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How to determine the power of current transformers?

Circuit elements which reduce the current flowing through power lines and electrical circuits to the level that measuring instruments, meters, relays and other devices operating with a similar technique can measure without damage and isolate these devices against high currents are called current transformers.

One of the issues to be considered in the selection of current transformer is that whether the secondary power of the current transformer can meet the receiver power and the requested powers or not. Power calculation can be practically calculated using the formula below.

Secondary power = Receiver power (measurement device etc.) + Connection cable losses + Contact Losses

The powers of some devices connected to current transformers:


Power (VA)
Ammeter (soft iron) 0.7… 1.5
Wattmeters 0.2 … 5
Cosⱷ meters 2 … 6
Counters (active and reactive) 0.4 … 1
Reactive power control relays 0.5… 1
Overcurrent relays 0,2… 6
Reverse current relay 1… 2
Secondary thermal relays

7.2… 9

Additional loads from copper cables:

Power loss in cable with secondary current 5 A (VA)

Cable (Cu) 2.5 mm 2 4 mm 2 6 mm2 10 mm2
1m 0,36 0,22 0,15 0,09
2m 0,71 0,45 0,30 0,18
3m 1,07 0,67 0,45 0,27
4m 1,43 0,89 0,60 0,36
5m 1,78 1,12 0,74 0,44
6m 2,14 1,34 0,89 0,54
7m 2,50 1,56 1,04 0,63
8m 2,86 1,79 1,19 0,71
9m 3,21 2,01 1,34 0,80
10m 3,57 2,24 1,49 0,89

Cable power loss calculation:

  • P = (Isn 2 x 2L) / (S X 56) VA
  • L: Cable length on the secondary side (m)
  • Isn: Secondary rated current (A)
  • S: Cross section of copper cable (mm2)
  • P: Power loss (VA)

Contact Losses:

Contact losses can be considered as approximately 0.5 VA depending on the number of connections to be made between the current transformer secondary terminals and the receiver / s. They are losses that represent copper losses that may occur in secondary terminals and receiver terminals.

For example; The load on an active-reactive counter and current transformer for 4 m / 2.5 mm2 cable is calculated as 1VA + 1VA + 1.43VA + 0.5VA = 3.93 VA. Here, the current transformer with 5VA secondary power may be sufficient.

Power calculations should be taken into account when choosing current transformers. The important thing here is that the requested power should not be more than the secondary full power of the current transformer or less than a quarter load. Otherwise, the current transformer cannot meet the power demanded by the receiver, it cannot fully respond to the need with incorrect measurements and faulty protection signals.

Federal Current Transformers are presented to users with busbar type and window type (without busbar) options in 30A – 4000 A rated current range. It provides error classes of 0.2, 0.2s, 0.5, 0.5s in the rated power range of 2.5 VA – 40 VA in different amps depending on the user requirement. According to user demand, special production can be made in 1,3 and 5 error classes. Secondary current in Federal Current Transformers is 5A as standard and secondary current is produced as 1A for user demand. Safety coefficient (saturation coefficient) is declared as Fs <5 in measurement current transformers. Current transformers are produced in accordance with the bar dimensions of 30x10mm… 125x58mm and cable diameters of Ø31… Ø125.


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