maximum demand kva 3 phase

[daveb1]

looking to get a bit of help got a job that currently has 8 flats and a landlord supply and a big office on the ground and basement

After using the OSG for diversity have come up with the following

118A per flat (no gas)
127A for landlord (lift etc)
180A for office (air con)

118x8=944+127+180=1251AMPS this seems way to much i know! but would rather cover myself so when the office are giving these to the supply company they are asking for kva?

At the moment they have a 200A 3 phase supply

The developers are looking to change the office into 3 more flats so 3X118=354amps more

so with the office gone we are looking at a total of 1425Amps (sounds like its getting confusing)

so my question is how can i get a kva value for the supply company? with 1425A 3 phase

Any help would be appreciated Thanks Dave

 

[Plonker 3]

Can you give a breakdown of the cicuits in each of the 3 different areas?

 

[Darkwood]

Considering most large houses happily run on 60/80/100 supply I'll be surprised if yo get a flat demanding 118amps, the Landlords supply sounds excessive too Im sure your miscalculating here.... by your reckoning you'll need a supply that would be adequate for most large factories !!!

 

[daveb1]

flats (off the top of my head, been a few years since been inside no access to flats )

kitchen ring
main ring
electric boiler
cooker
hob
light
light

office (no access)
over two floors, i know there is
electric shower
kitchen
air con
lights?
sockets?


landlords i could see
3p lift 40A
small lift dis board 32A
2x ring circuits
3xligts
fire fan control system

Thanks Dave

 

[dan86]

as per darkwood- me thinks there is a duff calculation somewhere. 118a for a flat sounds hugely escessive.

 

[daveb1]

i know its seems mental but it the way i was told how to do it by 2394/5 tutor
doing it this way the average house comes out at 145 ish, an its funny to think most are fused at 60/80

 

[Darkwood]

Either your tutor is crap or you haven't fully understood it.

Still lacking a lot of info to make comments ... is heating off peak to flats?
As you don't even have access to the offices how can you make a reliable calculation?

You need to do some on-site investigation - im afraid guess work because of limited access isn't an excuse - sort access out get a better idea of the installs

 

[wozz]

your tutor told you to use that as that the subject he is teaching. The stuff in the OSG isnt really any use in doing the diversity in a install such as yours.

 

[daveb1]

Yes you are right about access will get in to see all the distribution boards for myself
No there is no off peak heating

Ok i could have miss understood the point and the tutor could also be crap but we were working all this out from the on site guide, i know these are not real life in book. just want to cover myself thats all.

 

[daveb1]

Oh yes when i have done my homework got some proper info could anyone tell me the formula for kva? and do i calculate per phase or an overall value
Thanks Dave

 

[Darkwood]

Ball park figure 1P KVA is ( KVA x 4) so 25KVA will be 100amp
3P KVA is ( KVA / 0.73) so 73KVA will be 100amp per phase.

Transpose as needed... again a quick method for a ball park figure!

 

[GMES]

Dave as Darkwood says that sounds way to high for what you are proposing , I mainly do industrial/commercial and most factories don't have anything like 1200 amps coming in to them. time to do a full and precise spec on the job / site. plus it could save the customer a hell of a lot of money.

 

[i=p/u]

I done a 2 bed flat today and it worked out at , highest breaker + 40% of rest = 92A but don't forget the heating was on 16A breaker , 2 ring circuits, a plug top cooker on 32A

 

[i=p/u]

Where's .73 come from for 3ph ?

and for single phase isit 100A x230v =23kva

 

[Darkwood]

Where's .73 come from for 3ph ?

and for single phase isit 100A x230v =23kva

I said quick ballpark method.... so does 0.73 work or not to give Amps per phase in 3ph

 

[iamelectric]

i know its seems mental but it the way i was told how to do it by 2394/5 tutor
doing it this way the average house comes out at 145 ish, an its funny to think most are fused at 60/80

why where they teaching u how to calculate maximum demand on the testing course ?

 

[Plonker 3]

There not, they are showing them in the OSG the guide on how to work it out.

 

[daveb1]

why where they teaching u how to calculate maximum demand on the testing course ?



I don't know about you but when i getting shown something i have never asked why are you showing me this, guess someone must have asked! Thanks for your positive input anyway mate

 

[wozz]

why where they teaching u how to calculate maximum demand on the testing course ?

isnt there still a max demand box on test forms? so there must be some idea on how to guess.

 

[iamelectric]

yes the size of the main fuse

 

[iamelectric]

try rapidtables.com kva calculator , but in my opinion 200amp 3 phase supply is plenty and if you ask for a 400amp 3 phase supply when the builder gets the price in he will stick with the 200

 

[Flanders]

Try This Amps (A) to kVA conversion calculator

 

[Flanders]

Sorry Iameletric just noticed you posted the same web site I guess the prob you've got is having no gas I did some calcs for a 2 bed flat with no gas max demand was about 80amps just hope they don't cook a sunday roast with all the trimmings and then decide to have a shower with all the heater on and make a cup of tea :laugh3:

 

[i=p/u]

Isit not 1.73

 

[Darkwood]

Isit not 1.73

Try it on a calculator then confirm on the link a few posts back...

73Kva / 0.73 = 100amps (ballpark) actual long calc method approx 105amps

100amps x 0.73 = 73Kva

It doesn't work with 1.73 don't mix my quick method up with other methods where 1.73 is used as a cube routing factor of 3ph.
As i said it was a easy onsite method to use with one equation answer.

 

[i=p/u]

So if I have 125A on each phase , 125 x .73= 91Kva each phase ? Yes ? Ok

if I got a machine and it said 324kva over 3ph , would = 324 x .73 / 3 = 78.8kva per phase, ah fuk it lol

- - - Updated - - -

Il use my converter if I ever have too

 

[Darkwood]

So if I have 125A on each phase , 125 x .73= 91Kva each phase ? Yes ? Ok

if I got a machine and it said 324kva over 3ph , would = 324 x .73 / 3 = 78.8kva per phase, ah fuk it lol



- - - Updated - - -

Il use my converter if I ever have too

Machines which are 3ph will give a Kva rating per phase not across 3 phases so you above machine would be stamped 108Kva

Come on IPU i clearly give kva to amps as division and i know you must be on the wind now as i know from your time on here your not stupid :smilielol5:

If you convert KVA to amps you divide not multiply 0.73

So your sum if you ever did find a machine with a total rating across the phases -
324 / 0.73 / 3 = 147.95amps.

well its a quick method for most God help the generation when the internet breaks down and they have to go back to maths

 

[i=p/u]

By writing a load of jumble, I found out you would never get amachine with kva for 3ph.

 

[Megavolt]

Went to large flat last week. When I traced feed to CU it was fed from a 40mcb tenant been there 5 years never had any probs.

 

[Darkwood]

By writing a load of jumble, I found out you would never get amachine with kva for 3ph.

You do but not often usually found on imported Japanese machines sometimes or the likes it can at first be a pain to figure out the actual demand without load assessing the different parts of the machines... most machines will give max demand in amps but large machines with big heating banks tend to be Kva.

You stick to the original then
S[SUB](kVA)[/SUB] = √3 × I[SUB](A)[/SUB] × V[SUB]L-L(V)[/SUB] / 1000

I'll stick to my onsite quick calc' KVA / 0.73 = roughly same as above

 

[Engineer54]

X 0.4 of the proposed final circuit ratings and you'll have your ballpark figure, forget the the 100% of the highest rated protective device, all at X 0.4!! Oh and even then the figure will be airing on the high side. But that is about as accurate as your going to get, unless you're a design engineer with a good few years experience under your belt!!

Forget using those methods in BS7671 or it's guides they don't work and never have worked. They are only there to use as exam questions, then to be forgotten about!! ...lol|!!

 

[Risteard]

After using the OSG for diversity have come up with the following

The OSG is completely inappropriate for judging maximum demand for that. Diversity does not have to be calculated this way - a competent person should consider the load and work out what is/is not likely to be used together and for how long.

Looking at what is connected to the flats I can't see them ever using more than 40A and that is when severely loaded!

 

[telectrix]

what's the electric boiler? that could be anything up to 10kW.

 

[GLENNSPARK]

yes the size of the main fuse

no...not the size of the main fuse...

this is a common and incorrect statement that i often come across on forms..

you can use your intuition to guestimate what the max demand will/should be...for a average 3 bed house i put down 60A...for a smaller 2 bed house i use 50A-60A...for a flat or say a back to back on 2 lifts with shower i go for 40A-50A

you can also take a direct measurement using a current clamp with all the appliances/loads that could be expected to be on at the same time...so for instance the shower, cooker and stuff like TV and some lighting...you clamp the tails...take a direct reading.

or as has been said above already you can go for a calc such as the highest load (typically a shower where theres no diversity allowed for an instantanious water heater)...then 40% of the rest.....

either way you wont get 100A

 

[Silly Sausage]

Dave, a bit of a dark art, diversity!

My house, 100A Supply, 25mil tails, 3 bed, 5 people; no electric heating, showers or cookers; big fridge/freezer, TV/HiFi etc...
We average < 3 or 4 Amps over about 12 or 14 hours, the occasional spike when the kettle goes on.

This maybe of interest...

Estimation of actual maximum kVA demand - Electrical Installation Guide

 

[iamelectric]

no...not the size of the main fuse...

this is a common and incorrect statement that i often come across on forms..

you can use your intuition to guestimate what the max demand will/should be...for a average 3 bed house i put down 60A...for a smaller 2 bed house i use 50A-60A...for a flat or say a back to back on 2 lifts with shower i go for 40A-50A

you can also take a direct measurement using a current clamp with all the appliances/loads that could be expected to be on at the same time...so for instance the shower, cooker and stuff like TV and some lighting...you clamp the tails...take a direct reading.

or as has been said above already you can go for a calc such as the highest load (typically a shower where theres no diversity allowed for an instantanious water heater)...then 40% of the rest.....

either way you wont get 100A

are you serious?

 

[Clive P]

Hi,

Read this the other day for a ballpark figure, but can't say how accurate it is?

Loading--
Domestic 15W/Sq ft
Commercial 30W/ Sq ft
Industrial 100W/Sq ft

Diversity--
Domestic 0.4
Commercial 0.5
Industrial 0.8

Regards.

 

[daveb1]

Just out of interest after giving these figures to the supply companies do they use there own diversity to the values you give them???????

Also what would your average 80amp BS1361 actually operate at from over current built up gradually???

Thanks Dave

 

[GLENNSPARK]

are you serious?

are you with 100A/whatever the bullit is?

 

[Darkwood]

When the DNO do their own calcs for a block of flats i believe they only attribute 8-15Kva per flat.. obviously diversity between different flats is used but really give you a idea just how little demand multiple properties can have.

 

[daveb1]

But my question is do they use diversity for the value i will give them?

 

[iamelectric]

phone them up and ask them

 

[daveb1]

I dont need to! I know the answer its you lot that i am asking asking?

 

[iamelectric]

of course they do

 

[daveb1]

It is interesting seems like it is the right way of calculating it after all then.

 

[Old Jag Lover]

200A 1 phase services very common in oz now,but at a guess a 500kva tranny would be ok,diversity plus reputable English and yank stuff can handle 300 percent overloads for 10 hours at a time with no effects.200A service due to the 80 percent rule means 160 plus POCO here will usually still only install a 100A service drop,they have their own rules,free air that sort of thing

 

[i=p/u]

Well I think 0.4 for everything seems right .. From what I see. I normally use highest breaker + 40% of rest and it's too high ,but I write it in if under a 100A. But think il use 0.4 on everything now, which will equal around 50-60A for standard domestic ... I've learn from this thread.lol