Advanced Meter School August 18-20, 2015 INSTRUMENT TRANSFORMER SIZING 2015 David L. Johnson

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 Currents: over 200 amps High Voltage: over 240 volts High Voltage: over 240 volts

When ? Industrial Industrial Commercial Commercial Service greater than 200 Service greater than 200 amps amps

How ? Overhead Service Overhead Service Underground Service Underground Service Secondary Secondary Primary Primary

Window (doughnut) Window (doughnut) Styles

Spade

Primary C.T. Primary C.T.

P.T. or V.T. P.T. or V.T.288/2.4= 120 volts 2.4/1 P.T. 277/480 volt service 4-wire wye

P.T. or V.T. P.T. or V.T. 4/1 P.T. 480/4= 480/4= 120 volts 480 volts service 3-Wire Delta

C.T. Ratio 200/5 = 40/1 400/5 = 80/1 600/5 = 120/1 800/5 = 160/1 1000/5 = 200/1

2.4/1 277/2.4=116 volts2.4/1 277/2.4=116 volts 4/1 480/4=120 volts4/1 480/4=120 volts Potential Transformer (P.T.) 60/1 7200/60=120 volts60/1 7200/60=120 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 = 30C AMB RF = 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 amps200/5 with a R.F. 2.0 = 400 max amps 200/5 with a R.F. 3.0 = 600 max amps200/5 with a R.F. 3.0 = 600 max amps 200/5 with a R.F. 4.0 = 800 max amps200/5 with a R.F. 4.0 = 800 max amps 400/5 with a R.F. 2.0 = 800 max amps400/5 with a R.F. 2.0 = 800 max amps 400/5 with a R.F. 3.0 = 1200 max amps400/5 with a R.F. 3.0 = 1200 max amps 400/5 with a R.F. 4.0 = 1600 max amps400/5 with a R.F. 4.0 = 1600 max amps Etc…Etc…

200/5 with R.F :5A RF = 30C AMB RF = 55C AMB

1200/5 with R.F RF

Q: Is it important to consider R.F.s? A: Yes. Your billing depends on accuracy. accuracy.

KVA Converting KVA to amps KVA x 1000 / line to line voltage / sq rt of 3 (1.732) KVA x 1000 / line to line voltage / sq rt of 3 (1.732) Examples: 500 x 1000 / 480 / = 601 amps500 x 1000 / 480 / = 601 amps 500 x 1000 / 208 / = 1387 amps500 x 1000 / 208 / = 1387 amps 1500 x 1000 / 480 / = 1804 amps1500 x 1000 / 480 / = 1804 amps 1500 x 1000 / 208 / = 4163 amps1500 x 1000 / 208 / = 4163 amps

Transformer Rated Meters Meter Form # 3s: Class 20 Meter 3s : Single Phase Service. Service Type: Secondary 120 volt 2-Wire.Service Type: Secondary 120 volt 2-Wire. Service Type: Single-Phase Primary C.T. and P.T.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.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 LegService Type: 120/240 4-Wire Hi Leg Service Type: 240 volt 3-Wire.Service Type: 240 volt 3-Wire. Service Type: 480 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 WyeService Type: 120/208 volt 4-Wire Wye Service Type: 277/480 volt 4-Wire Wye.Service Type: 277/480 volt 4-Wire Wye. Service Type: Primary C.T. and P.T.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 / 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