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

2. 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.

3. Why ? When ? How ?

4. Why ? High Currents: over 200 amps High Currents: over 200 amps High Voltage: over 240 volts High Voltage: over 240 volts

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

6. How ? Overhead Service Overhead Service Underground Service Underground Service Secondary Secondary Primary Primary

7. Window (doughnut) Window (doughnut) Styles

8. Spade

9. Primary C.T. Primary C.T.

10. 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

11. 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

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

13. 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

14. 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.

15. Name Plates 200 :5A RF = 4.0 @ 30C AMB RF = 3.0 @ 55C AMB

16. 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).

17. 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…

18. 200/5 with R.F. 4.0 200 :5A RF = 4.0 @ 30C AMB RF = 3.0 @ 55C AMB

19. 1200/5 with R.F. 1.5 1200 RF 1.5 @30°C

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

21. 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 / 1.732 = 601 amps500 x 1000 / 480 / 1.732 = 601 amps 500 x 1000 / 208 / 1.732 = 1387 amps500 x 1000 / 208 / 1.732 = 1387 amps 1500 x 1000 / 480 / 1.732 = 1804 amps1500 x 1000 / 480 / 1.732 = 1804 amps 1500 x 1000 / 208 / 1.732 = 4163 amps1500 x 1000 / 208 / 1.732 = 4163 amps

22. 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.

23. 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.

24. 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

25. 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.

26. 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…

27. Questions / Answers