Advice on maximum demand and diversity on a domestic

[scottyboyiow]

Good morning guys and gals. Looking for abit of advice on the following domestic situation. Sorry its a bit long. I am a qualified sparks, but I'm in the industrial field, where as you can imagine MD is abit more straight forward!
Anyway the heads up is, I have been asked by a family friend if I would be willing to re-wire their kitchen. I'm not registered so this will be done under building regs notice when the time comes. So they want a new ring main put in, and a new 5.5kW double oven and a new 6.3kW hob (both seperate), where they had gas before. However my concern is the load that would put on the 100A main fuse. They have a split 2-RCD board feeding the following for a large 3 bedroom bungalow:
- Shower 40A
- Bedroom ring main 32A
- Lounge/Dining room/Hall ring maom 32A
- Kitchen Ring main 32A
- Boiler house feed 32A (Also feeds sockets for washing machine, tumble dryer)
- Attic socket and light 16A
- Bedroom Lights 6A
- Rest of house 6A

And a seperate RCBO feeding the garages rated at 32A.

Using diversity guidlines and my own adjustments I have come up with the following:

Shower = 40A
Bedroom ring/Lounge/dining ring = 12.8A (I think the two ring mains are for convenience if there is a fault rather than the load)
Kitchen Ring main = 12.8A
Boiler house feed = 32A
Attic socket/light = 10A
Bedroom Lights (Approx 300W) 1.3A
Rest of house (Approx 300W) 1.3A
Garage Feed (Couple of sockets and a light) 12.8A

Total = 123A

As you can see with the new hob and double oven the MD is going to be well over the 100A main fuse. Do you think my calcs are ok? Or a bit over estimated? So should installing the new oven and hob be a problem? There could be a solution that they are having a new combi boiler installed, so I could get them to run a thermostatic mixer shower, rather than an electric shower, thus 'freeing up' the 40A shower feed, is this the best route to go?

I look forward to any response,

Thanks Scottyboyiow

 

[bernardgreen]

they are having a new combi boiler installed, so I could get them to run a thermostatic mixer shower, rather than an electric shower, thus 'freeing up' the 40A shower feed, is this the best route to go?

Not if they want to have decent showers with consistant flow rates.

Will there be 2 Kwatt being used in the attic while the electric shower is in use at the same time as the kitchen is in full use ?

That is very unlikely so while the calculated 123 Amps might be "accurate" in reality the chances of 123 Amps actually being used is very small.

 

[scottyboyiow]

So Bernard, are you saying with the additional 11.8kW load for the hob and oven taking the MD upto something more like 174A, in reality the chances of these items all being used at the same time for a prolonged period (as 100A main fuse will take more amps for a short period), I should be able to just wire in the extra load?

 

[riveralt]

You also want to remember what the max load is of the consumer unit you are using - most are 100Amps.

 

[riveralt]

You haven't applied diversity to your oven hob arrangement but that would still bring the load to around 150amps.

As Bernard says the chances of everything being on at once are minimal. However, I would split the tails with a Henley block and put two consumer units on the system or if one already fitted then add a separate one for the kitchen etc under RCBO's.

 

[scottyboyiow]

Yes riveralt, i did think about that! As you say most consumer units are 100A, but i reckon most consumers demands are actually higher than the 100A in theory. Can common sense be used here then, to say although in theory MD is 174A, the chances of it ever rising near to that for prolonged period are minimal, so it should all be ok? After all it is only a couple living there!

You wrote too quickly ok I will look at that

 

[holmslaw]

..

 

[PrenticeBoyofDerry]

If the boiler circuit is also a RFC
The MD will be somewhere near 112A
Three RFC@32A
Two Lighting Circuits@12A
Shower@40A
Rad@16A

RFC+Rad demand =32+(80x0.4)=64A
Lighting=12x0.66=7.92
Shower=40A

As holmslaw states, the fact that the main fuse can withstand a overload for a considerable time, there should be no real issues.

 

[holmslaw]

..

 

[JohnD]

diversity of kitchen ring is a bit iffy

lots of people, on washing day, will be running the tumble drier and washing machine for hours at a time. Add a dishwasher, plus short-term devices like kettle and toaster, and load shoots up

It's a pity they want an electric hob. Gas hobs are much better.

This is a different topic, but a combi boiler with no hot water storage is not really suitable if anyone will be having a shower or bath while someone else is running a hot tap. And the total amount of water coming into the house will be spread between a shower (whether electric or combi) plus any other hot or cold taps that are being run in the house and garden at the same time. A larger water supply pipe to the road will help with flow, but may also need plumbing changes inside the house. Lots of houses only have about 12 litres/min from the main.

 

[JohnD]

Two Lighting Circuits@12A

surely not

 

[ricicle]

, but I'm in the industrial field, where as you can imagine MD is abit more straight forward!

Are you sure about that

 

[PrenticeBoyofDerry]

Two Lighting Circuits@12A

surely not

Sorry John I jumped a head of myself, there are two lighting circuits protected by 6A devices, a total of 12A (Two Lighting Circuits@(=)12A) to be factored by diversity at 66%.
The rest adds up, just a typo

 

[scottyboyiow]

@ricicle - when I say straight forward, installing a plant where you know exactly what the most is going to be that is running at one point, thats abit easier then guessing what one, two, three or four people are planning on doing at certain times in there daily routines. Or the numerous possibilities that they could do. That was the point I was trying to make. Sorry.

 

[timtheenchanter]

diversity of kitchen ring is a bit iffy

lots of people, on washing day, will be running the tumble drier and washing machine for hours at a time. Add a dishwasher, plus short-term devices like kettle and toaster, and load shoots up

but the OP says

" Boiler house feed 32A (Also feeds sockets for washing machine, tumble dryer) "

so those demands are not on the kitchen ring. (do we even still have washing days??)