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Topic Title: 10mm supply
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Created On: 21 February 2014 05:10 PM
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 21 February 2014 05:10 PM
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dazum1

Posts: 4
Joined: 21 February 2014

Hi,

I have a job where it is physically impossible to upgrade or change the current supply for the customers consumer unit. Currently it is a 10mm T/E

The job itself is to install a new CU and add a couple of new circuits for various things.

I am a little worried as the cable is estimated to be 25m from the meter to the consumer unit (this is large house turned into 10 flats hence why we cannot change the supply) given worst case scenario this would give us 46A total.

I have 1 x 32a sockets, 1 6a x lights, 1 x 32a supply for 2 x immersion heaters, 1 x 6 or 16A supply for pump (depending on what the plumbers eventually install), 1 x 32a supply for 6kw boiler and a 40A supply for the cooker. Now with all of that you can see my concerns...


Obviously another issue is the main earth is only the earth of the 10mm T/E cable...

And another issue is the main switch for the consumer unit, the smallest I have found for the consumer unit is 63A 30ma???

Has anyone any advice or suggestions on what to do?

Thanks in advance

Darren
 21 February 2014 05:28 PM
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alanblaby

Posts: 283
Joined: 09 March 2012

Without doing any calculations, total demand would be a concern. Then volt drop - there seems to be a lot of equipment going in, will the 10mm conductor cover that?
You need to add up you expected demand, and see if the volt drop will comply.
What is the bonding like?
How about the main earthing conductor?
Presumably this is a sub-main, so what is protecting the sub-main?
You may be struggling, put in a little diversity, and you may be OK, but check out the earthing first,
 21 February 2014 05:54 PM
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dazum1

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The main earthing is from the 10mm T/E which is a concern, as this is 4mm? There is just no way of getting a new earth from the basement, around the flats and into this flat, it is basically the furthest flat.

The earthing arrangements are perfect in the basement.

We need to add bonding within the flat which we will be using 10mm earth

I have spoken with the client to see if the plumbers can reduce their demand at all. The flat is only small, 1 bed, 1 bath, 1 kitchen, 1 lounge and a hallway, there are about 10 sockets in total and 1 light in each room so there isn't going to be much demand from the sockets or lighting. I am just concerned about all the plumbing requirements + the electric cooker/hob + the small amounts on the sockets and lighting.

I am unsure whether the boiler will demand the 6kw constantly or just ever now and then!

Such a head scratcher!
 21 February 2014 06:49 PM
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burn

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It sounds to me as if the "boiler" in a night storage unit, supplying hot water to radiators during the day. Even if the pump needs a 16A MCB, it's running current will probably only be a fraction of that.
The two immersion heaters should be on two separate timeswitches, the main (lower) one to take advantage of cheap night tariff to heat the whole tank and the other to reheat the top half as a short evening boost if required.
This leaves just the cooker, any appliances plugged in the sockets (plus occasionally the immersion boost) during the day & evening.
I would expect a 63A supply to be perfectly ok

If you can get a 10mm bonding conductor to the flat I don't see why you can't do the same for an earth.

burn

burn
 21 February 2014 06:52 PM
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weirdbeard

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Hi dazum1, welcome to the house of fun......

My first point is you say it's impossible to install a new supply cable....thats never true...do you mean it's just not easy?

2 - how long has the existing supply cable been in place? if it's not showing signs of overheating then it must have been man enough to date to provide the necessary services required for a one bed flat?

3- Why 2 X immersions, this is only usual where there is on peak and off peak metering - is this what there is?

4- 16A pump????? for what? a huge water feature???

5- 40A cooker for a 1 bed flat.....if this is going full bore there'd be no need for space heating! 13A micro/grill/hob combis aren't unusual in 1 bed flats!

6- 63A 30mA - unless thats an RCBO, it sounds like an RCD you're looking at which only provides 30mA RCD protection, the 63A isn't really relevant?

7,8,9 etc to follow.......

 21 February 2014 06:54 PM
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anastasis

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I'm a bit confused why there is a 6kw boiler and 2 immersion heaters... but on together they will overload your supply (assuming 3kw imm heats) and that's before the cooker is switched on.

Is the boiler actually a shower?

And do the two imm heats need to be on together? Could it be a sink/bath setup - one or the other? Maybe the intention is for it to be economy 7 with the bottom one on overnight.

Remember that the regs require you to avoid small overloads of long duration - you could easily have this here.
 21 February 2014 06:59 PM
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weirdbeard

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Originally posted by: burn



The two immersion heaters should be on two separate timeswitches, the main (lower) one to take advantage of cheap night tariff to heat the whole tank and the other to reheat the top half as a short evening boost if required.



Bear in mind that for any consumer to take advantage of the percieved cheaper off peak/on peak tariff, they must be using at least 50% of their electric on the off peak - such as a good sized house with storage heaters - if not using this then the customer is better off using well controlled heaters when they actually need them on a standard tariff.
 22 February 2014 09:33 AM
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broadgage

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Without more detail, I very strongly suspect that the existing cable is very inadeqaute.

Regardless of the exact arrangements, full use of a 40 amp cooker on a 46 amp capacity cable effectively prohibits use of ANY other significant load at the same time.

Use of the cooker and the 6KW "boiler" at the same time would be a significant overload in itself without use of even one of the immersion heaters. And what about a tumble dryer, hair dryer, kettle or toaster.

Some diversity is of course allowable but not that much with potentialy long hour heating loads.
The 6KW boiler is probably for wet central heating, in which case it could run for hours at a time.
Likewise an immersion heater is a long hour load that could well run for several hours and at the same time as the boiler.
Those two loads would be about 40 amps in total.
6 amps would seem a reasonable estimate, after diversity, of the total consumption for lighting, TV, refrigeration and other low loading appliances.
So that is the cable fully loaded by long hour loads that are likely to be used at the same time.
Use of the cooker at the same time would be a significant overload, and what about a washing machine, microwave, hairdryer, kettle, or toaster ? most unlikely that all those would used at the same time, but even the use of just one at a time could add about 10 amps average.

And they might reasonably add a tumble dryer which though only 10 amps typicly can run for a long time unlike a kettle, toaster or microwave oven.
 22 February 2014 09:45 AM
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OMS

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As this is one of 10 flats, then get a power logger on a few of the other flats to get an idea of baseline demand ?

That said, a 10mm2 would be good for about 60A clipped direct, so it's probably not a real issue given the nature of the loads.

That's up to you to ascertain the loads however - your description needs, perhaps, a bit more detail to comment further.

Regards

OMS

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 22 February 2014 11:30 AM
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alancapon

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I wouldn't initially worry about your 63A RCD main switch on the existing supply, as it should be fused at no more than 60A in the basement on the other end of the 10mm supply cable anyway.

How does the boiler work? Does it use both immersions while it is pumping water, or does it use both during the night (on an E7 supply for example) and only one during the day? How are you planning to signal to the boiler than the off-peak tariff is available (assuming you have an E7 supply)?

Are you sure that the incoming supply is suitable for an additional 6kW load? Have you asked the local DNO this question? It is possible (particularly if there is no gas) that the DNO's supply is already pretty well loaded.

What type of supply is it? TN-S, TNC-S or TT? If it is TNC-S, then your earth in the submain is probably going to be undersized.

What is your maximum demand? I would suspect that you maybe the wrong side of 60A!

Many more questions than answers really . . . .

Regards,

Alan.
 22 February 2014 12:44 PM
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dazum1

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In truth, I have no idea about the plumbing side of things, and trying to get answer from them is like trying to draw blood from a stone. I don't think they even know themselves! All the information I can get from them is they need a supply for 6kw, 2 x supplies for 2 Immersion heaters and a supply for a pump - these are all going on 13A FCU's, fuses will be changed accordingly when more is known - the 6kw supply is 45 DP FCU.

I have asked the customers if they can see if the plumbers can change what they are installing, to reduce demand on the system, and when purchasing a new cooker/hob, to get one that comes with a plug fitted - which means no more than 13A. So I am hoping with that we should be okay as that is cutting out a lot of potential demand.

When I say it is "impossible" to run a new cable, of course its not "impossible" but it would mean routing a cable from the basements, through the flat above, through the hallway/stairway, though another flat, and into the flat where we need it. This would mean pulling people carpets/laminate flooring up, lifting floor boards up, puling the carpet tiles in the hallway up, who knows what else in the other peoples flats - all that disruption would not go down well with the flat owners.

The main thing with all the flats is, they all originally had electric heaters, 5 in total, and all in the region of 1kw, the bathroom one would hardly have been used - same as the kitchen and hallway. So realistically you may have had 2-3kw of consumption, which means they would have had able consumption for all the other items within the flat.

As I say - the 46A is worst case scenario, and not being able to trace the supply fully, I cannot say to sure what the cable is running through, it is within the ceiling voids, and I know for sure it is bunched with various other cables where it heads out of the flat.
 22 February 2014 02:19 PM
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anastasis

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45A DP FCU - are you sure? Never seen anything of the sort, other than a Wylex 104 switchfuse!

Generally, from what you've said, this job has a good chance of going pear-shaped in a big way (running costs as well as supply capacity) unless you sort out the communication problems and clarify the specification. Quite frankly I'm not at all convinced by the design.

Using a 6kw electric "boiler" to feed radiators is unheard of IMO. There's no point in using electricity to heat water, and then moving the water round to rads with a pump. Just stick electric heaters (panel / storage / fan as appropriate) in the rooms.

And if a new supply is needed, do some lateral thinking - if it can't go on the inside, what about the outside? Could you clip some SWA to the wall?
 22 February 2014 02:39 PM
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redtoblackblewtopieces

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I wonder if the electric boiler is the Aztec type which would send the heated central heating water through the coil of the tank (as with s plan systems) and the immersions are for back up only. Hope its on econemy 10 -reconnected a 9kw replacement one of these in a two bed flat for a plumber just recently.
Kevin

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 23 February 2014 01:14 AM
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alancapon

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Originally posted by: dazum1
. . . All the information I can get from them is they need a supply for 6kw, 2 x supplies for 2 Immersion heaters and a supply for a pump - these are all going on 13A FCU's, fuses will be changed accordingly when more is known - the 6kw supply is 45 DP FCU.

Are you now talking 12kW? 6kW for the boiler and two 3kW immersion heaters?


Regards,

Alan.
 23 February 2014 07:56 AM
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broadgage

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Originally posted by: alancapon

Originally posted by: dazum1

. . . All the information I can get from them is they need a supply for 6kw, 2 x supplies for 2 Immersion heaters and a supply for a pump - these are all going on 13A FCU's, fuses will be changed accordingly when more is known - the 6kw supply is 45 DP FCU.


Are you now talking 12kW? 6kW for the boiler and two 3kW immersion heaters?

Regards,

Alan.


That was my understanding when I estimated that the cable was too small.
I presumed that only one of the presumed 3KW immersion heaters would run at the same time, but together with the 6KW boiler and a post diversity load of 6 amps for lighting and low loading appliances, that is the cable fully loaded without any use of the cooker or other appliances.
If BOTH of the presumed 3KW immersion heaters can run at the same time, then it is even worse !
 23 February 2014 12:05 PM
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alancapon

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Originally posted by: dazum1
. . . I have asked the customers if they can see if the plumbers can change what they are installing, to reduce demand on the system . . .

To be honest, 6kW is fairly small in terms of a central heating boiler. Most gas or oil boilers are rated at between 15kW and 24kW. A better option is going to be for your quote and plan to cope with the load proposed, which you have to sort out the maximum demand and diversity for. Isn't this why the householder chose to use an electrician to do the work, rather than the plumber or "the man from the pub"?

The landlord of the building will have rights over the common riser areas for a new cable to be installed - you really need to be talking to the landlord rather than planning to hack the other flats about for your cable.

As I said earlier, what enquiries have you made with the DNO to ascertain that the supply to the property can cope with the additional load you are planning to connect?


Regards,

Alan.
 24 February 2014 12:38 PM
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AJJewsbury

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I can get from them is they need a supply for 6kw, 2 x supplies for 2 Immersion heaters and a supply for a pump - these are all going on 13A FCU's, fuses will be changed accordingly when more is known - the 6kw supply is 45 DP FCU.

Perhaps a load shedding relay or two might help?
- Andy.
 24 February 2014 12:43 PM
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AJJewsbury

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We need to add bonding within the flat which we will be using 10mm earth

While undoubtedly good practice (to reduce touch voltages within each flat), as far as I know there's no BS 7671 requirement to repeat main bonding within smaller parts of of a bigger overall building - i.e. you could look at it as one big installation with main bonding just at the main intake positions. (Presuming no extraneous-conductive-parts enter the building directly into the flat.) So no real requirement for a 10mm2 c.p.c./bonding conductor to each flat.

- Andy.
 24 February 2014 05:56 PM
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Crazycolours

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It would be interesting to know all the supply parameters for a start?

Was it changed when the House was converted to 10 flats?
What type of supply is it?
How many phases?
What type of DB is in the basement.
What is the total load?

Others have asked these questions but the OP hasn't given all the details? (unless I've missed a post).

Regards, C.

Edited: 24 February 2014 at 06:02 PM by Crazycolours
 24 February 2014 06:39 PM
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dazum1

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Thanks for all the replies.

The original job was to rewire the flat, but due to trying to save costs the customer got a survey done and the electrics were deemed to be fine, so the job changed to a CU change and to add a couple of circuits - as we do a lot of work for this particular customer, we took his word on that but now I am questioning it. I have pulled off the job today for now, I met with the plumbers and the flat owner to discuss what the flip is happening. Explained that the 10mm cable is not anywhere near sufficient for the proposed plumbing demand, let alone anything else within the flat and that the survey should have picked up on that, having known that a wet system was being installed.

I have told the flat owner that if he really wants the wet system instead of the original storage heaters etc, then he must look at getting a new 2nd supply into the flat. Upon further inspection in the basement, there is a 2nd supply that is currently coming into the flat but is actually on a timer switch and is only live between 6pm and 8pm - obviously this is of no use to anyone and is only a 6mm T/E. I have advised for him to get in touch with the electricity board and have them come out and do what they need to do. The meters, timers and even main fuses down in the basement look to have been there for 30+ years, the main fuse is the old BILL type lever switch which I have never come across and ideally the whole electrical system could do with upgrading - which I am sure the board will say when they come out to have a look.

The main issue is the customers and the plumbers not really talking between one another and to us. A quick 30 minute meet up today and we all know where were at.

To answer a few questions...

I originally thought the house was converted into flats around 10 years ago, but it would seem a lot longer due to the current fittings in the basement
It is a TN-S system
3 phases

As you say AJJ, it is not a requirement but we do it for good practise.

The only option is to bring another supply into the flat, minimum another 10mm, along with a 16mm earth. It will be ran along the outside of the building, have 2 separate smaller consumer units. The flat owner has had to bite the bullet on that one. I think it is a lesson well learnt for him he has tried to cut costs back and scrimp, but inevitably it has come back to haunt him (I did warn the customer at the time), and for me to not to take his word again and to double check, BUT from day 1 I did say we need to check the supply and we may need to change this.

After speaking with another flat owner today - he even told me he had told the customer about the supply cable and that it would not be sufficient, and he still ignored it!

Hopefully the job will now finally go as planned!
IET » Wiring and the regulations » 10mm supply

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