Transformer Quick Reference
TRANSFORMER QUICK REFERENCE
VOLTAGE RISE IN TRANSFORMERS DUE TO CAPACITOR BANK:
-
The voltage drop and rise on the power line and drop in the transformers. Every transformer will also experience a voltage rise from generating source to the capacitors. This rise is independent of load or power factor and may be determined as follows:
-
% VOLTAGE RISE IN TRANSFORMER=(KVAR / KVA)X Z
-
Kvar =Applied Kvar * Kva = Kva of the transformer * z = Transformer Reactance in % * Example: 300 Kvar bank given to 1200 KVA transformer with 5.75% reactance. * % Voltage Rise in Transformer=(300/1200)x 5.75 =1.43%
STANDARD SIZE OF TRANSFORMER (IEEE/ANSI 57.120):
Single Phase Transformer
Three Phase Transformer
5KVA,10 KVA,15 KVA,25 KVA,37.5 KVA,50 KVA,75 KVA,100 KVA,167 KVA,250 KVA,
333 KVA,500 KVA,833 KVA,1.25 KVA,1.66 KVA,2.5 KVA,3.33 KVA,5.0 KVA,6.6 KVA,8.3 KVA,10.0 KVA,12.5 KVA,16.6 KVA,20.8 KVA,25.0 KVA,33.33 KVA
3 KVA,5 KVA,9 KVA,15 KVA,30 KVA,45 KVA,75 KVA,112.5 KVA,150 KVA,225 KVA,300 KVA,500 KVA,750 KVA,1MVA,1.5 MVA,2 MVA,2.5 MVA,3.7 MVA,5 MVA,7.5MVA, 10MVA ,12MVA,15MVA,20MVA ,25MVA, 30MVA,37.5MVA ,50MVA ,60MVA,75MVA,100MVA
 STANDARD SIZE OF TRANSFORMER:
Standard Size of Transformer
KVA
Power Transformer (Urban)
3,6,8,10,16
Power Transformer (Rural)
1,1.6,3.15,5
Distribution Transformer
25,50,63,100,250,315,400,500,630
Â
Â
IMPEDANCE OF TRANSFORMER (AS PER IS 2026):
MVA
% Impedance
< 1 MVA
5%
1 MVA to 2.5 MVA
6%
2.5 MVA to 5 MVA
7%
5 MVA to 7 MVA
8%
7 MVA to 12 MVA
9%
12 MVA to 30 MVA
10%
30 MVA
12.5%
SIZE OF CABLE ON SECONDARY SIDE OF TRANSFORMER (11KV/433V)
REF: KSEI HANDBOOK
Rating of T/C (KVA) Primary current (Amp) Secondary Current (Amp) Min. Size of Neutral Earthing Conductor (mm2) Minimum Size of Cable (mm2) 63 3.3 84 25X3 50mm2 100 5.25 133.3 25X3 95mm2 or (2×50 mm2) 160 8.4 213.3 25X3 185mm2 or (2×95 mm2) 200 10.49 266.6 25X3 300mm2 or (2×120 mm2) 250 13.12 333 25X3 2×185 mm2 315 16.53 420 31X3 or 25X4 (2×300 mm2) or (3×185 mm2) 400 21.80 533 38X3 (3×300 mm2) or (2×400 mm2) 500 26.20 666.5 25X6 (3×400 mm2) or (4×240 mm2) 630 33 840 31X6 4×400 mm2 750 39.36 1000 50X4 Bus Bar Trucking (min. Isc 50KA) 1000 52.50 1333 210mm2 Bus Bar Trucking (min. Isc 50KA) 1250 65.50 1667 290mm2 Bus Bar Trucking (min. Isc 50KA) 1600 83.98 2133 380mm2 Bus Bar Trucking (min. Isc 50KA) 2000 105.00 2666 450mm2 Bus Bar Trucking (min. Isc 50KA)
Â
HT FUSE ON PRIMARY SIDE OF TRANSFORMER (11KV/433V)
Rating of T/C (KVA) Primary current (Amp) Secondary Current (Amp) HT Fuse Min (Amp) Max(Amp) 63 3.3 84 10 16 100 5.25 133.3 16 25 160 8.4 213.3 16 40 200 10.49 266.6 25 40 250 13.12 333 32 40 315 16.53 420 40 63 400 21.80 533 40 63 500 26.20 666.5 50 100 630 33 840 63 100 750 39.36 1000 75 160 1000 52.50 1333 100 160 1250 65.50 1667 100 200 1600 83.98 2133 160 250 2000 105.00 2666 200 250
Â
ACCURACY CLASS LETTER OF CT:
Metering Class CT
Accuracy Class Applications
B
Metering Purpose
Protection Class CT
C
CT has low leakage flux.
T
 CT can have significant leakage flux.
H
 CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)
L
 CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.)
 ACCURACY CLASS OF METERING CT:
Metering Class CT
Class Applications 0.1 To 0.2 Precision measurements 0.5 High grade kilowatt hour meters for commercial grade kilowatt hour meters 3 General industrial measurements 3 OR 5 Approximate measurements
ACCURACY CLASS OF PROTECTION CT:
Class Applications 10P5 Instantaneous over current relays & trip coils: 2.5VA 10P10 Thermal inverse time relays: 7.5VA 10P10 Low consumption Relay: 2.5VA 10P10/5 Inverse definite min. time relays (IDMT) over current 10P10 IDMT Earth fault relays with approximate time grading:15VA 5P10 IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA
SIZE OF CAPACITOR FOR P.F CORRECTION:
For Motor
Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor)
For Transformer
< 315 KVA 5% of KVA Rating 315 KVA to 1000 KVA 6% of KVA Rating >1000 KVA 8% of KVA Rating