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

I want formula to calculate cable size as per load given in kw & amp.I searched many sites but didn't right answer.Plz reply me asap.

41 Answers   Armco, BEL, BGS, Biocon, BRB Cable, Crystalline Chemical Industries, DLF, Doshion, Electrical, Gold Roof, L&T, Lanco, MEIL, Oman Cables Industry SAOG, PK Services, Polycab, Sabic, SLV, System Electrical, Tata Motors, UEM, UMIC, Vega, Vertex, Voltas,

---

how to calculate the cable size depending on the amps?

1 Answers   Polycab,

---

if we want to increase the back up time of an UPS what should we do ? if we want to decrease the charging time of a battery/batteries attached with the UPS what should we do ?

6 Answers  

---

what will be d.g. capacity to cater together for 14hp A.C. load & 3kw lighting load

0 Answers  

---

how to diffrentiate between ac & dc motors from a distance

5 Answers  

---

please I wont know how to calculate the vector group for power transformer ZNyn11 best regard.

0 Answers   Al Rasikh,

---

Why all voltage level are multiples of 11, like 220V, 440V, 1100V instead of 200V, 400V, 1000V?

9 Answers   Indian School of Mines,

---

in elevator industries , how to calculate the required KVA for the elevator (elevator spec- 40A module , motor of 13.5KW, 20A)

1 Answers   DLF, Tata Steel Limited,

---

What is transient Phenomenon?

0 Answers  

---

What are the advantages and disadvantages of short-pitched winding?

0 Answers  

---

In three phase supply why we use R Y B Colour not other.

4 Answers  

---

in a three pin socket y the pin of earthng is longer n bigger...?

3 Answers  

---