Opinions please RE: Current demand & diversity for 5 bedsits / flats.

[S60TEM]

I just need some opinions for calculating my demand.

I've put my estimate in for a building which is to be converted into 5x Bedsits/flats. Each one of the unit is to have the following

Shower - 40A circuit, 10mm, 9.5kw.

Cooker - 32A circuit, 6mm, 6kw.

Sockets - 32A circuit, ring final

lights - 6A circuits, 1.5mm, 4x100w assumed

smokes - 6A

Calculating rule of thumb that DNO's use adding the breaker sizes up then multiplying by 0.4 = Demand is 46.4A. mmmmmm!!

Using OSG for diversity:

Shower - 100% FL = 39.5A

Cooker - 10A + 30% remainder of current = 14.5A

Sockets - Rated current of circuit = 32A (which is impossible imo!)

Lights - 66% of total current demand = 1.6A (negligible)

Smokes - 1A max! (negligible!)

So worst case using OSG would be 88.6A

Now to me, a single person living in the bedsit will not use the demand as they wont be showering, cooking, watching TV etc etc all at once!

Would we assume a % of full load? Maybe 75% which would make it 66.45A

Trouble being is that the supply needs upgrading and Northern Power Grid are asking for total demand in kVA which works out at:

kVA= V x I / 1000 (single phase) = 240 x 88.6 / 1000 = 21.2kVA or 240 x 66.45 / 1000 = 15.95kVA

Now I've advised them that they NEED a 3 phase supply to be installed but was just wanting the opinions of the more experienced designers amongst us please.

mucho grassy a r s e.

 

[NozSpark]

How are they heating the properties?

 

[S60TEM]

they'll have gas central heating.

The agent wanted me to quote for panel heaters too until I told her how much they were and how much to run them and she was dead against storage heaters and economy 7 for some strange reason.

The building has gas supply and it has 2 single phase supplies at present as it has a ground floor office space which we're gonna use this supply for the communal areas and boiler etc etc. The 3 Phase i've suggested will run the 5 flats.

 

[ProDave]

Well a "normal" domestic installation is "rated" at 16KVA by the DNO, at least that's what Scottish Hydro do.

If you want more than 16KVA then you have to pay a lot more for the installation of the supply.

Now that does not mean the supply is limited to 16KVA. My house is nominally a 16KVA supply, but I've registered 90A being drawn with 2 showers and other stuff running without issue. The 16KVA is the rating, applying their diversity rules for the property.

In your situation, I would think a nominal 16KVA supply for each bedist is more than adequate. I suspect if you request a 21KVA supply for each it will raise questions and / or increase the cost significantly.

Like most things, DNO's make up their own rules, including how to calculate diversity.

 

[S60TEM]

cheers for that Dave, yeah I know normal is 16kVA and thats why I sugested a 75% of full load to get it beneath the 16kVA.

 

[samson337]

I always put smokes on light circuits, but I surppose you can put them on there own, if they trip and battery not working then your in a pickle

 

[S60TEM]

Each flat will have its own heat and smoke detectors but they are also protected with a detector from a grade b ld2 fire alarm system.

 

[Manator]

This is one of my pet hates to be honest, and this is no disrespect for the OP, its more on the standard of training and the myths associated with electrical design. If you look at diversity in detail it is based on guidelines for those who have not undertaken advanced courses in electrical design. For most of us, this would work out at around 90% of practising electricians, so you are not on your own.

Electrical design engineers would use the characteristics of any CPD and main switch coupled with any configuration of DNO supplied fittings/fuses.

Worst case scenario for you would be, all residents arrive home at the same time and take a shower, so without doing any calculations lets say they all spend an average of 10 minutes in the shower, and lets say that before they shower 4 of them put on their ovens to preheat, now thats a lot of demand, but only lasts for 10 minutes, now look at the test curves for the CPD's and any main switch, some may run above the rating for an hour or more before they cut out. Designers can use this, based on allowance for overloading, for short durations and cool down times in between.

I think it was Special Location who posted a very very good post about how total demand could never change after a consumer unit change, for instance an old installation with 4 basic circuits changed to a new dual RCD unit with 8 circuits is never going to change the total demand.

 

[SPECIAL LOCATION]

Read the whole of Appendix A in OSG its only pages 109 - 112...

It basically says the info give is just some suggested guidance AND..

it CANNOT give accurate figures for calculating diversity as it requires specialist knowledge of the installtion.....

and the experience of the person designing the installation....

Single person per bedsit??

I would think 10kVA tops per flat!

consider...

Three 25A loads all running at different times of the day...

Max demand is not 25A x 3 =75A

nor is it (25A x 3) x 0.4 = 30A

Three 25A loads all running at different times of the day...

IS a max demand of 25A

Whereas...

Three 25A loads all running continuously 8 hours a day from 8:00am till 4:00pm..

IS a max demand of 25A x 3 =75A!!

Not (25A x 3) x 0.4 = 30A

Adding up breaker values has got to be one of the most stooopidest ways of trying to consider max demand IMHO!

headbang

You have GOT to include times of day and the number of users....

Two identically wired houses with identical appliances and identical fuse boxes and circuits

could quite easily have a vastly different max demand...

if one property has a single elderly widow lady in it

and the other has 6 university students living in it!!!!!

You have to apply your expertise and specialist knowledge of the installation your are designing from he specifications given by your client.

if there are unknowns..

You will need to discuss the maximum budget the client has available ...

and either put limits on the possible power..

or you will increase the costings and install bigger supplies.

Dunno why people struggle with this concept....

:C

unless they are just looking for ready made answers in the regs books!!!

:_|

 

[Manator]

I hate that special location guy, he lets out more secrets of the trade than that magic circle guy with death threats