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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details. |
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22nd Feb 2023, 10:36 am | #1 |
Heptode
Join Date: Feb 2022
Location: Leicestershire, UK.
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1960's Meter Tails
My better half has just ordered a 9.5kW electric shower & she thinks it will be an easy job to install it .... however:-
No room on existing Wylex CU & meter tails seem to be 10mm2 so clearly not so easy. Bearing in mind I was a C&G 14th then 16th edition electrician, I intend to swap out the old Wylex CU for a Protek one with split RCD's etc. which will then allow me to remove the existing 60A RCD that takes out all sub circuits on the existing installation. I will also remove the bonding clip on the supply cable and transfer on to the suppliers earth terminal because I understand it can deform the lead sheath possibly leading to a big bang ! All of the above fairly straightforward, but one thing I fear is doing battle with EON in getting the supply tails & fuse upgraded especially as I do not want them using this as an excuse to swap my existing 100A meter for a smart one. Then I remembered when I was a trainee, asking another electrician why were meter tails were so thick when the actual fuse wire protecting them was so thin ? Aah he answered, that's because the fuse wire is only a short length meaning the resistance is very low allowing it to pass a large current. So here is my paradox i.e. meter tails are normally very short in length so is this taken into account by the design bods in determining the CSA versus anticipated current loading / heating effect ? Rog |
22nd Feb 2023, 11:28 am | #2 |
Octode
Join Date: Dec 2019
Location: Owston Ferry, North Lincolnshire, UK.
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Re: 1960's Meter Tails
Supply cable sizes such as meter tails and feeds to outbuildings are supposed to be calculated to handle the full possible load, so in my opinion the existing meter tails need to be replaced with a minimum of 16mm and posibly 25mm depending on the max load at your location. The total load will need to be added up and then once you know that figure, you can look it up in the IEE regs book for the type of cable in question.
Dave
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22nd Feb 2023, 11:36 am | #3 | ||
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
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Re: 1960's Meter Tails
Meter tails are sized for the current just the same as any other cable. Technically there's no disparity between the cable size and the fuse size; a 100A service fuse required tails that are rated to carry 100A, no more no less. However, because the tails interconnect the supplier's equipment with the customer's, it is logistically more complex to organise changing them than any other cable in the premises. Therefore there is a tendency to spec the largest possible tails to reduce the likelihood of them needing to be upgraded, so it is common to find tails rated for 100A on a 60A fuse. This would be a needless waste in a distribution circuit within the property but because the tails are short and incur modest cost, there is little objection to over-speccing them. In practice these days that means 25mm² is used by default, because most terminals will accept it, even if 16mm² is acceptable.
Returning to the subject of length, the only situation in which the length of a conductor has an impact on its current rating is where there is thermal coupling between the terminals and the furthest point along the conductor from them. A strap two inches long between two substantial busbars would be able to conduct heat from throughout its length into the bars, and hence be able to carry a higher current than a one-foot cable of the same CSA. But this is a trivial situation and is not considered in normal design. Conducted dissipation is however important in the design of fuse elements. The two underlying reasons for the ratio between the CSA of a wire fuse element and the cable it protects are that the permissible working temperature of the element is much higher than that of a typical cable, under continuous full load, and that for a smaller conductor the surface area to CSA ratio is larger hence the current density can be higher for the same temperature rise. The fuse has to run hot at its rated current in order to obtain a suitable fusing factor (ratio of must-fail current to rated current) so that it provides protection against overload. If instead one looks only at the short-circuit situation where the dissipation in both the fuse and cable is adiabatic (i.e. all the energy dissipated during the S/C event is still present in the conductor when it ends) then it will be seen that a much smaller conductor is permissible for any given size of fuse. One can sometimes take advantage of this where OCPD is only required to provide fault protection, not overload. Quote:
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Last edited by Lucien Nunes; 22nd Feb 2023 at 11:47 am. |
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22nd Feb 2023, 11:38 am | #4 |
Moderator
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Re: 1960's Meter Tails
One wonders what size the DNO fuse is.
I was surprised when one of my work colleagues told me his incomer was only 60A I had mistakenly thought that 100A was the norm for older domestic properties. Cheers Mike T
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22nd Feb 2023, 11:45 am | #5 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
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Re: 1960's Meter Tails
Where a service terminal taking BS1361 fuses is fitted, i.e. most cutouts that are not legacy wire fuses, one finds 60, 80 and 100A fuses somewhat randomly. Modern concentric service cables are invariably rated for 100A and many older TN-S services are 0.0225 sq. ins. and suitable for 100A. But a 60A link is still often found, perhaps from 50 years ago when a 40A meter was installed. If the cable is adequate then you can request a larger fuse for an EV charge point install or whatever.
With smaller service cables, e.g. 0.015 sq. ins., an upgrade to 100A requires a new cable, the same with looped supplies which have to be unlooped usually at the DNO's expense. |
22nd Feb 2023, 12:19 pm | #6 | |
Heptode
Join Date: Feb 2022
Location: Leicestershire, UK.
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Re: 1960's Meter Tails
Quote:
Supply is Lead Sheathed TNCS with individual stranded copper Phase & Neutral conductors of approx 10mm2 - based on inspection of one from a neighboring property that had to be replaced. Meter is rated at 100A. Not clear in my piccy but there is a Supply Earth terminal via a connector & 6mm uninsulated wire soldered onto the lead sheath. That should easily take a 16mm from the CU. Rog |
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22nd Feb 2023, 12:56 pm | #7 | ||
Octode
Join Date: Jul 2004
Location: Dundee, UK.
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Re: 1960's Meter Tails
Quote:
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These are two important points which Lucien makes, Are the existing tails double insulated - they look a bit thin for that. I would suggest that the first requirement in making an alteration is the assessment of the service head and judging by the photograph, some updating is needed. The BS951 clamp is definitely wrong and should be at least a C2 fault on an EICR. Since the main earthing cable is connected to the lead sheath of the cable, it looks to be a TNS system - this should really be confirmed with your DNO. There is a separate earth busbar, which is a good thing to have - and to keep. What is the white enclosure to the right of the consumer unit? Is it a consumer's isolator? If anything has to be done the tails, it would definitely be a good idea to have a consumer's isolator as any future work can then be done without disturbing the service cut-out. EON is your energy supplier, not the DNO. I think you will find they are separate. If so, it is the DNO you need to contact to have the service head brought up to standard. The DNO will probably be less concerned about the meter but yheu wouldn't want you touching the earth to the cable sheath. Does the installation of a new circuit not come under Part P in England? If that's the case you definitely want to get everything correct, there could also be insurance considerations. I don't know how familiar you are with the 18th Edition (Amd 2) - there's quite a difference with the 16th Edition. PMM |
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22nd Feb 2023, 1:29 pm | #8 | ||
Rest in Peace
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Re: 1960's Meter Tails
Quote:
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22nd Feb 2023, 1:50 pm | #9 | |||
Heptode
Join Date: Feb 2022
Location: Leicestershire, UK.
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Re: 1960's Meter Tails
Quote:
Yes I concede that supply is TN-S should have known better but in mitigation I have been out of the trade for nearly 50 years. The enclosure PMM queries to is a 60A RCB protecting all of the installation which clearly needs to go once decision made on using a spare split RCB CU I have, or buying a new one with RCBO's. Part P would apply but there is the paradox i.e. Despite my TNCS confusion I am qualified as C&G 14th then later 16th Edition electrician so it does pain me to have to employ another for what is just a box ticking excercise. Also, previous research has confirmed I need to apply to the company that bllls me for leccy for the supply upgrade i.e. EON & not the DNO (which would be much more preferable). Rog Last edited by Roger Ramjet; 22nd Feb 2023 at 1:51 pm. Reason: duplicate word |
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22nd Feb 2023, 2:31 pm | #10 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
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Re: 1960's Meter Tails
Part P is a Building Regs. thing so check what your local council would charge to certify your work. It varies wildy hence it is usually cheaper to find someone who is a member of a body that can bypass the council.
Last time I checked the council charges vary from £50 to £500 which is an outrage. |
22nd Feb 2023, 4:08 pm | #11 |
Nonode
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Location: Spalding, Peterborough, Cambridgeshire, UK.
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Re: 1960's Meter Tails
About 30 years ago I was installing a supply to a new build outbuilding rated as agricultural use. I had a full time electrician friend who guided me.
His comment was similar to just do the calculations and use a size of tails in excess of it and all would be fine. So, due to the tails needing to go through and along the wall and a shortish distance to the fuse box, 25mm came out as a little undersized. So, I used 35mm tails. When the East Midlands Electricity chap arrived he really had a go at me, asking why on earth I had used 35mm as they were extremely difficult for him to connect. I showed him my calculations for 35mm, he agreed they were correct and he then said I should have ignored them as it wouldn't have mattered one bit. Should have used my common sense and 25mm rather than make his job difficult! But, this was 30+ years ago. Rob
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22nd Feb 2023, 4:16 pm | #12 |
Nonode
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Location: Warsaw, Poland and Cambridge, UK
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Re: 1960's Meter Tails
Anecdotally, the BS951 clamp may not be a problem though it's technically wrong. Our house in. Cambridge has one clamped around the incoming cable. I put it there a couple of decades ago when I bought the place and found there was no earth connection at all! This time last year I had an EICR done and the consumer unit changed by a reputable local firm. The electrician commented on the earth clamp and I explained its origin. He said it wasn't a problem as long as it was secure and the earth loop impedance was OK. Both of those things are true, so it stayed.
Chris
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22nd Feb 2023, 4:41 pm | #13 | |
Heptode
Join Date: Feb 2022
Location: Leicestershire, UK.
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Re: 1960's Meter Tails
Quote:
Mine has been OK & the irony is that an supply earth has been provided albeit via a 6mm un insulated cable soldered onto the lead sheath. The conflict for me is if the tails are updated to 25mm, am I still OK to link onto a supply earth of 6mm with a 16mm wire ? Rog Last edited by Roger Ramjet; 22nd Feb 2023 at 4:42 pm. Reason: typo |
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22nd Feb 2023, 6:04 pm | #14 |
Moderator
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Re: 1960's Meter Tails
Due to the safety, regulatory and legal aspects of modifying household wiring it has been decided after a long discussion between the mods that this is off topic for the forum.
Such work should be carried out only by those qualified with the requisite insurance and experience to do so. So this thread will be closed. Cheers Mike T
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