3-phase power calculations

[Davey345]

Hi, I am a trainee electrician and have some questions concerning a 3-phase installation. I have been asked to calculate the maximum load of a supply at a factory and have the following information. I realise the best option is to use a clamp meter and measure the voltage and current load across each phase.. this is something i intend to do on our next visit to the site.

My question is this.. There is a 3-phase 70Kva service line installed on 35mm swa.

there are 4 main lathe machines and several single phase loads.
the machines power and current ratings are:

1) Hardridge Gx150 power 21Kw, load current 38 A
2) Hardridge Gx600 power 21Kw, load current 42 A
3) Bridgeport power 19Kw, load current 40 A
4) Bridgeport power 10Kw, load current 30 A
5) Proteus power 12Kw load current 63A

I am using the following formula for KVA: = (V*I*√3)/1000 where √3 = 1.732
the kva figures I obtained are 26.25, 26.25, 23.75, 12.5 and 15.2 respectively.
Making a total of 103.87Kva....???

This is way over the 70Kva plus if I add all the figures I have calculated fo rthe single phase boards, the total Kva goes up to 173.45 Kva

This does not make sense to me can someone please explain or show me where i have gone wrong..

many thanks..

 

[plugsandsparks]

Not looked at all the calcs but you probably have full load numbers for each machine. This won't happen in practice. Normal diversity applies.

 

[Knobhead]

I get 83KW without taking any diversity in to account. Non of the machines will run at FLC and it would be highly unlikely they all run at the same time.

Taking the 3 highest rated machines running at 80% FLC and .9PF I get 63.3A