Diversity for multiple occupancy?

[homespark]

Hello gents
I have been asked to quote for two separate jobs: one is a large house being converted into six flatlets, each to have their own CU, and all fed by a single-phase incomer, currently fitted with a 100A cutout. Each flat will probably have a maximum demand of 60A, protected by its own 60A cutout.
The second job is an existing house that has been converted to 5 flats in the past (long ago: very old cloth-covered distribution wiring). I have not yet managed to gain access to the very old & rusty distribution board, but expect to find 60A brakers (maximum) inside. The main cutout may be 150A (not definite as yet).
The main issue I'm concerned with is diversity, and whether it applies to this situation, i.e. if each flat has a max demand of 60A (say), can diversity be applied to all 5 or 6 flats to arrive at a max demand & cutout value for the main incomer? I haven't found anything in the regs or OSG specifically relating to this situation and wonder where I can find more information on this. Cheers

 

[Adam_151]

Its beyond the scope of the OSG, you'll have to use engineering judgement, A clamp meter might be usful, a recording one even more so.

On a slightly relevant note... what do you reckon the DNO rate the diveresified demand of a 4 bedroom house with gas CH at when working out demand for a whole housing estate?

(not that I'm suggesting you use their figures, they are based on a much larger number of dwellings, and a larger tolerance for high but relativly short duration overloads!)

 

[ColJack]

I've read this one....

it's 16A.....


I'd get the DNO to upgrade the supply to 160A 3 phase.. and get them to install meters for each flat seperately.. that way each tenent has to sort out their own bills and suppliers..

 

[homespark]

I've read this one....

it's 16A.....

Here goes...are you being serious?? Somehow I think you might be...

Yes, I agree, ideally the property would be converted to 3-phase with separate cutouts and meters for each tennant. Sounds expensive and I get the feeling my potential client is used to getting it done cheaply somehow...maybe I should steer clear of this one, but I'd be interested in knowing more about how such calculations are made.

 

[DESL]

I would work out max demand at each CU apply diversity using OSG

The total supply demand would be the sum of all the diversity values for each flatlet.

It really needs a new 3P supply

 

[kai]

Once I saw six 60amp fuses feeding 6 small (gas heated) flats, running off ONE 100amp EDF single phase service fuse incomer!

 

[homespark]

If there is no further diversity that can be applied to all six flats together (or is there?), then I guess it's quite simple...each flat's going to have a 60A demand (it's good for most houses I've looked at), so that makes a total demand of 360A. Sounds like 3-phase to me...but won't that be expensive? My client is definitely looking for a cheaper option (like 6 x 60A on a 100A incomer!). And he wants to go ALL electric! Oh dear, it ain't gonna fly, is it?

 

[homespark]

By the way - long time no see, Adam_151 (well, it's me that's been away) How are you doin? And I see BAS is back! Yeah, OK, I've been away ages (soaked up a lot from these forums (OK,fora) and have been a busy little bee, but now I'm a bit stumped.

 

[ColJack]

just run the feed cables round all the walls of the flats.. they'll keep it nice and warm, no need for heating then...

120A 3 phase gives you 60A per flat..

 

[homespark]

Good plan, ColJack I'll suggest it to client.

If we were to do it all 'by the book', then nearly every installation should be on a 100A cutout - minimum. Well, I can tell you that the large house I live in (not mine, sadly) has 5 bedrooms, 2 studies, 2 living rooms and at least 4 ring mains and it's happily sitting on a 60A supply. In fact, before my sister & her husband bought it, it had been converted into five or six flatlets, still on the same 60A fuse. Admitedly, heating is gas, but still..

So I can well believe that 6 x 60A flats were all on one 100A incomer, and probably still are without any problems. But I certainly don't want to design an installation that way. I can see me walking away from this one. Unless he's realistic and accepts the 3-phase route. He'll just need to increase the rent a bit...

 

[Adam_151]

Yo,

I'm not bad thanks, just finished 2330 level 3 and now got to somehow get some experience in the real world...

I think there is a degree of additional diverisity that can be applied to the group of flatlets as a whole if you use engineering judgement, but its not as simple as applying a multiplyer from the OSG , but 6 on a 100A would be daft unless its just a few lights and the odd S/O in them...

Whats the loading likely to be in them? heating and cooking gas or electric? electric shower? space for washer and dryier, or is there a commomn laundry room?, floor area of flatlets?

a 100A fuse will hold quite a bit more than 100A for longer than you'd imagine, but its a relativly poor design and you don't want 6 ****ed off resisdents when it does let go...

The DNO thing, I beleieve is nearer 2kw for a 4 bed detacted property with gas heating, if you get one of you past bills and work out the average draw, it'll be below this (ours is large and averages out at ~1kw), over a large housing estate it averages out due to the number of houses and the long integration period of half an hour (which covers for loads of kettles going on when the football ends!), and gross overloads can be tolerated as long as the average is in check due to the fact that all that metal and oil gives the local tx a high thermal capacity, and cables are in freeair or buried in the ground., and the largest demand usally occurs when the temperature outdoors is cold anyway!

It obviously wouldn't work on such an extreme basis for a block of 6 flats, but is an interesting concept

 

[Stoday]

It can cost you a lot of money in DNO connection charges if you ask for a greater capacity than you need. In 2005 the connection charge methods changed and you now have to pay the full cost of connection work.

As for the flats, it depends on floor area and the fuel used for space and water heating. Electric on-peak space heating is a viable option for small new flats built to 2007 part L standards, but the connection charge may be high.

For the six flats, assuming gas heated, up to 80 sq m or so, you could easily supply on a three phase 100A connection. Two flats per phase, of course. To put the six on one single phase connection would mean upgrading to 150A at least.

 

[homespark]

Thanks gents, will get back to you this evening as have to dash out to work. Well done on passing your 2330 level 3, Adam. I finished mine this summer and am busy working in south London (and elsewhere). Plenty of work, but lose a lot to as not cheap enough for some folk! I think this client might leg it when I start talking sense about his requirements...

 

[homespark]

Talked to EDF at length today, and to my client (who seems aware of likelihood of an upgrade). EDF do a relatively cheap upgrade to 3 x 100A for between £1K-£3K. This would probably do fine if we can get all flats on 60A demand (some flats may now have gas heating; there'll be no electric showers).
Stoday - I'm interested in how you made your calculation: if 6 flats can go on 3x100A incomer (seems reasonable), how can the same six go on 1x150A? Just curious... (as the existing cutout may be 150A). Am going round next week with an experienced sparks to have a better look.

 

[homespark]

By the way, thanks Adam for your comments on diversified demand for large groups of houses - I missed it earlier (where do you get this knowledge from? Are you working for a large electrical contractor?).