primary rating which is just below the full load
current of the serving power transformer, i.e., for
a 500 kVA transformer with a full load of 1387 amps
at 208 volts - select a 1200/5 CT ratio; for a 750
kVA transformer with a full load of 902 amps at 480
volts - select a 800/5 CT ratio.
2. Select a continuous-thermal-current rating
factor (RF) in accordance with the following table:
RATIO
RF at 30 degrees C
200/5
4.0
300/5
3.0
400/5
4.0
600/5
3.0
800/5
2.0
1200/5
1.5
1500/5
1.5
2000/5
1.5
3000/5
1.33
3. Select an ANSI Metering Accuracy Class in
accordance with the following table:
Primary Amp Rating (of CT)
Accuracy Class
200
0.3 thru B-0.1
300-400
0.3 thru B-0.2
600-1200
0.3 thru B-0.5
1500
0.3 thru B-0.9
2000-3000
0.3 thru B-1.8
**************************************************************************
Current transformers shall conform to IEEE C57.13. Provide current
transformers with a metering accuracy Class of 0.3 through [_____], with a
minimum RF of [_____] at 30 degrees C, with 600-volt insulation, and 10 kV
BIL. Size current transformers as indicated. Provide butyl-molded window
type current transformers mounted [on the transformer low-voltage bushings.
Route current transformer leads in a location as remote as possible from
the power transformer secondary cables to permit current measurements to be
taken with hook-on ammeters.] [in the current transformer cabinet.]
2.8.3
Voltage Transformers
**************************************************************************
NOTE: See TM 5-811-1 for guidance regarding voltage
transformers. Minimum standard potential
transformer accuracies for metal-clad switchgear are
not listed in IEEE C37.20.2. Accuracy classes as
listed in IEEE C57.13 are 0.3, 0.6, and 1.2.
Standard burdens for each accuracy class are W, X,
Y, Z, ZZ, and M. The designer should check the
burdens connected to determine the actual accuracy
class and burden required. In general, ANSI C12.11
requires 0.3 accuracy class for up to Y burdens,
except for voltages of 5 kV and below. Where
metering potential transformers are provided, a 0.3
accuracy class should be specified, if available for
SECTION 16311A
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