help with cable calculation

[magpiemorgans]

Hi guys ,

I need a little help to do a cable calculation .
I have got to install a swa cable to a new merlin gerin 8 way 3-phase d/b via a new 520/415V transformer about 70KVA with a secondary output of around 80A
The cable will be a 4 core (3 phase+earth )
The length of cable is 80 metres , 20 m of which is in a 4 inch underground duct the rest is run along a perlin on the inside of a warehouse , there are about 3 other cables run along this perlin too .
The supply voltage is 520v 3 phase+earth
63A bs88 fuses are fitted in the distribution board feeding the transformer .
From the transformer there will be again a 4 core (3 phase+neutral) with seperate earth .
The merlin D/B will be fitted in a rittal enclosure in an outside compound .
The main switch is a 125A and all circuits will be RCBO's
At present there are only going to be 4 ciruits supplying 4 portakabins for contractors .
I'm not to good with cable calculations but have tried and also taken into account the volt drop , I came up with a 25mm cable .
Please could someone tell me if I am right with my answer or if I need to alter the cable size

 

[Knobhead]

70KVA @ 415V FLC = 97.3A There’s your starting figure.

And you’ll be pushing your luck with the supply fuses 525V = 77A

 

[magpiemorgans]

cheers tony , the fuse carriers in the supply D/B only rated at 63Amps so looks like thats going to be a non-starter .
I have a 520V supply and transforming it down to 415V , so when I am doing the eqution for the design current ( Ib ) do I divide by 520V to give me the total amount of Amps ? is it 70kva/520 =amps?
when working out the volt drop , from 25mm and over there are 3 columns r , x , z how does that fit into the equation when you are multiplying mV x Ib x length / 1000 ?

 

[Knobhead]

Line Voltage / √3 gives phase voltage
KVA * 1000 / 3 / phase voltage gives current per leg of the transformer.
You have to remember there’s 3 legs each with it’s share to do.
The calculation works the same for both Delta and Star

If you PM me with your e-mail address I can send you an Excel spreadsheet for working out motor and transformer FLC’s