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How to wire current transformer

How to wire current transformer

Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to meters, protective relays, and other instruments. Current transformers deliver to the secondary current proportional to the primary current and adapted to the rating of the associated device. A current transformer also isolates the measuring instruments from what may be very high voltage in the primary circuit .

Types of current transformers
Cable through CT Bar through CT Split core CT Bar primary

Polarity markings and connections

Current transformer has terminals labeled :
  • Primary winding  P1 and P2 ( K and L)
  • Secondary S1 and S2 ( k and l)

Fig.1 CT terminal markings

The convention is that, when primary current enters the P1 terminal, secondary current leaves the S1 terminal, as shown by the arrows in Fig. 1. Or, when current enters the P2 terminal, it leaves the S2 terminal.

It is essential with certain instrumentation that the current transformer is physically positioned correctly on the conductor. P1 must face the supply feeder, and P2 must face the load. It is also important to ensure that secondary connections are made in accordance with instrument diagrams.

The secondary terminals of the current transformer must not be open circuited on load as dangerously high voltages may be presented under these conditions. Energized but unused CT’s must be kept short-circuited. It is also recommended that one side of the secondary windings is earthed.


The most common CT secondary full-load current is 5 amps which matches the standard 5 amp full-scale current rating of switchboard indicating devices and power metering equipment. however, CT’s with a 1 amp full-load value also available.
CT ratios are expressed as a ratio of the rated primary current to the rated secondary current. For example, a 160:5 CT will produce 5 amps of secondary current when 160 amps flows through the primary. As the primary current changes the secondary current will vary accordingly. With 80 amps through the 160 amp rated primary, the secondary current will be 2.5 amps. When the rated primary amps is exceeded, which is usually the case when a fault occurs on the system, the amount of secondary current will increase but, depending on the magnetic saturation in the CT, the output may not be exactly proportional.

How do I find what wire to use ?

When wiring a current transformer, it is important to consider the power absorbed by the cables connected between the CT secondary terminals and the measuring instrument. The resultant cable burden should be added to the equipment burden, and the total should not exceed the available VA of the CT.

Secondary lead burden ≤ Cable burden + Equipment burden

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