME on page 28 of the IEE On-Site Guide. Then looking at table 10a on page 161 gives a wider picture, and it all rather implies the supply must be 70mm^2 (or larger?).
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.
If the supply cable is really 70mmsq then am I reading On-Site Guide table 10a (pg 161) correctly that a 35mmsq earth cable should be specified for the run earth terminal-to-Consumer Unit? Or should it be sized according to the 25mmsq cable coming from the switch-fuse (16mmsq if I read pg 161 aright)?
BTW I enquired about 25mm^2 earth cable at a TLC branch only to be greeted with astonishment, so if a heavy guage earth cable turns out to be needed, where can you buy it?
many thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Dunno why they should be astonished - it's in their cat, CA25S at 1.20 per metre. If it was uncommon, they wouldn't offer cut lengths.
--
*Taxation WITH representation ain't much fun, either.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

I was astonished that the sales clerk was astonished as I was reading from the catalogue! He knows a lot about elec wiring & said he hadn't sold any 25mmsq in yonks, but could proably get it in.
Poking about it rather appears that TLC is more of a buying organisation or cooperative & individual outlets are fully or semi-independant. Also TLC deals mainly with domestic sized gear, so to be fair it is quite understandable.
If it was uncommon, they wouldn't offer cut lengths.
Yes cut is in the catalogue, but the mail order operation seems to be run from the centre, not from the shops.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Think they actually stock about the same range as most ordinary wholesalers. And are rather better than some. But you can't expect every branch to stock the entire range - same really as anywhere.

Well, yes. But the shops will get in anything you need from 'head office' pretty quickly.
--
*Succeed, in spite of management *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I've ordered over the internet and the order was definitely dealt with by the local branch and came in a TLC branded van, not via Courier.
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It must depend on where you are, given that TLC aren't nationwide. Yet.
--
*I'm pretty sure that sex is better than logic, but I can't prove it.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"jim_in_sussex" wrote in message

When you say "from the cutout" do you mean the individual 'tails' to the meter, or the incoming multi-core cable from the street? If the wires or cable are recent theyr'e likely to have the size embossed on the sheath somewhere: look carefully.

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)?

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?

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

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.
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. 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. 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?).

That doesn't say much for TLC (whom I've never used). Any decent wholesaler will have it, or be able to get it - try somewhere like Newey and Eyre. Most wholesalers will sell you cut lengths of the larger sizes - but watch the price, it's easy to get ripped-off on cable where stupidly high list prices tend to be quoted. Make sure you get the 95% discount!

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



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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

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

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.

<snip>
Please, can anyone identify the

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?

Another key point I missed, thanks

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

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

that's the answer I want to hear, 16mmsq it is, many thanks.
OTOH the main bonding for the house

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

Thanks, I thought it had posted first time but right at the end my computer went into a deep contemplation of its navel
cheers
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"jim_in_sussex" wrote in message

OK :-)

Ooh, I've never seen CT metering in a house before. (Might "mansion" be a better description?) It used to be common (standard?) for anything over 100 A, but, round here anyway, whole-current meters are normal up to 200 A/phase.

Or it might be because I made a mistake - see below.

Err, my head, for the no. of strands; the rest is just simple arithmetic. Actually I've seen 35mm^2 wires in both 7 and 19 strand versions, but I think the other sizes are always as described. The no. of strands always follows the sequence 1, 7, 19, 37, ... which follows from the geometry of the situation, so to speak.

Correct so far

^^ Drat, that 70 should be a 50 (& relates to your comment above). Basically for phase sizes greater then 35mm^2 the earth has to be at least half the phase size.

IMO the OSG is a model of how _not_ to produce an accessible reference manual.

Is the plastic section entiely within the house? If it is then the galv. pipe ought to be main-bonded (sizes as above). OTOH an outside tap should definitely not be earthed to the PME terminal because it's outside the equipotential zone and could become live under fault conditions. Some more plastic pipe would be useful here.

In the circumstances described (i.e. the pipe isn't going to be metallicaly connected to the water mains or to any other electrical installation) I'd say that was entirely adequate.
--
Andy



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I`ve never seen a whole current meter rated above 100A before (Manweb area)
--
Please add "[newsgroup]" in the subject of any personal replies via email
--- My new email address has "ngspamtrap" & @btinternet.com in it ;-) ---
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Colin Wilson" wrote in message

area)
Well in premises where I used to work there were two 3x 200 A meters, big square-ish chunky things fitted with electronic MRUs. 15 revs/kWh on the disc, IIRC. I can't remember the manufacturer - Siemens, perhaps. This is the former Eastern Electricity area, BTW.
--
Andy



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

In my building, we pull well over 1MW. The meters are driven via current transformers though, the main feeds don't go directly through the meters. I think we've now got 3 feeds into the building, as each time we needed more power, it was done by adding another feed rather than upgrading the existing ones.
--
Andrew Gabriel

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I`m guessing you have your own HV ring then, rather than taking it directly off the local REC`s network...
--
Please add "[newsgroup]" in the subject of any personal replies via email
--- My new email address has "ngspamtrap" & @btinternet.com in it ;-) ---
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Originally, we were on the 1MVA transformer feeding the local residential properties. With the first upgrade, a second transformer was added to the pad. I guess our feeds must now be split between the two transformers since the total exceeds the capacity of either one, but I don't know which feed is from which transformer. We managed to trip our main 630A incoming 3-phase breaker whilst we were waiting for the new transformer to be installed for the first upgrade. Funny thing was a company electrician was on site it the time, and it tripped just as he fitted a replacement lamp in low voltage downlighter;-)
--
Andrew Gabriel

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 8 Jul 2004 08:38:15 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) strung together this:

Some of those cheap imported bulbs are useless, try fitting a type C 630A MCB. ;-)
--

SJW
A.C.S. Ltd
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Or a thick steel nail. Much the same thing, really!
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 8 Jul 2004 13:04:38 +0100, "Christian McArdle"

I think a short length of galv\copper pipe would be better. The nail will probably melt quicker than the MCB would blow!
--

SJW
A.C.S. Ltd
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Andrew Gabriel" wrote | In my building, we pull well over 1MW.
See folks, an instant electric shower /can/ beat a high flow combi :-)
Owain
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You can get 24kW instant electric heaters, provided you can spare a 100A phase for one. I wouldn't attempt to apply diversity given that it will draw the full 100A when operational. Alternatively, I believe you can get 3 phase ones that will only draw 33A off each phase.
These could be a good solution for a shower liking person with lots of electricity and no gas or oil. They would be particularly suitable for a holiday house that is left unattended, as water tanks can be a worry for leakage and accidental heating for long periods (such as over winter).
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.