New Development connection help

[LeoMc]

Hi

I was wondering if anyone could help with the following question, as it's been a long time since I have dealt with electrical design theory.

A local developer has submitted an application for connections to a proposed 200 plot housing development (each house requiring a maximum load of 200A). The nearest sub station is 400m away and has a 500kVA transformer with a demand of 350kVA. There is an existing 11kV overhead line crossing the far corner of the site, and several 11kV circuits laid in the footpath that fronts the site together with a single low voltage cable supplying street lights.
………………………………………….
When I have calculating the total load of the housing development do I add each individual houses load requirements, which would make a total load of 40kA?

If that as the case I would require a new substation with a transformer of around 30mVA with a demand of around 27.7mVA, taken from one of the 11kV circuits taken from the footpath that sfronts the site.

I am aware I am likely to be way off the ball here with my workings out, but thought it was better to show where I was in my line of thought. Any help is welcomed.

 

[malcolmsanford]

I would be more concerend on the excessive loading of 200amp per house, is this a course project.

 

[LeoMc]

This is a hyperthetical question yes.

What load would you think be more suitbale for 3/4 bedroomed houses with gas heating?

 

[Wiredspark]

This is a hyperthetical question yes.

What load would you think be more suitbale for 3/4 bedroomed houses with gas heating?

with GCH 63 to 80A max

 

[malcolmsanford]

The maximum load that a DNO would supply under normal circumstances to a large house would be 100amps, if as a developer you required a larger load then I think you would be in 3 phase negotiations with the DNO

Never once in your post have you alluded to 3 phase. I have not tried any of your calcs out, but they don't look at first glance to be 3 phase,s and this is what you need.

Also you don't seem to have applied any diversity to your clacs either. With 200 plots your going to have to factor in diversity, as often houses will be empty, there will be spikes in the morning, and early evening, but the rest of the time it will be very low usage.

I would also look to factor in PV. solar, and ground pump heating in to this. With the building regs for an instance Part L 75% of all lighting in new builds need to be low energy.

It is not just a simple case of 200 plots, 200amps therefore 350kva, you will need to estimate load diversity and start from there.

 

[LeoMc]

Ok if I based it on each household having a maximum load of 80A. Am i correct in multiplying 200 properties by an 80A max load, giving a total load for the development of 16KA?

Therefore giving that it would be a 3-phase supply 415V supply a 12mVA transformer with a demand for 11.08mVA

 

[Wiredspark]

No, as Malc said you will have to apply diversity in your calcs.

 

[malcolmsanford]

I'm not a DNO engineer but I'm pretty sure they would have a formula for sizing there TX and such.

I've know through experience that a very very rough rule of thumb is that you add up the protection devices in an installation and then apply a multification factor of 0.4 for domestic 0.6 for commercial ie offices hotel, and 0.8 for industrial.

So if you had a house that had 3 RFC, @ 32amp each Cooker 40amp shower 45amp and 2 lighting circuits @ 6 amps then

32+32+32+40+45+6+6 = 193 X 0.4 = 77amps sp they would supply a 80 amp fuse.

Of course as our industry changes so do formulas and I'm sure that with the advent of electrric showers, induction hobs, and Electric CH installation will get bigger, some of this will be offset of course by PV and solar, but not all.

 

[LeoMc]

No, as Malc said you will have to apply diversity in your calcs.

Sorry had not refreshed the page before replying.

Thanks for the help so far. I'll check the regs on diversity and let you know my findings.

 

[malcolmsanford]

Leo there is no diversity formula or guidance in the regulations sorry to say. There is a section of the On Site guide appendix 1 page 95 that does give you an outline on how to apply some sort of diversity, but unfortunately diversity is an rather intangible science, that boils a lot down to experiencew.

 

[LeoMc]

Leo there is no diversity formula or guidance in the regulations sorry to say. There is a section of the On Site guide appendix 1 page 95 that does give you an outline on how to apply some sort of diversity, but unfortunately diversity is an rather intangible science, that boils a lot down to experiencew.

The following guidance is from the 'On site guide'

Lighting – 66% of total demand – 2x 6A circuits = 7.92A
Cooker – First 10A + 30% of remaining load = 10 + 9 = 19A
Shower – 100% = 45A
Standard arrangements = 100% of largest demand + 40% of remaining = 32 + 12.8 + 12.8 = 57.6

Giving a total of 129.52A (130A to make things easier)

Therefore 130A x 200 properties gives 26kA.

As advised I then need to factor in diversity for the whole development, as it is unlikely for the estate to be fully occupied, and using there full load allowance at the same time. Therefore I propose that 40% of the total demand is allowed for. Giving a total load of 10.4kA

Giving that the supply is a 415V 3-phase supply, this would then require a transformer of 8 mVA with a demand of approx 7.2 mVa.

Does it look like I am on the right track here?

 

[LeoMc]

Following a bit more studying, I've found that I divide the load by 3, and still maintain desired maximum load per household.

So the total amps per phase required would be 3.47kA, so a transformer of 3mVA with a demand of approx 2.5mVA.

Can anyone confirm, I'm on the right line, or alternatively let me know if I'm making a massive mistake?

 

[LeoMc]

I've just seen a sentance in the onsite guide that show how electricity utilities factor in diversity for residential properties. It states that they allow for an average of an 2KW load per customer.

So 2kW / 240V = 8.33A load needed per household.

So 200 properties fed from a 3-phase supply would need (200 x 8.33) / 3 = 555A per phase. And therefore would require a 500kVA transformer, with a demand of approximately 385kVA.

This now looks about right now, unless anyone can see any obvious mistakes.

Thanks for the help of everyone that has contributed.

 

[Knobhead]

You’ve calculated the loading for each house. There’s still diversity for the distribution system. Meal times etc. vary from house to house. Only the DNO could answer this for you.

 

[Engineer54]

LeoMc,

Please, it's MVA not mVA!!!!