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CTs are used in determining the line current. Theprimary winding is connected in series with theconductor leads and the secondary leads are

connected to the current coil of the meter. The currenttransformers usually have a secondary side ampacity of5A.

The marked ratio of a CT is the ratio of primary current tosecondary current.

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Before the CT and VT ratios can be determined,the following information is needed:

i. Customers reserve capacityii. Supply voltageiii. Customers transformer configuration if high -voltage

meter

=tage xSupplyVol3

CapacityReserve

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ExampleCustomer Reserve Capacity= 365kVA

Supply Voltage=12000V

IL =

= 17.56AThis represents the current in the primary of the CT

120003

365000

x

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CT ratios for LVMetering

200:5

400:5

CT ratios for HVMetering

25:5

50:5100:5150:5200:5300:5

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From the exampleCT with the ratio of 25:5 would be chosen

The rating factor of CT is also noted.

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VT are only used for Metering on the HV.

If a four-wire metering unit is used, the reference wire

used is the neutral wire, this would result in phasevoltages being imposed on the voltage transformer.

For the three-wire metering units, the reference wire

used is a live phase and as such phase-to-phasevoltages would be imposed on the voltage transformer.

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ExampleSupply voltage =12000VPrimary is 4W star

The phase voltages would be measuredPhase voltage=6900VTherefore, 3VTs would be required.VT with a ratio of 7200:120 would be chosen.

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Supply voltagecharacteristic

VTR used

6.6 kV (3-wire) 7200:120

12 kV (4-wire) 7200:12012 kV (3-wire) 12000:12033 kV (3-wire) 33000:11066 kV (3-wire) 66000:110

132 kV (3-wire) 132000:110

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The value of the current to be measured woulddetermine the type of meter to be used.

Less than 200A- Non transformer type201-500A- Low-Voltage Meter Installation, CTmetering is to be done.Above 500A- Three-phase 115/230V, and 230/400V

installation requiring over 500A are to be metered athigh-voltage (Metering Units)

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Form Wires Circuit Class

1S 2 115V, 1 200

2S 3 115/230V1

200

16S 4 115/230V230/400

3

200

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Form Wires Class Circuit CustomerReserve

Capacity16S 4 200 115/230

3 Delta50-80kVA

230/400V3 Star

50-138kVA

10A(Used if I L >

500A)

4 20 115/2303 Delta

80-199kVA

230/400V3 Star

80-350kVA

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Form Wires Class Circuit CustomerReserveCapacity

10A 4 200 6.6kV/12kV/33kV/66kV

Exceeding350kVA andless than4000kVA

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AvailabilityFor domestic use

Character of ServiceSingle phase: 2-wires or 3-wires 115/230 voltsBilled on energy consumed ( kWh ).

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Maximum demand of 50 kVA.Single phase: 3-wires 115/230 voltsThree phases: 4-wires 115/230 volts or 230/400 volts

Three phases: 6.6, 12 kilovoltsBilled on energy consumed ( kWh ).

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Maximum demand greater than 50 kVA but notexceeding 350 kVA.Single phase: 3-wires 115/230 volts

Three phases: 4-wires 115/230 volts or 230/400 voltsThree phases: 6.6, 12, 33 kilovoltsBilled on energy consumed ( kWh ).Meter Used: Form 9s, 10A, 16S

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Maximum demand greater than 50 kVA but notexceeding 350 kVA.Three phases: 4-wires 115/230 volts or 230/400 volts

Three phases: 6.6, 12, 33 kilovoltsBilled on energy consumed ( kWh ) and demand ( kVA).Meter Used: Form 9s, 10A, 16S

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Maximum demand greater than 350 kVA but notexceeding 4000 kVA.Three phases: 6.6, 12, 33, 66 kilovolts

Billed on energy consumed (kWh

) and demand (kVA).Meter Used: Form 9s, 10A, 16S

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Maximum demand greater than 4,000 kVA but notexceeding 25,000 kVA.Three phases: 6.6, 12, 33, 66 kilovolts

Billed on energy consumed (kWh

) and demand (kVA).Meter Used: Form 9s, 10A, 16S

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Sixty (60) Hertz Alternating currentMaximum demandexceeding 25,000 kVA.

Energy Usage less than 25,000,000 kWh per month.Three phases: 66, 132 kilovoltsBilled on energy consumed ( kWh ) and demand ( kVA).

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Maximum demand greater than exceeding 25,000 kVA.Energy Usage greater than 100,000,000 kWh per month.Three phases: 66, 132 kilovolts

Billed on energy consumed (kWh

) and demand (kVA).