Transformer - KVA Rating & Load Calculation

I am here explaining how to calculate the Ampere/Load Capacity of a Transformer from given Parameters.

A you know, the Transformers ratings are generally displayed only in KVA / MVA ratings and its Primary/Secondary Voltages. Inorder to find out the Primary nd Secondary currents please follow the below steps.

Formulae to be Remembered 

P = (Root 3) X V X I   
                       (In gen case, we are not considering Power Factor for Transformers)
P = 1.73 X Primary V X Primary I
P = 1.73 X SecondaryV X SecondaryI


           Mva/Kva                    =    Primary Current
   1.73 X Primary Voltage

           Mva/Kva                     =   Secondary Current
1.73 X Secondary Voltage


Lets Assume a Transformer with following Paramateres

100Kva     11KV/415V


From  Example :-


          100,000                   =  5.25 Amps  (Primary Current)
      1.73 X 11,000

          100,000                   =  139.28 Amps  (Secondary Current)

      1.73 X 415

So the Secondary side have a capacity of 139.28 Amps which can feed a maximum load of 100KW (through its total of 3 Phase ).

Voltage Drop Calculation for Cables

Voltage Drop Calculation is one of the key calculation we have to do when we design the load and finalize  cables.
In usual practice, if the distance increase over 15mts, we will be doing a voltage drop calculation.

Different countries have different values for cables (voltage drop/mtr). I'll be explaining here as per Qatar Standards (Since am working in Qatar :-)) and the calculations are based on Qatar General Electricity & Water Corporation (Kahraama) rules.

VOLTAGE DROP CALCULATION for Cables


Basic Equation , Vd = Lx I x (Vd/Amp/M)
                                           1000

  Vd = Voltage Drop
     L = Length of Cable
     I = Current
     Vd/Amp/M  = Voltage drop/Amp/Mtr

Percentage of Voltage drop = (Vd/V) x 100


How to Calculate ?

I would prefer an example to show you how we calculate.

Consider a 3Phase Load of 90Kw which is 100mtr apart from the feeder panel.
We need to calculate the cable size?

So as per  formula Vd = = Lx I x (Vd/Amp/M)
                                                    1000

Length = 100Mtr
I = ?  (We need to find that )

P=1.73VICosPhi     (To know this calculation, please refer my previous post. i explained there how to calculate current )

90,000 = 1.73 x 415 x I x 0.8    (assuming V = 415v , Power-factor = 0.8)

I  =      90000   
       1.73x415x0.8

  =  156.69Amps

Please find the attached table for 3/4 core XLPE SWA cables as reference                           (Qatar's Standard Chart)






I will assume the cable is laid in ground and for 156.69 Amps, we can choose 70mm2 cable from the chart which have a current carrying capacity of 164Amps when laid in ground.

Actual calculation

Now lets consider the 70mm2 cable will be suitable or not using voltage drop formula.
voltage drop/Amp/Mtr of 70mm2 cable as per the above table is 0.61Mv

Vd  = Lx I x (Vd/Amp/M)
                     1000

  Vd = 100 x 156.69 x (0.61)
                        1000

        = 9.55809

 Percentage of Voltage drop = (Vd/V) x 100
                                            = (9.55809/415)  x  100
                                            = 2.303%

This is acceptable since it doesn't cross 2.5%. But considering the maximum load and for safer purpose we will be choosing the next cable size ie., 95mm2 with voltage drop of 0.45mv


Vd = 100 x 156.69 x (0.45)
                     1000
       = 7.05105


 Percentage of Voltage drop = (Vd/V) x 100
                                            = (7.05105/415)  x  100
                                            = 1.699%

This value is far more consistent and safer. so we will be going for the 95mm2 cable.


Nb:- Also Please find the excel file i've prepared for easy calculation under the link

click for excel file