INSTRUMENT
TRANSFORMER
SIZING
2012
David L. Johnson
Advanced Meter School
August 21-23, 2012
What is an instrument
transformer?
A current transformer (CT) is used for measurement of
electric currents. Current transformers, together with voltage
transformers (VT) or potential transformers (PT), are
known as instrument transformers. When current in a circuit
is too high to directly apply to measuring instruments
(meters), a current transformer produces a reduced current
accurately proportional to the current in the circuit, which can
be conveniently connected to measuring and recording
instruments (meters). A current transformer also isolates the
measuring instruments (meters) from what may be very high
voltage in the monitored circuit. Current transformers are
commonly used in metering and protective relays.
Why ?
When ?
How ?
Why ?
• High Currents: over 200 amps
• High Voltage: over 240 volts
When ?
• Industrial
• Commercial
• Service greater than 200
amps
How ?
• Secondary
• Primary
• Overhead Service
• Underground Service
Styles
Window (doughnut)
Spade
Primary C.T.
P.T. or V.T.
2.4/1 P.T.
288/2.4=
120 volts
277/480 volt
service
4-wire wye
P.T. or V.T.
4/1 P.T.
480/4=
120 volts
480 volts
service
3-Wire Delta
C.T. Ratio
•
•
•
•
•
200/5
400/5
600/5
800/5
1000/5
=
=
=
=
=
40/1
80/1
120/1
160/1
200/1
Potential Transformer
(P.T.)
• 2.4/1
277/2.4=116
• 4/1
480/4=120
• 60/1
7200/60=120
volts
volts
volts
Understanding Ratios
The relation between two similar
magnitudes with respect to the number
of times the first contains the second:
the ratio of 200 to 5, written 200:5 or
200/5.
Example:
200/5: For every 200 amps that flow
through the primary winding (window),
the meter will receive 5 amps. From the
secondary side of the C.T.
Name Plates
200 :5A
RF = 4.0
RF = 3.0
@
@
30C AMB
55C AMB
R.F.
Rating factor : The amount by which
the primary load current may be
increased over its name plate rating
without exceeding the allowable
temperature rise. Temperatures are
measured in C (celsius).
Rating Factors
Examples:
•
•
•
•
•
•
•
200/5 with a R.F. 2.0 = 400 max amps
200/5 with a R.F. 3.0 = 600 max amps
200/5 with a R.F. 4.0 = 800 max amps
400/5 with a R.F. 2.0 = 800 max amps
400/5 with a R.F. 3.0 = 1200 max amps
400/5 with a R.F. 4.0 = 1600 max amps
Etc…
200/5 with R.F. 4.0
200 :5A
RF = 4.0
RF = 3.0
@
@
30C AMB
55C AMB
1200/5 with R.F. 1.5
1200
RF 1.5 @30°C
Q: Is it important to consider R.F.s?
A: Yes. Your billing depends on
accuracy.
KVA
Converting KVA to amps
KVA x 1000 / line to line voltage / sq rt of 3 (1.732)
Examples:
•
•
•
•
500 x 1000 / 480 / 1.732 = 601 amps
500 x 1000 / 208 / 1.732 = 1387 amps
1500 x 1000 / 480 / 1.732 = 1804 amps
1500 x 1000 / 208 / 1.732 = 4163 amps
Transformer Rated Meters
Meter Form # 3s: Class 20 Meter
3s : Single Phase Service.
•Service Type: Secondary 120 volt 2-Wire.
•Service Type: Single-Phase Primary C.T. and P.T.
Transformer Rated Meters
Meter Form # 4s: Class 20 Meter
4s: Single Phase Service.
•Service Type: 120/240 3-Wire with 2 C.T.
Transformer Rated Meters
Meter Form # 5s: Class 20 Meter
5s : Three Phase Service.
•Service Type: 120/240 4-Wire Hi Leg
•Service Type: 240 volt 3-Wire.
•Service Type: 480 volt 3-Wire.
*Meter also known as Form 45s
Transformer Rated Meters
Meter Form # 9s: Class 20 Meter
9s : Three Phase Service.
•Service Type: 120/208 volt 4-Wire Wye
•Service Type: 277/480 volt 4-Wire Wye.
•Service Type: Primary C.T. and P.T.
How to Determine the Correct Size C.T.
Transformer Rated Meters are rated for 20 amps
 Transformer Size: KVA
 Power Formula: KVA x 1000 / line to line voltage / 1.732
 Rating Factor: 2, 3, 4
 Meter: 20 amps max
 C.T. Ratio: 200, 400, 600, etc…
 Wire Size: 2/0, 4/0, 250, 350, etc…

Questions / Answers