Maximum demand question

[chris day]

A client contact me as they are getting the supply upgraded to a block of flats and UKPN need a figure adjusted for diversity for each flat.
The original contact didn’t give a crap about the supply capacity.

x6 flats have been refurbished and during this process x6 15kw electric boilers for each flat has been added. (No gas in flats).

The issue I am having is after diversity from the OSG I am getting a crazy number per flat (150a roughly) or maybe I am missing something.

With my calculations I feel I am overkill and a substation will need to be installed, how common are the electric combi boilers as they are a big problem and not accounted for by the original fit out contractors, I am close to telling client they can’t have them and need an alternative as a substation is 100k

UKPN have said you need a sub station installing if each flat consumes 150a... these bloody boilers, I suppose it’s not my problem but I want to feel I am being realistic with my advice atleast.

Each flat DB is the following:
CCT1 Boiler supply 63a
CCT2 Ring 1 32a
CCT3Ring 2 32a
CCT4 lights 1 6a
CCT5 lights 2 6a
CCT6 Oven 40a
CCT 7 Hob 16a

 

[Nigel]

Are you applying diversity to the MCB rating or to what is actually going to be installed?

 

[chris day]

Are you applying diversity to the MCB rating or to what is actually going to be installed?

I am going by MCB with the exception of the boiler circuit as each boiler is 15kw on a 63a breaker.

 

[Nigel]

I am going by MCB with the exception of the boiler circuit as it’s 15kw on a 63a breaker.

Well at least do it properly. If there is say 5 LED pendants on that lighting circuit that could be 15w (0.07A) and you are putting that as 4A (66%).

Same for sockets. Why put through 20A for a ring final when in all honesty the installation is probably going to be half that for both rings.

 

[GBDamo]

Ifeel your pain with this, load calculations are the thing i feel least confident with. When ever i follow OSG diversity theyre always "overloaded" its then a case of rationalising to get the number down which feels like a fiddle.

Is this MCB undersized for 15kW? I get that to 65A.

 

[Wilko]

Hi - the 15kW water boiler is a big boy and if I recall correctly we're not given any diversity allowance for heaters. Modern electric boilers can be limited to a max kW by config and manufacturers quote times for hw heating (like 6 min for 50l at 15kW and 30min at 3kW).

Can I ask what make and model is used?

 

[buzzlightyear]

http://www.napitonline.com/downloads/CP 4 07 P 10-11 16th Diversity.pdf they will come round and dance for you .

 

[Shoei]

40A looks a lot for the oven, most new ovens are pretty efficient, spec a 13A connection and and must be A or B energy rated. It would be reasonable to use that for the calcs.

The boilers are the problem, bit late now but heat pumps might have been a solution. There's a thread on the same subject here..

 

[J.C.E]

I have same problem
1st fixed 14x 1bed flats with 12kw boiler in each

I calced max demand around 72a per flat so would of thought looking at a 245kva new supply

however SSEN are saying they want to install a 130KVA supply and it will be enough

I have installed the 14x switch fuse isolators (luckily the Lewden ones come with 60 80 and 100a fuses and all sub mains are 25mm 3core

and SSEN are supplying/fitting the Ryefield

So up to them if they want a callout for a popped main head!

I have it all in writting via email so its their call now

 

[Ian1981]

I always do 100% demand of the largest load in this case the boiler then add 40% of the remainder, having applied diversity to the remaining circuits first.

 

[snowhead]

however SSEN are saying they want to install a 130KVA supply and it will be enough

SSEN will be applying diversity to the whole block of flats, something they are probably quite experienced at.

 

[chris day]

Of corse wilko thank you.

Gabarron MAC15

I am thinking about asking a heating engineer to dial them all down to 3kw and see if they even work, that’s the only way I would feel comfortable

 

[buzzlightyear]

diversity, sod it ..bang it in .

 

[GBDamo]

http://www.napitonline.com/downloads/CP 4 07 P 10-11 16th Diversity.pdf they will come round and

diversity, sod it ..bang it in .

And keep cranking it down til it stops?

 

[GBDamo]

Maybe a stupid question but...

On new installs...

With the advent of low power appliances is there more sense in putting rings on 20A MCB ( mad but just for discussion, using 1.5 T&E)

On a typical install with up/down and kitchen sockets, instead of diversified 32+(32*0.4)+(32*0.4)=57.6A you have 20+(20*0.4)+(20*0.4)=36A.

Please destroy by all means but interested in the reasons why not.