Block of flats. Max Demand

[bartkusal]

Wonder if I can get some guidance, on max demand in block of flats. It always was hard for me as it always comes to massive demands, and I red here that it all comes with years of experience and that it is like an art. This is first bigger project I am taking on. So please don’t shoot.

I need to determine the max demand for the job, so the correct supply can be brought in.
Basically it is an old mansion being converted to a block of 11 flats.

Each flat will have: 2 lighting, 2 ring, 1 shower 8.5kW (37A), 1 cooker 4.5 kW (19.6A).

Option 1: Going by OSG diversity: 4A lights+4A lights+32A ring+8A ring+37A shower+16A cooker=101A (crazy)

Option 2: Going by 0.4 factor diversity: (6+6+32+32+37+19.6) x 0.4 =53A (more like it)

For the whole house:
Can I treat flats as circuits in this instance and apply 0.4 factor again? Then:

Option 1: ((101A x 11)+ 20A landlords board) x 0.4 =452A (3Ph), Supply current per phase 452/3=150A. So I need then 103.5kW 3ph+N supply.

Option 2: ((53A x11) +20A landlord board) x 0.4 = 241A (3ph). Supply current per phase 241/3=80A. So I need then 55.2kW 3ph+N supply.
I find myself thinking to come up with Option 3, :banghead:: (option 1+ option2) /2. =80kw 3ph + N supply.

So what you think? Am I somewhere near with my thinking? Obviously if I can’t work it out I will go with safest but unfortunately most expensive option 1, it just seems it will be rather large supply cable 50 mm2 or maybe higher, it is what it is I suppose. Not sure about ratings on supply cables, just had a look in regs book, 50mm2=135A and 70mm2=167A, SWA XLPE reference D, obviously its different cable.

After and if I will get this confirmed by more experienced, I then will share with my calcs and plans of action for the customer’s installation.

Thanks in advance for your help and guidance.

Al

 

[Sharpend]

How many rooms in each flat to require 2 ring finals? I'd doubt very much that you'd be pulling 32a on the 2nd circuit? Would need some idea of loading in each flat not just the circuits.

 

[bartkusal]

2 bed flats. 1 separate ring for 4 kitchen sk and all appliances, 1 ring for the rest areas( lounge 6sk, bedrooms 4 sk each and hallway 1.

 

[bartkusal]

On option 1 , second ring was calculated as 25%, I think that what it says in OSG if i remember exactly.

I quickly drafted my OP from the top of my head, and have to go now and do some work will be back in the evening to read all the replies, thanks again.

 

[Silly Sausage]

Max Demand & Diversity = Nightmare!

A case study (my house)
3 bed detached, me, wife & 3 daughters.
80A Cutout Fuse
3 RFCs
2 Lights
1 Smokes
No electric shower or electric heating.
Computer, TV, Hifi, Fridge/Freezer, Lights all active for 10 hours.
Combiboiler, Washing machine & dishwasher now and again.

I'm using about 75kWh a week @ 13.4p/kWh
So, on average, over 10 hours usage/day...

75/7 = 10.7kWh/day
10.7/10 = 1.07kW
1.07/230 = 4.4 Amps (AVERAGE)

So, apart from 10A spikes (very often) from the kettle, the whole house is pulling sod all.


I hope that's right, please correct me if it's not!!! :smilielol5:

 

[Spoon]

wife & 3 daughters.

You poor sod. 4 against 1....

 

[bartkusal]

Any more input?:icon9:

 

[Knobhead]

To follow on from Archy’s post.

Like Archy gas heating and cooking. I have an electric shower but I like a bath, so gas again.
A fridge/freezer and a freezer. The kettle gets some hammer, microwave used occasionally.

No one will live with me! (Not if they’ve got any bloody sense).

I re-jigged the CU for my flat. Everything running through a 16A RCBO. Never tripped for the trial period of a fortnight.

The stupid thing is the biggest power users in my flat are the PC’s. If I’m at home and awake they are on.

 

[bartkusal]

To follow on from Archy’s post.

Like Archy gas heating and cooking. I have an electric shower but I like a bath, so gas again.
A fridge/freezer and a freezer. The kettle gets some hammer, microwave used occasionally.

No one will live with me! (Not if they’ve got any bloody sense).

I re-jigged the CU for my flat. Everything running through a 16A RCBO. Never tripped for the trial period of a fortnight.

The stupid thing is the biggest power users in my flat are the PC’s. If I’m at home and awake they are on.

Hello, thanks for replies. Yes I do understand what you Tony and Archy are saying, it’s easy to say when you know exactly what you use and how much.
In my instance I needto design an adequate system, where I don’t know the exact load, what I knew I did put it in the OP. What I know, is that it’s going to be 11 modern flats done up for sale or for rent, so I don’t know who and how many people living in them, or what they likely activities and patterns of usage. Kitchen will have all the modern appliance’s imaginable built in.
I think I would be safe to go with option 3 or there bouts, what you guys think. I know it will be ok, its just that I lack confidance, I think?

 

[Richard Burns]

I would tend to look at this from a practical point of view considering the capability of fuses to take over current.
In general you could consider on average a house may take about 15A, this is considering multiple users in different houses using power simultaneously. For a flat this would be reduced to maybe 10A to use as an average.

For each flat in your situation lighting can be discounted because it will add very little to the overall loading (comparatively).
You normal socket circuits may take on average 5A considering 11 flats and multiple usage it is probably somewhere close.

Kitchen circuits would be likely to take more because of high power appliances and usage of kettles / toasters in the morning could be simultaneous between flats. So there may be a peak here but only for maybe ten minutes. So say take this as 15A.
Showers will take a high peak and may be on simultaneously and for on average say about 20 mins so you need to account for a 35A loading for a short while.
Cooker again could be on simultaneously across flats but diversity level of 16A should be OK.

So you are looking at 36A per flat with a twenty minute 100% overload so a 40A fuse would handle this but be a bit close to the wire. However you are averaging out across the flats and so a 40A demand for each individual flat should be OK to consider as an average ( I would tend to use a 60A protection for each flat just in case).
Therefore you are looking at say 10kW per flat the split would be 4, 4, 3+landlord so each phase would need 40kW or 170A.
This is all just estimates and guesswork but comes out similar to your option two, I would have thought from a basic set up that this should be OK.
However this is in no way a professional assessment so you would need to make your own judgement on what is required.

 

[bartkusal]

Hi Richard, thanks a lot, all that detailed sharing of experience I was waiting for, invaluable, you are my top spark now.

Sorry to be a pain, just a little clarification again. When you say I was close in option 2. I did have 53A per flat, but taking all flats in consideration I applied diversity again and had 80A per Ph. So it seems that you are not applying diversity anymore between the flats, just added them up.Is this correct?

If that’s the case, its seems a lot, but then it is what it is, I suppose.

Thanks again, its most appreciated.

Al

 

[Richard Burns]

My approach here is not applying diversity as such. It is more looking at the reality of the situation and considering likely loads from normal loading of houses in general. Taking into account the high load appliances that affect maximum demand.

Because I am looking at average values for a flat I do not apply diversity again because the reductions are fully incorporated already.

As a basic rule over a large number of dwellings the demand is likely to be about 15A per house, however as you get closer to a single dwelling variations in demand mean that you cannot apply these individually to dwellings and it is likely that peak demands could cause an overload, so the protection increases.

I got 40kW you got 55kW, overall I would not really like to supply 4 flats with electric showers through a 100A fuse but it would probably be OK in most cases.

 

[bartkusal]

My approach here is not applying diversity as such. It is more looking at the reality of the situation and considering likely loads from normal loading of houses in general. Taking into account the high load appliances that affect maximum demand.

Because I am looking at average values for a flat I do not apply diversity again because the reductions are fully incorporated already.

As a basic rule over a large number of dwellings the demand is likely to be about 15A per house, however as you get closer to a single dwelling variations in demand mean that you cannot apply these individually to dwellings and it is likely that peak demands could cause an overload, so the protection increases.

I got 40kW you got 55kW, overall I would not really like to supply 4 flats with electric showers through a 100A fuse but it would probably be OK in most cases.

Sorry Richard, where this 100A fuse came from? In previous post you came to 170A per phase. Is it because there is no BS 1361 and BS 88-3 fuses higher then 100A? What other options DNO have, perhaps BS 88-2 160A? Or is this only for motors and inductive loads? Thanks.

 

[Richard Burns]

Look this is really something that you need to be sure about yourself.
I have said 100A fuse because a basic 100A three phase supply is going to be easy and (relatively) cheap to install, going above this will have the DNO charging more and taking longer.
Looking at my breakdown for the flats if each one is able to run at the limit on a 40A fuse then for four flats you would have 160A giving a bit more leeway takes you to 170A, however from the above posts from others giving you some useful realistic information it is probable that four flats would never take out a 100A fuse as you would need a consistent 200A for more than 20 minutes to blow the fuse.

Again you need to be confident that what you say will be suitable.

The DNO could probably supply whatever you want but may charge you £50,000 for a new transformer!

 

[Silly Sausage]

We all agree that the fuses will handle being overloaded for short periods of time.
What is the general consensus of opinion about cables being overloaded occasionally, but regularly, eg shower usage?
re Shower, design the cct to take the current, I mean the supply calbles.

 

[driverman]

Re flats: Generally now all meters to each flat have to be on the ground floor altogether in a community room whereby each tenant/ owner has 24 hour access. The DNO would have to install a suitable sized mains cable (possibly 95cne) into a Ryefield box to cater for 11 flats plus "Landlords supply" for corridor lighting and fire alarm. From the Ryfield box a separate cable to a 60/80amp cut out (header) fuse is supplied for each flat. The supplier will the connect a separate meter to each and every flat. A suitable sized switch fuse is then fitted by the electrician, running a separate sub main to each flat consumer unit. To accommodate all the DNO's equipment , meters, switch fuses etc a large area is required. Basically all the cut -out fuses and meters are next to each other along with the switch fuses.