Help needed with max demand calculation

[Paulgwat]

Hello all,

i posted this in the electrical theory section but with no responses I thought I would post here as well - since originally posting I have spoken to ELECSA and they have basically said I can use any basis I want to calculate diversity so long as I document what I used and why which doesn't really push things forward.

i would welcome any thoughts people have on how to approach this

"
I am hoping for some guidance on the maximum load calculations on a studio flat and also the block in general.

Background is I am working on a single studio flat in a block of 16, the landlord is gradually going to redevelop the whole block and add an additional two stories making a total of 24 flats. Building is standard 60’s block andbrick.

Electric is the only utility apart from water entering the building so all heating, cooking and water heating is going to be electric.

Incoming supply is 3 phase TNS – haven’t heard back fromWPD what the supply into the block is rated for as yet.

Each flat is served via a service cupboard on that floor (not sure on the phase split as 4 floors currently) through a 60A DNO cutout and then a 60A KMF protecting the tails which run for approx 8M at a guess (somewhere in the fabric of the building and pop out of the wall above frontdoor).

Given the whole flat is going to be electric I have contacted WPD for an estimate as to what the building incoming will support as in my opinion from looking at Appendix A in the OSG a 60A supply won’t cut it with the following fixed loads.

9.5kwelectric shower - 40A – According table A2 stays as 40A
3kwinstant water heater – 12.5A – stays at 12.5A
4kwelectric radiator -16.67A – diversity from A2 gives 12A
6kwelectric hob – 25A – diversity gives 14.5
2kwsingle oven - 8.33A – No diversity as under 10A so 8.33A

In addition,there will be a cold fill washing machine, cold fill dishwasher and all of the other electrical items such as kettle etc to take into account but even without those my calculator makes the calculated demand 87.33A

Now it isn’t exactly a large flat but there are families living in some of them (two adults and a small child) although I think the landlords target market is single occupancy.

So my question is, in the real world do the calculated number stack up or should I be looking at a different way of calculating things– before I did the calcs I was thinking along the lines of an 80A supply but that number is implying a 100A supply is needed which to me seems somewhat crazy given it’s a 35sqm flat!

Also how do I look at diversity between the flats? I know this is the job of WPD but it would be good to understand where they might becoming from when trying to work out overall supply into building.

If I have missed anything glaringly obvious please shoutas this is my first experience of self-contained flats and I am pretty sure I must have missed something!!
"

Many thanks in advance for any and all inputs

Paul

 

[Martyc]

60A should be fine, the vast chunk of that load is the shower, what are the odds that all appliances will all be on at the same time and drawing current (as the hob, oven and heater will cycle in and out and unlikely they'll all be drawing at the same time). You may get occasions where 60A temporarily is exceeded but that won't affect the cut out unless it's for substantial periods of time .


Sent from the moon using telepathy

 

[Simonslimline]

Do you have a mentor you work with?

 

[Paulgwat]

I have been working with another electrician who is recommending upgrading the tails from 16mm to 25mm and going for an 80A cut out.

treating the hob and cooker as one unit was suggested by ELECSA which would drop things down to just over 80A which I believe was where the other electrician was coming from with his 80A recommendation.

I may be being naive but I thought the whole point of the factoring in the diversity calculation was to account for the fact that an oven say is only really drawing a much lower amount than the ratings plate would suggest?

 

[davesparks]

What additional load are you adding to the existing installation? What was the actual demand of the existing installation (assuming someone was able to, and actually did, measured it)

Who designed an installation which, on paper at least, will overload the available electricity supply? In my opinion they need to sort themselves out and design an installation which works.

Cooking loads - apply diversity to the whole cooking load, not each individual appliance (I thought that was stated in the osg, but may be wrong)

How on earth is a 4kW electric radiator going to magically only draw 12A? Assuming it is a single radiator then it will either draw it's full rated current or nothing (or equivalent reduced current if lower power settings are available)

Based on the instinctive educated guess method of calculation I would expect the installation to have a measured average demand around the 4/5kW mark and sit quite happily on a 45A HRC fuse.




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[Paulgwat]

Davesparks, the flat is an empty shell, someone else has previously gone in and ripped everything out apart from 3 light fittings so no opportunity to load test and to be honest I don't believe much had been changed since the flat was built in the 60's. The previous heating was storage heaters, no electric power shower and only a vented hot water cylinder for hot water with a couple of Kw element in it.

The "installation" as stands is the developer and architects wish list for what they want in each flat, they hadn't thought about whether the existing electrical infrastructure would actually support it until I asked if they had confirmed with the DNO that it could be supported.

Cooking loads I think I misinterpreted the OSG and agree it makes sense to jump it into one item for the sake of sanity and calculation.

Heating - again I may have misread the OSG table A2 but I am pretty sure it said 100% of the first 10A and 30% of the remainder which works out at 12.5A, is this not correct?

I thought I was supposed to be making the calculation of peak demand and working from there, if I understood your last sentence correctly I should really be designing for average demand and making use of the Fuses characteristics to cover the peak demand?

Many thanks for the help with this, my background is semiconductor electronics where the rules were to design based on min/max and not typical or average numbers

Cheers

Paul

 

[davesparks]

Then the developer and architect need to get their heads out of the clouds and actually think about things properly!
It should be fairly obvious to anyone with a brain cell or two that a building originally designed to have a stored water soloution and storage heaters is not going to handle a completely different and power hungry soloution very well.

It sounds like what it needs is an electrically heated hot water storage system, something like it had before. Though a modern unvented system would be far more efficient.

For the heating load calculation you have stated that it is a 4kW radiator, if this is the case then how do you expect to divide that load by a percentage figure from a book?
You need to actually think about what these figures and calculations actually mean and how they work. It should be obvious that a single 4kW resistive load controlled by a thermostat is only ever going to be on or off, it will not magically draw a percentage of 4kW because your book says it does.

My last sentence was based on instincts, educated guesses and experience and took all of 30seconds to come up with. I would carry out a more detailed analysis of the job before committing to any particular solution.

You do need to consider peak demand, but that doesn't necessarily mean your fusing must be greater than your peak demand.

In your situation I would have had a sensible conversation with the architect, developer, other contractors to establish what is and is not possible.
But I served my apprenticeship at an old fashioned company, generally working with old school architects who actually listened to us and took the time to design things properly.
We even had accurate scale drawings (hand drawn on some jobs) we could work from, none of this 'check dimensions on site' rubbish!

 

[Paulgwat]

Many thanks again - my assumption was that these diversity calculations from the book were in effect duty cycle calculations to better reflect a more realistic use model I.e. The heater is clearly only on or off but the diversity factors in that for instance the heater is only on 2/3rds of the time therefore the lower number.

it sounds like I am not using them in the correct way.

re the architect, in the last week 3 sets of drawings have been produced due to errors as basic as a shower tray that was bigger than the wall it was supposed to be on and a kitchen that had cupboards that had no access to them.

what I was looking for in asking this question was some ammunition to go back to the developer and architect and have the discussion without being shot to pieces in front of the client if I have things wrong.

From the answers I have I see there is a huge gap between the text book and the real world in this area and it seems that the installation will work on a 60A supply in reality.

many thanks

Paul