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Discuss Have I got a big problem!? in the Commercial Electrical Advice area at ElectriciansForums.net

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Hi,
i'm currently working on a project where 10 flats are being built on one side of an old silk mill. We are having a new cable put in and the origin upgraded etc. This type of work has to be done by NERS registered electricians so an outside company recommended by the DNO came yesterday and asked for what each flat will be using current wise so they can do a cable calc for the supply, which is the following.
Boiler 9kw 50am breaker, ring 32a, ring 32a, oven 6.2kw 32a, heating 20a, lights 6a
This is then multiplied by 10 for the ten flats. Looking at diversity tables theres a strong possibility diversity cant be applied unless we could use column 2, heating & power - If not the cable size is going to be ridiculous as it needs to be in trunking, ratings go down etc.. What are peoples thoughts?
 
Is the incoming supply going to be 3 phase? if so the loads will be split over each phase so roughly 3.5 flats per phase.
Also you state that there is going to be 2 x RFC per flat?
 
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snowhead

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Boiler 9kw 50am breaker, ring 32a, ring 32a, oven 6.2kw 32a, heating 20a, lights 6a
If you're using those figures as is, then;

I wouldn't want to pay the electric bill for any of those flats.
And I wouldn't want to be inside any of them with all the excess heat from those appliances running flat out 24/7.

I'm sure the NERS guys are sitting down somewhere chuckling to themselves.
 
  • Thread Starter Thread Starter
  • #4
Is the incoming supply going to be 3 phase? if so the loads will be split over each phase so roughly 3.5 flats per phase.
Also you state that there is going to be 2 x RFC per flat?
incoming is 3phase and yes 2 x RFC per flat
 

MDJ

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There is only so much that can be plugged in, a basic flat next to nothing
 

Spoon

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Boiler 9kw 50am breaker, ring 32a, ring 32a, oven 6.2kw 32a, heating 20a, lights 6a

Out of curiosity, who specified this equipment?
 

Spoon

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  • Thread Starter Thread Starter
  • #14
Sorry, I should have been more clear.



Who specified that the flats should have the above?
its an old mill and they didnt want to, or couldnt put gas in so the plumbers themselves will have supplied the boiler, like the kitchen fitters the oven/hob etc..
 

Spoon

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its an old mill and they didnt want to, or couldnt put gas in so the plumbers themselves will have supplied the boiler, like the kitchen fitters the oven/hob etc..
Ok, so the boiler & oven/hob are already fitted?
 

davesparks

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Boiler 9kw 50am breaker, ring 32a, ring 32a, oven 6.2kw 32a, heating 20a, lights 6a
This is then multiplied by 10 for the ten flats. Looking at diversity tables theres a strong possibility diversity cant be applied unless we could use column 2, heating & power - If not the cable size is going to be ridiculous as it needs to be in trunking, ratings go down etc.. What are peoples thoughts?
Why do you say there's a strong possibility diversity can't be applied? Over 10 dwellings diversity can definately be applied!
Your loadings seem very odd to me, it's almost as if you've counted the ratings of circuit breakers instead of actually calculating Ib.

If you've got a 9kW boiler why do you need another 20A circuit for heating?
 
B

Bobster

I think you need a better electrical contractor.

Although diversity for a block of flats isn't exactly beginner stuff, the information you have provided (I presume has been provided by you by the contractor you've employed) is pretty duff.
 
  • Thread Starter Thread Starter
  • #18
Why do you say there's a strong possibility diversity can't be applied? Over 10 dwellings diversity can definately be applied!
Your loadings seem very odd to me, it's almost as if you've counted the ratings of circuit breakers instead of actually calculating Ib.

If you've got a 9kW boiler why do you need another 20A circuit for heating?
they are breaker ratings and the 20a for heating is just a radial going to a potterton economy 7 controller for controlling the boiler. I wondered if there was a diversity figure you could apply to there being 10 flats, its not something i have seen as yet though, thanks for the reply
 
  • Thread Starter Thread Starter
  • #19
I think you need a better electrical contractor.

Although diversity for a block of flats isn't exactly beginner stuff, the information you have provided (I presume has been provided by you by the contractor you've employed) is pretty duff.
According to you it is...
 

davesparks

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they are breaker ratings and the 20a for heating is just a radial going to a potterton economy 7 controller for controlling the boiler. I wondered if there was a diversity figure you could apply to there being 10 flats, its not something i have seen as yet though, thanks for the reply
Then you need to use the design current, not the mcb ratings, the reason for this is hopefully obvious.
This 20A heating circuit is a good example, it has a 20A mcb but the only thing connected to it is a timeswitch which will use a fraction of an amp. Over 10 flats that's 200A if you add up the mcb ratings, or maybe 2A if you add up the actual load.
 

Spoon

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Its ok, i have sorted it, thanks for mostly everyones helpful replies
Glad you got it sorted mate.
Next time it would be better to chat to an electrician and you can go through the design and what is needed together, rather than just bypassing them.
Every day a learning day...
 
Oven and hob for 6.8kw seems a bit light. Whilst modern ovens can just about run on a 13A plug (say 3kw), a typical 4 ring induction hob will be rated up to about 7kW.
 

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