Answer / faiyaz

We know that, Transformer always rated in kVA.  Below are two simple formulas to find the rating of Single phase and Three phase Transformers.
Rating of Single Phase Transformer:
P = V x I.
Rating of a single phase transformer in kVA
kVA= (V x I) / 1000
Rating of a Three Phase Transformer:
P = √3. V x I
Rating of a Three phase transformer in kVA
kVA = (√3. V x I) /1000

  but lets me try to explain.
Here is the rating of Transformer is 100kVA.
But Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.
And Primary Current on High Voltage side is 5.25 Amperes.
Also Secondary voltages or Low Voltages (L.V) is 415 Volts
And Secondary Current (Current on Low voltages side) is 139.1 Amperes.
In simple words,
Transformer rating in kVA = 100 kVA
Primary Voltages = 11000 = 11kV
Primary Current = 5.25 A
Secondary Voltages = 415V
Secondary Current = 139.1 Amperes.
 Now calculate for the rating of transformer according to
P=V x I (Primary voltage x primary current)
P = 11000V x 5.25A = 57,750 VA = 57.75kVA
Or P = V x I (Secondary voltages x Secondary Current)
P= 415V x 139.1A = 57,726 VA = 57.72kVA
Once again we noticed that the rating of Transformer (on Nameplate) is 100kVA but according to calculation…it comes about 57kVA…
The difference comes due to ignorance of that we used single phase formula instead of three phase formula.
Now try with this formula 
P = √3 x V x I
P=√3 Vx I (Primary voltage x primary current)
P =√3 x 11000V x 5.25A = 1.732 x 11000V x 5.25A = 100,025 VA = 100kVA
Or P = √3 x V x I (Secondary voltages x Secondary Current)
P= √3 x 415V x 139.1A = 1.732 x 415V x 139.1A= 99,985 VA = 99.98kVA
Consider the (next) following example.
Voltage (Line to line) = 208 V.
Current (Line Current) = 139 A
Now rating of the three phase transformer
P =  √3 x V x I
P = √3 x 208 x 139A = 1.732 x 208 x 139
P = 50077 VA = 50kVA

what will happen if instead of neutral we use earth wire as return conductor?if we can use ,then why we are not using in our day to day appliances?please justify the answer

11 Answers   NTPC, ONGC,

---

What r the specification furnishid in motor name plate?

5 Answers  

---

How to decide the rating of CBCT? We have 6.6Kv switchgear & having 50/1 cbct ,i want to know how this 50/1 A came? On what basis we decide CBCT reting?

1 Answers   Cross,

---

In CDG type IDMTL realy O/L , E/F, O/C which are used for out going feedrs,on R-phase o/c element, Y-phase , e/f elecment and on b -phase o/c element will be connected . Why the earth fault on center phase only, why not on other two phases?

2 Answers  

---

What are the raitings of Mcb,mccb and Acb in detail ??

0 Answers   Engineering Vision,

---

What is the importance of Form B to state Electricity Boards?

1 Answers  

---

why we decide alternating wave form in ac system and not in dc system

2 Answers   GMR,

---

How the speed of synchronous motor changes with the variation of load?

1 Answers  

---

What is the meaning of capacity of a 63 Amps TPN MCB? Is it the per phase capacity or the totol capacity?

1 Answers   Mother Dairy,

---

What is actual name of Transformer oil.

6 Answers  

---

What is difference between SMC supportg and Glass Fibre Reinforced Plastic Support.

0 Answers  

---

When we giving the 3 phase supply to ups system how to coming netural?

0 Answers   Juniper Networks, Tata Motors,

---