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Resources 17
The protection CTs are specified to provide operation on the higher end of expected
current flow to accurately capture (over current) fault events.
NOTE: A fourth CT can be used to directly measure neutral current, however most
meters and relays are capable of accurately calculating neutral current without a
physical CT being installed.
PTs step down the interconnection voltage to a range that the meter’s inputs can accept
… typically a nominal 120V. As an example, the ratio of a PT (e.g. 60:1) will result in a
voltage at the meter of 119.993V for a 12.47kV interconnection.
The Math:
The Interconnection Voltage: 12.47 kV * 1000 = 12,470V L-L
The Line Voltage: 12,470V L-L / sqrt(3) = 7,199.558V L-N
The PT’s Output Voltage: 7,199.558V L-N / 60 = 119.993V
Most meters will accept PT voltages up to 300V or even 600V, so sizing a PT to achieve
120V is more of a rule of thumb that a requirement … ensuring the PT has appropriate
accuracy throughout the range of expected current flow (0 to the Export Limit or your
Facility) is more important.
CTs similarly step down the interconnection current flow to a range that the meter’s
inputs can also accept … typically a nominal 5A. As an example, the ratio of a CT (e.g.
300:5) will result in current at the meter of 5A for 300A of current flow.
The Math (Expected Current Flow for a 12.47kV Interconnected, 5,000kW Facility):
The Interconnection Voltage: 12.47 kV * 1000 = 12,470V L-L
The Line Voltage: 12,470V L-L / sqrt(3) = 7,199.558V L-N
Facility Wattage: (5,000kW * 1000) = 5,000,000W
Facility Total Amperage: 5,000,000W / 7,199.558V L-N = 694.487A
Line Amperage (3-Phase System): 694.487A / 3 = 231.496A
Now that we know are maximum expected current flow is 232A, we can choose an
appropriate CT ratio.
For metering, we may choose a 300:5. The 232A falls into the mid-range of the 300A
rating, however we could also safely choose a 200:5 so long as the CT has an
appropriate “rating factor” which will typically allow a CT to operate at 1.5 or 2.0 of its