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Insulated ceiling ring final nightmare.
jimmyhorns
14 Posts
Question
I am having a bit of a nightmare trying to adequately size cable for a ring final. The existing circuit is 2.5mm2 T&E which runs in the ceiling void between ground floor and first floor (this is a residential property). Currently there is no ceiling insulation installed. An extension means a new ceiling and significant modification to the ring final, and also (to meet regs) sound proofing insulation installed in the ceiling. This has a similar U value to thermal insulation in most cases.

100mm of insulation is to be installed throughout the new ceiling - which is 50% old circuit.

My first thought were to re-wire the whole circuit in 4mm T&E as with 2.5mm2 and the 18th ed tables its method of installation in all places wont give me 20A on each leg of the ring.

Great I thought, problem solved with plenty of overhead. More of a pain at 2nd fix, but not the end of the world.

Except now when I am looking through it seems like 4mm2 might not be enough if its going to run the risk of being covered with insulation. Joists are 170mm, so while the cable will not be covered completely the insulation will tend to balloon around it, so it looks like I will have to de-rate by a factor of 0.5 which would mean needing 6mm2 cable which seems crazy.

Any thoughts? Where am I going wrong in my assumptions / calcs?

Thanks in advance

James
33 Replies
Alcomax
280 Posts
Before anything else, what does the ring final serve? i.e. the real likely demand. Also, are you going to be running any other new circuits through this soon be be insulated ceiling?
Traditionally, first fix of cables has always been though centre of floor joists. Fine when no insulation, but later on any lateral runs are potentially to be covered by insulation. Similar in a way to lofts. Yes it is a problem. 16 amp or 20 amp ring reduction can help. With 4mm ring if I remember without calculating, a bit of a get out is a 25 amp OCD, though not all switchgear manufacturers do a 25 in some domestic CUs, though Hager has a full range that fits commercial and domestic. Not sure about other makes.
mapj1
3144 Posts
In practice I suspect most folk do not worry so much and there are no issues, but if you do want to do so, some options.

Clip cables to joists - I know it seems odd, but in rockwool the wood is the better heatspreader than 'free' cable. Dress cables either above or below insulation as far as possible, so insulated on one side only.
Battens and plasterboard for an empty  suspended ceiling a few 10s of mm under the sound proof one ?
Run round the walls in the 6 inch zone below the ceiling.
Mike,
Sparkingchip
3803 Posts
Make it two 16 amp radials with 2.5 twin and earth cables.
Ah the building is going to be flats, is that why you have the acoustic insulation? You are making an assumption about the acoustic insulation, using standard Rockwool insulation does not give much soundproofing. The acoustic product comes in panels about 50mm thick, which have considerable density. This is necessary to absorb low frequencies which are acoustically much more troubling, but they offer good high-frequency attenuation too. Commonly such is carried out with the slabs fixed to the ceiling with very tight joins, and then covered with 2 layers of high-density plasterboard, joins all staggered. All this has quite a high acoustic attenuation, and also the necessary level of fire control. The slabs just between the joist are difficult to fit and lose a considerable proportion of the sound attenuation. All this should be in the hands of an acoustics designer, and if done in this manner you still have an insulation-free ceiling void. It would stop any fire damage above unless there is a very considerable fire, burning for a long period, hours, as the acoustic insulation is very effective against heat transfer too.
Chris Pearson
2252 Posts
Which RM/IM are you using? 2.5 mm² should be good for a 32 A ring, but 4 mm² may not be for a radial. I have considered boxing in the wiring to get round this problem.

(AFAIK all new builds require sound insulation.)
AJJewsbury
2333 Posts
100mm of insulation is to be installed throughout the new ceiling
Provided you can position the cable next to a joist or plasterboard - it sounds like method 100 to me - which table 4D5 gives 2.5mm² T&E a rating of 21A.
   - Andy.
Alcomax
280 Posts
OK.  The OP has quite correctly come to the conclusion that some of the existing RFC 2.5mm will be sandwiched between 2  lots of 100mm insulation type material. So imagine 2.5 mm twin/earth in pipe lagging, but the lagging is 100mm rockwool or similar.
Yes, some will be resting on the plasterboard, some may be clipped to side of joists, but  likely a significant amount is routed through joist holes for some distance between joists. This is all existing. Suddenly this cable is derated. I know many wish that away, but it derates all the same. If you are adding to this circuit or altering it, you need to take this into account. After all you are aware that insulation is going to be installed? 
If this was new work you could use all sorts of exotic and time consuming construction methods, or you could install as normal , holes through joist but wire the ring final in 4mm with a 25 amp OCD . 2 x 4mm twins in a socket is no big deal; many contractors routinely use 4mm for rings now.  25 amp covers all bases, 32 amp is fine with careful routing of cable.
jimmyhorns
14 Posts
Alcomax:
Before anything else, what does the ring final serve? i.e. the real likely demand. Also, are you going to be running any other new circuits through this soon be be insulated ceiling?
Traditionally, first fix of cables has always been though centre of floor joists. Fine when no insulation, but later on any lateral runs are potentially to be covered by insulation. Similar in a way to lofts. Yes it is a problem. 16 amp or 20 amp ring reduction can help. With 4mm ring if I remember without calculating, a bit of a get out is a 25 amp OCD, though not all switchgear manufacturers do a 25 in some domestic CUs, though Hager has a full range that fits commercial and domestic. Not sure about other makes.


Thanks for the reply:

The ring final currently supplied a living room and two bedrooms. I didnt want to overcomplicate the already slightly complicated first post but can see more clarity would be useful.

The groundfloor is being reconfigured and the living room becomes a kitchen, the extension the new living room. First floor is also reconfigured from 2 bedrooms to 3 bedrooms and an ensuite.

Loads are likely to be relatively high so ring reduction below 32 Amps probably isnt going to be sufficient, certainly not for the kitchen component.
Total floor area served will be 90 square metres though I am mindful that two ring finals would probably be a better solution given the appliances that are  likely to all be in use at the same time (washing machine, dishwasher, heat pump tumble drier, coffee machine, toaster, hairdryer etc). Although the figures suggest I can just sneak in under the 32 Amps I would much prefer a belt and braces approach.

In the new area (extension) section I can run above the insulation clipped to joists as the joists are 225mm but to get to there I have to route along 170mm joists.

As you say the first fix is indeed through the 'centre' of floor joists.

Would a solution be to install substantial noggins between joists adjacent to the hole in joists and therefore be able to have the cable clipped and in contact with the noggins or joist throughout its path, provided I run 4mm2?

If I understand this correctly I would then apply reference method A, which for 4mm2 in T&E gives me 25 Amps per leg.
If I do this on alternate sides of the noggin this would give me 2 x 32 Amp ring finals 

Any flaws to my logic here?

Chris Pearson
2252 Posts
By my reckoning, you have 70 mm to play with above the wadding. If the floorboards are 20 mm, that gives you a zone of 40 mm in which you can clip the cables. It will be a fiddle, but feasible.

I have faced this challenge in a house with wet underfloor heating, so leaving aside the thickness of the floor, there is (should be) no risk of penetration by nails from above. With web joists, it was possible to clip everything up out of harms way.

The only misgiving that I have is that I have stated IM 100# on the EIC, but the wadding was put in after I left so I don't know what is actually in there now.
Alcomax
280 Posts
Assuming you may need two circuits, if you did use the existing 2.5 ring, what would that serve at the end of the day?
Edit: looks like a kitchen!
A 2.5 ring on a 20 will work, but not on a large kitchen. Wood supports between joist to have it " clipped" is interesting, but it would still be surrounded by insulation anyhow.  If the existing 2.5 was just two beds and a living room 20 amp would work. With 4mm the trade off 25 amps or 32 amps may not be a big deal. only 7 amps; think the maximum demand of the whole house and the number of circuits,  many rings are not going to be a problem with a 25 amp circuit breaker.
jimmyhorns
14 Posts
davezawadi (David Stone):
Ah the building is going to be flats, is that why you have the acoustic insulation? 

No - normal residential property (2 story house). Building control are adamant that there has to be some form of insulation in the ceiling void. Architects technician agrees its required to meet regs. I'm not up for the fight to show that it isn't - its a case of picking your battles. I cant find anything in the regs other than 'reasonable provision to reduce sound transmission' which leaves it completely open to interpretation - in this case building control's.

 

jimmyhorns
14 Posts
Alcomax:
Assuming you may need two circuits, if you did use the existing 2.5 ring, what would that serve at the end of the day?
Edit: looks like a kitchen!
A 2.5 ring on a 20 will work, but not on a large kitchen. Wood supports between joist to have it " clipped" is interesting, but it would still be surrounded by insulation anyhow.  If the existing 2.5 was just two beds and a living room 20 amp would work. With 4mm the trade off 25 amps or 32 amps may not be a big deal. only 7 amps; think the maximum demand of the whole house and the number of circuits,  many rings are not going to be a problem with a 25 amp circuit breaker.

I think I am already at the stage of starting again with 4mm2. There is so much of the existing circuit that would need to be modified with the added complication of junction boxes that I can do without.

There is a natural split down the middle of the kitchen, effectively dividing it into two halves. From the point of view of balancing out the two ring finals better, supplying 1 side of the kitchen, 1 side of the living area (kitchen and living area are open plan 50 sq metre, square dimensions) and 2 bedrooms on one ring final, and the other half on the other seems like the best option. Like you say demand from living area and bedrooms will be relatively low, and the kitchen demand substantially higher.

Looking more carefully I think that reference method 100, if I can clip to wood consistently (it mentions joist but if the noggin is the same size as the joist I cannot see there being any difference with heat transfer):

 

Reference Method 100

Flat Twin and Earth cables - See Table 4D5 for current-carrying capacities:

Installation methods for flat twin and earth cable clipped direct to a wooden joist, or touching the plasterboard ceiling surface, above a plasterboard ceiling with thermal insulation not exceeding 100mm in thickness having a minimum U value of 0.1 W/m2K.

If the insulation is >100mm then its method 101 however this is still >20 Amps per leg if I use 4mm2.

It looks like the solution is my mitre saw, a couple of lengths of 2x6, a reel of 4mm2 T&E  and some graft..........

I am beginning to think that something is very wrong here. Thermal insulation between floors of a single dwelling? Certainly, I cannot think of this requirement, and the BRegs are being rewritten at this moment so this is not something yet present. Ask Building control where and what this is. The envelope of buildings is the controlled thing, and any stairs will make this idea completely redundant anyway unless there are thermal doors top and bottom.

Now the Electrics.
From your description, I assume that the existing property has a single ring serving both floors, in the ceiling void of downstairs. Please advise if this is not the case. You wish to extend it to provide all the power for the extended property. I ask why, I would expect a new circuit at least for the kitchen. Is the ground floor solid? If so run a new circuit in a chase in the walls for downstairs. You will have a plasterer, it is easy to repair. I suggest another circuit for the kitchen only, same installation method, you have to chase the walls anyway so a bit more length is no problem. How will you feed the cooking appliances? It sounds as though the whole conversion/extension is a DIY job. Fine, not a problem, except you will need an Electrical Installation Certificate from a competent source (person) to get the BR completion certificate.

Asking here for a design is OK too, except that we only give advice on the regulations, not a final design. Plenty of ideas here (that is why the forum has members) but ideas are never the complete answer, or the design. Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it. I have suggested an alternative method, much better than 200mm of Rockwool between floors. This will be thermally better, and much easier on the labour front, both the electrician and labourer and Boss will cost more than you putting up some nice slabs and 1 plasterboard. The loss through the joists makes 100mm useless, so overall only 100mm.

I may have drawn some incorrect conclusions from your data presented. Fair enough, present the real situation, why you want to do A or B is important. I am sure that you will then get the best free advice available.
As I got interrupted I missed the last post. You could do as you say but it is quite a lot of work, reduces the insulation u value of the floor, and will be very difficult to change and test in the future. This is much more than you seem to suggest, it would help if you describe the whole design, including how you intend to cope with partP of the Building Regulations. I trust the BI has told you about that too.
Regards
David CEng.
 
Chris Pearson
2252 Posts
davezawadi (David Stone):
Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it.

David, it is all in Part E. Although it seems rather onanistic, there are good reasons for it. For example, at one stage Daughter and her beau had a semi-basement flat in Town. Unfortunately, every little tinkle, splash, and flush from the loo in the landlady's flat above was audible. 😝

I note what you say above about acoustic insulation. I have not yet found a value for its U value, but presumably it is a little less than that of thermal insulation. In the meantime, provided that cables are kept to the edges, I think that IM 100# is the best that we can do.

jimmyhorns
14 Posts
Chris Pearson:
davezawadi (David Stone):
Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it.

David, it is all in Part E. Although it seems rather onanistic, there are good reasons for it. For example, at one stage Daughter and her beau had a semi-basement flat in Town. Unfortunately, every little tinkle, splash, and flush from the loo in the landlady's flat above was audible. 😝

I note what you say above about acoustic insulation. I have not yet found a value for its U value, but presumably it is a little less than that of thermal insulation. In the meantime, provided that cables are kept to the edges, I think that IM 100# is the best that we can do.

Thermal conductivity values are very similar - looking at rockwool - 100mm thermal is 0.044 vs acoustic 0.038 W/mK - and it is on this basis that I am applying the rules for installation in or adjacent to thermal insulation. Im going to run the design past the person issuing the electrical installation certificate before commencing work, but wanted to get it right before submitting it and also for my own piece of mind. I like to work through problems logically and better my understanding through doing so. Thank you for your time in helping with my understanding - I am grateful to all that have contributed to this thread. 

jimmyhorns
14 Posts
davezawadi (David Stone):
I am beginning to think that something is very wrong here. Thermal insulation between floors of a single dwelling? 

Now the Electrics.
From your description, I assume that the existing property has a single ring serving both floors, in the ceiling void of downstairs. Please advise if this is not the case. You wish to extend it to provide all the power for the extended property. I ask why, I would expect a new circuit at least for the kitchen. Is the ground floor solid? If so run a new circuit in a chase in the walls for downstairs. You will have a plasterer, it is easy to repair. I suggest another circuit for the kitchen only, same installation method, you have to chase the walls anyway so a bit more length is no problem. How will you feed the cooking appliances? It sounds as though the whole conversion/extension is a DIY job. Fine, not a problem, except you will need an Electrical Installation Certificate from a competent source (person) to get the BR completion certificate.

Asking here for a design is OK too, except that we only give advice on the regulations, not a final design. Plenty of ideas here (that is why the forum has members) but ideas are never the complete answer, or the design. Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it. I have suggested an alternative method, much better than 200mm of Rockwool between floors. This will be thermally better, and much easier on the labour front, both the electrician and labourer and Boss will cost more than you putting up some nice slabs and 1 plasterboard. The loss through the joists makes 100mm useless, so overall only 100mm.

I may have drawn some incorrect conclusions from your data presented. Fair enough, present the real situation, why you want to do A or B is important. I am sure that you will then get the best free advice available.
As I got interrupted I missed the last post. You could do as you say but it is quite a lot of work, reduces the insulation u value of the floor, and will be very difficult to change and test in the future. This is much more than you seem to suggest, it would help if you describe the whole design, including how you intend to cope with partP of the Building Regulations. I trust the BI has told you about that too.
Regards
David CEng.
 

Thanks David for your very detailed reply.

This is acoustic insulation to satisfy part E. There is an argument that it doesnt apply to extensions, but the water here is muddy and both local planning control's ascertion is that it does, as does the independent building control company that I am using for the build (licenced). Being completely honest I am quite pleased that there will be acoustic insulation present as with 3 boys in the house whose version of 'no running in the house' seems to be the complete opposite there may be significant benefit from it!

The extension itself is a project managed professional job. I am only tackling some aspects of it myself - agreed with the team performing the majority of the work. I do not have the skills for the majority of the work, or at least to do it to the standard required and the speed required. We wouldnt cope with the roof half off for 2 months while I did it, but a week is do-able!  I would class myself as a pretty well read DIYer when it comes to both the plumbing and the electrical works. I am reasonably bright, have been through the on site guide and read IET Wiring Regulations: Electric Wiring for Domestic Installers and a couple of other books to understand the theory behind the regulations. Other hobbies involve electronic and microcontroller design mostly for RC model aircraft which involves similar basic principles. I am doing this work because I want to rather than have to - I enjoy the learning and understanding that comes with it even though I will not be using it professionally (Im a surgeon by trade). It would of course be a lot simpler to leave this headache to someone else and pay them for their time, and will indeed be paying someone competent to provide the relevant certificate for building control.  I appreciate that I will not be able to do the job as efficiently as someone with formal training, but with enough care and attention and research I should be able to do the work to an acceptable standard - and if not I will be pulled up on it quite quickly. I hope that helps with where I am coming from on this - not an arrogant 'its easy I can do that' but more a 'I enjoy learning how to do this and it gives me satisfaction to know that I have made some of our home work and work safely'.

So back to your post:

From your description, I assume that the existing property has a single ring serving both floors, in the ceiling void of downstairs. Please advise if this is not the case.

Not quite. The existing property has two 32 Amp ring final circuits on 2.5mmT&E. The first supplies both floors at the front of the property (Study, Kitchen excluding cooker, hallway on GF, hallway on FF, one bedroom). The second supplies both floors at the rear of the property (Living room, two bedrooms).
Lighting is by 2 x 6 Amp radial circuits on 1mm2 T&E, one per floor.
The existing cooker is supplied by a 40 Amp radial on 6mm2 T&E


The extension is at the rear of the property, and two stories. The old kitchen becomes a 'snug' which seems to be the in word for another small living room. No changes need to happen to the front of house ring final to meet new demand (demand will actually reduce).

The old living room becomes the kitchen, the extension the new living room. Upstairs there is the addition of an extra bedroom and ensuite, although the layout changes and existing socket points no longer have a wall to be attached to so will need to be moved.

I am at the point of drawing up the design. preliminary calcs suggest that I could do the whole rear of the house on a single 32 Amp ring final, but in reality I suspect use will push beyond this at times and would rather have the certainty of being well within spec rather than on the edge and risk several thousand pounds of corrective work.

I'm on call tonight, but let me post up the design and calcs once I run through them and then I welcome the scrutiny. Im not expecting people to do this work for me, but a gentle nudge or more if I am going off piste. As a quick summary current plan (in my head) is to replace the rear of property ring final with 2 x ring finals one for each side of the house. cooker will be supplied by a 40 Amp radial as previously but on 10mm2 T&E (this is already in but of course not connected). Part P will be dealt with through the electrician who will be inspecting and the building control company I am using.

Once again thanks for your help

Sparkingchip
3803 Posts
Sparkingchip:
Make it two 16 amp radials with 2.5 twin and earth cables.
2 x 16 amp circuits require cables rated at 16 amps to provide 32 amps capacity.

1 x 32 amp circuit requires cable rated at 20 amps to provide the same 32 amps of capacity.

So if using 2.5 mm twin and earth it can be derated by 20% for the 16 amp circuits from the 20 amps required for the 32 amp ring circuit.
Ok, Jimmy.
I have looked at Part E quite carefully and can find no reference to this for existing buildings. I don't know where this came from, saying it is "muddy" is not true, the section on domestic floor acoustics is specifically about new builds.
webarchive.nationalarchives.gov.uk/20141202105407/http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_E_2010.pdf page numbered 66.
I suggest you ask your independent BI for the exact details he thinks are there, because if you really want noise isolation this very simple design is not the way to do it! It is also fiddly to install, needing all the floor up, or if you really want to make it tricky the ceiling down!
Have a look here www.rockwool.com/uk/advice-and-inspiration/why-stone-wool/acoustic-capabilities/, there are several useful videos.
Rockwool only seems to consider conversions to flats, which is obviously where the acoustic properties are most important.

I should point out that I have been involved in the design of quite a few sets of acoustic treatment and as these are for broadcast and performance spaces, the results have to be good and have been!
I'll give a quick rundown on the theory. I suggest the treatment of the ceiling (above) in your house for minimum noise transmission is the best solution because most of the low-frequency transmission through a timber joist floor is via the joists, not through the space between. Higher frequencies do travel through the air space but one of the important points for attenuation is that the absorber is mechanically lossy (to reduce transmitted energy and turn it to heat) and obstructs as much of the path as possible, to ensure the air movement causes mechanical loss. This is why acoustic products are dense, non-reflective as far as possible, and very mechanically lossy. This is not true of thermal insulation where just keeping "trapped" air fairly still is all that is needed for efficiency so the fibres can be much less dense or constrained. Coupling to the air makes no difference thermally. Two layers of plasterboard are heavy and stiff enough to work as membrane absorbers for any low frequencies passed through the acoustic absorber panels from the movement of the entire floor structure from bass music, or jumping about. Note that the attenuation you are likely to achieve is not perfect, but may be of the order of 30dB if you do a good job.

I hope that description, at least, will give you a bit of traction with the BI and perhaps focus your thoughts. It will greatly decrease transmission upwards and also reduce reverberation in the downstairs room.

With no insulation to worry about in the floors, the job will be simple and quick, and may well pay for the materials for the ceiling treatment in reduced electrician costs, so probably a win-win situation. Two persons can easily treat a ceiling although the work is quite heavy, sheets of high-density plasterboard, but a couple of the prop gadgets help greatly to hold and position them. You will need some long drywall screws and an electric screwdriver! The slabs will not need many fixings as they will be clamped between the ceiling and new boards, you will need very tight joints and you might decide to glue them to the ceiling for convenience, but screws too will work well.

I look forward to the electrical notes.

Regards
David

 
Zoomup
2277 Posts
M.I.C.C.

Z.
Chris Pearson
2252 Posts
I can understand the satisfaction obtained from doing the work oneself and as a surgeon, jimmyhorns presumably likes fixing things with his hands.

That's the easy bit!

It occurs to me that the existing narrower ceiling does not need acoustic insulation because it must be no worse than it was previously. (R.4(3) of BR 2010). If the building control wallahs want it in the new bit, best to put it in, but the taller void will make it easier.

If you simply extend the existing circuits, they are not notifiable under Part P; but any new circuit would be.

If you put in one or more new circuits, the existing CU needs to be suitable. Specifically, it needs to have space for any additional MCBs; the appropriate ones need to be still obtainable; and RCD protection is required.

I do not see why there should be any significant increase in power consumption - there is no mention of an electric shower in the new en suite bathroom.
Have you looked him up on the medical register Chris? Perhaps the name is a giveaway. I think that is the correct interpretation of your comment, it could refer to you though!
jimmyhorns
14 Posts
Chris Pearson:
I can understand the satisfaction obtained from doing the work oneself and as a surgeon, jimmyhorns presumably likes fixing things with his hands.

That's the easy bit!

It occurs to me that the existing narrower ceiling does not need acoustic insulation because it must be no worse than it was previously. (R.4(3) of BR 2010). If the building control wallahs want it in the new bit, best to put it in, but the taller void will make it easier.
 

My thoughts were exactly this - the old ceiling does not need 'improvement' as it will not be any worse than it was previously and is not dangerous. I conceed to David's superior knowledge of the soundproofing issues. Due to ceiling height considerations dropping it further in the new kitchen section would not be practical - what you say does make perfect sense in terms of the difference in mechanism of actions of the differing types of insulation.

Building control's approach on this is that it is a new single room, rather than an old room and a new room. The existing rear wall of the property, (apart from 2 x 700mm piers that come out from the wall to support the 6 metre steel) disappear. On the basis that this is a new room, and this area of the room has a change of use into a kitchen, now requires different ventilation / extraction and fire detection I can live with the notion that this is 'new' rather than existing. I also note that part E only applies to new builds. 

I have already had conversations regarding the above and BC have been very reasonable about some other aspects of the build that could have ended up being very expensive to solve but common sense prevailed. I am not up for the fight on this if there is now a solution, even if this means a bit of extra work for me... life is just too short :)


Well the first bit of the electrical notes:

Supply and Main intake:

Single phase, 230V 25mm2 supply into 100 Amp cut out (Henley series 7) with TN-C-S Earth system

25mm2 meter tails from cut out to 80 Amp (non-smart) Meter

25mm2 consumer tails and 16mm2 Earth to Switch Fused unit (100Amp) and then into 100Amp Dual RCD consumer unit.

Planned circuits with diversity calcs:

Lighting 1 - 20 x 10W luminaires (All LED) = 200W = 200/230 = 0.87 Amps, applying 66% diversity =    0.56 Amps
Lighting 2 - 30 x 10W luminaires = 300W = 300/230 = 1.3 Amps, applying 66% diversity =                     0.86 Amps

Immersion Heater - 3000W = 3000/230 = 13 Amps. No diversity =                                                           13 Amps
Cooker - 4800W = 4800/230 = 21 Amps, applying diversity 10 = (0.3x11) = 10 + 3.3 =                            13.3 Amps

Ring final 1 = 32 Amps full load. No diversity applied                                                                                 32 Amps
Ring final 2 = 32 Amps full load, applying 40% diversity = 32 x 0.4 =                                                        12.8 Amps
Ring final 3 = 32 Amps full load, applying 40% diversity = 32 x 0.4 =                                                        12.8 Amps
Garage radial = 32 Amps full load, applying 40% diversity = 32 x 0.4 =                                                    12.8 Amps

Total maximum demand applying diversity =.                                                                                             98.12 Amps




So the first issue I get to is that it is unclear whether the supply is rated for 100 Amps (though the cabling is, the meter isnt and the DNO may not specify a 100A supply)

Any flaws so far?

 

Chris Pearson
2252 Posts
jimmyhorns:
Any flaws so far?

Yes, if you plugged everything in and switched on, how much current would it add up to?

If you had only 2 ring finals in the house, you would subtract 12.8 A from the above, but the overall load would be the same.

32 A is a lot for a garage unless you have an EVCP. Could be 1 x 3 kW heater, but then what?

Try dividing your annual consumption by 365, then 24, and finally 0.23: that will give you your average load. It won't be much.

jimmyhorns
14 Posts
Chris Pearson:
jimmyhorns:
Any flaws so far?

Yes, if you plugged everything in and switched on, how much current would it add up to?


32 A is a lot for a garage unless you have an EVCP. Could be 1 x 3 kW heater, but then what?
 

I thought that was the purpose of diversity - to correct for the fact that everything is unlikely to be on at the same time.

To answer your question, with everything plugged in and at full capacity, a lot of Amps, but in practice I cant see that happening.

For example, the immersion heater 'never' gets used etc.

In reality the cooker, washing machine, tumble dryer, hairdryer, full garage load and the lights are all that would be on simultaneously which would give:

21+13+8+32+2 = 76 Amps

The garage is more of a workshop and garage. 3kw heater + 4kw compressor + Mill + lathe + Plasma cutter + (insert any of the other toys here). Average demand is actually very low, but if its cold, the compressor kicks in while I am using the plasma cutter you are looking at 13 + 20 + 13 amps so already over spec - I'm careful with what runs simultaneously of course limiting it to one machine if the heater is running or two machines if it isnt.

jimmyhorns
14 Posts
On the plus side I have called the DNO and confirmed it is a 100 Amp supply 

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