Current Transformer Basics

The Current TransformerC.T. ), is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary.

Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line using a standard ammeter. The principal of operation of a current transformer is no different from that of an ordinary transformer.

typical current transformer

Typical Current Transformer

Unlike the voltage or Power Transformer looked at previously, the current transformer consists of only one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped around the core or just a conductor or bus bar placed through a central hole as shown.

Due to this type of arrangement, the current transformer is often referred too as a “series transformer” as the primary winding, which never has more than a very few turns, is in series with the current carrying conductor.

The secondary winding may have a large number of coil turns wound on a laminated core of low-loss magnetic material which has a large cross-sectional area so that the magnetic flux density is low using much smaller cross-sectional area wire, depending upon how much the current must be stepped down. This secondary winding is usually rated at a standard 1 Ampere or 5 Amperes for larger ratings.

There are three basic types of current transformers: “wound”, “toroidal” and “bar”.

  • • Wound current transformers – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer.
  • • Toroidal current transformers – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.
  • • Bar-type current transformers – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.

Current transformers can reduce or “step-down” current levels from thousands of amperes down to a standard output of a known ratio to either 5 Amps or 1 Amp for normal operation. Thus, small and accurate instruments and control devices can be used with CT’s because they are insulated away from any high-voltage power lines. There are a variety of metering applications and uses for current transformers such as with Wattmeter’s, power factor meters, watt-hour meters, protective relays, or as trip coils in magnetic circuit breakers, or MCB’s.

Current Transformer

current transformer


Generally current transformers and ammeters are used together as a matched pair in which the design of the current transformer is such as to provide a maximum secondary current corresponding to a full-scale deflection on the ammeter. In most current transformers an approximate inverse turns ratio exists between the two currents in the primary and secondary windings. This is why calibration of the CT is generally for a specific type of ammeter.

Most current transformers have a the standard secondary rating of 5 amps with the primary and secondary currents being expressed as a ratio such as 100/5. This means that the primary current is 100 times greater than the secondary current so when 100 amps is flowing in the primary conductor it will result in 5 amps flowing in the secondary winding, or one of 500/5 will produce 5 amps in the secondary for 500 amps in the primary conductor, etc.

By increasing the number of secondary windings, N2, the secondary current can be made much smaller than the current in the primary circuit being measured because as N2 increases, I2 goes down by a proportional amount. In other words, the number of turns and the current in the primary and secondary windings are related by an inverse proportion.

We know from our tutorial on double wound voltage transformers that its turns ratio is equal to:

transformer turns ratio

from which we get:

current transformer turns ratio


As the primary usually consists of one or two turns whilst the secondary can have several hundred turns, the ratio between the primary and secondary can be quite large. For example, assume that the current rating of the primary winding is 100A. The secondary winding has the standard rating of 5A. Then the ratio between the primary and the secondary currents is 100A-to-5A, or 20:1. In other words, the primary current is 20 times greater than the secondary current.

It should be noted however, that a current transformer rated as 100/5 is not the same as one rated as 20/1 or subdivisions of 100/5. This is because the ratio of 100/5 expresses the “input/output current rating” and not the actual ratio of the primary to the secondary currents. Also note that the number of turns and the current in the primary and secondary windings are related by an inverse proportion.

But relatively large changes in a current transformers turns ratio can be achieved by modifying the primary turns through the CT’s window where one primary turn is equal to one pass and more than one pass through the window results in the electrical ratio being modified.

So for example, a current transformer with a relationship of say, 300/5A can be converted to another of 150/5A or even 100/5A by passing the main primary conductor through its interior window two or three times as shown. This allows a higher value current transformer to provide the maximum output current for the ammeter when used on smaller primary current lines.

Current Transformer Primary Turns Ratio

current transformer primary turns ratio

Current Transformer Example No1

A bar-type current transformer which has 1 turn on its primary and 160 turns on its secondary is to be used with a standard range of ammeters that have an internal resistance of 0.2Ω’s. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. Calculate the maximum secondary current and secondary voltage across the ammeter.

Secondary Current:

secondary current

Voltage across Ammeter:

voltage across ammeter


We can see above that since the secondary of the current transformer is connected across the ammeter, which has a very small resistance, the voltage drop across the secondary winding is only 1.0 volts at full primary current. If the ammeter is removed, the secondary winding becomes open-circuited and the transformer acts as a step-up transformer due to the very large increase in magnetising flux in the secondary core. This results in a high voltage being induced in the secondary winding equal to the ratio of:  Vp(Ns/Np) being developed across the secondary winding.

So for example, assume our current transformer from above is used on a 480 volt three-phase power line. Therefore:

current transformer secondary voltage


This 76.8kV is why a current transformer should never be left open-circuited or operated with no-load attached when the main primary current is flowing through it. If the ammeter is to be removed, a short-circuit should be placed across the secondary terminals first to eliminate the risk of shock.

This is because when the secondary is open-circuited the iron core of the autotransformer operates at a high degree of saturation, which produces an abnormally large secondary voltage, and in our simple example above, this was calculated at 76.8kV!. This high secondary voltage could damage the insulation or cause electric shock if the CT’s terminals are accidentally touched.

Handheld Current Transformers

clamp type current transformer

There are many specialized types of current transformers now available. A popular and portable type which can be used to measure circuit loading are called “clamp meters” as shown.

Clamp meters open and close around a current carrying conductor and measure its current by determining the magnetic field around it, providing a quick measurement reading usually on a digital display without disconnecting or opening the circuit.

As well as the handheld clamp type CT, split core current transformers are available which has one end removable so that the load conductor or bus bar does not have to be disconnected to install it. These are available for measuring currents from 100 up to 5000 amps, with square window sizes from 1″ to over 12″ (25-to-300mm).

Then to summarise, the Current Transformer, (CT) is a type of instrument transformer used to convert a primary current into a secondary current through a magnetic medium. Its secondary winding then provides a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions.

A current transformers primary coil is always connected in series with the main conductor giving rise to it also being referred to as a series transformer. The nominal secondary current is rated at 1A or 5A for ease of measurement. Construction can be one single primary turn as in Toroidal, Doughnut, or Bar types, or a few wound primary turns, usually for low current ratios.

Current transformers are intended to be used as proportional current devices. Therefore a current transformers secondary winding should never be operated into an open circuit, just as a voltage transformer should never be operated into a short circuit.

Very high voltages will result from open circuiting the secondary circuit of an energized CT so their terminals must be short-circuited if the ammeter is to be removed or when a CT is not in use before powering up the system.

In the next tutorial about Transformers we will look at what happens when we connect together three individual transformers in a star or delta configuration to produce a larger power transformer called a Three Phase Transformer used to supply 3-phase supplies.

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38 Responses to “The Current Transformer”

  1. Mannu Kumar

    if the amp meter having range from 0 to 100 with CT 100/5 having scale 0, 20 40 , 60 ,80 ,100 then for calibrating the amp meter at 20 amp how amp source is required 20 or 1.

  2. Abdelsalam Elhaffar

    The CT example connected to a 480 V supply is not correct. The primary winding is in series with the power system not in parallel and its voltage in the range of few millivolts (mV).
    During an open-circuit secondary, the CT magnetization core and primary current will decide the amount of the high secondary voltage. The whole of the primary ampere-turns act on the core as an excessive exciting force. The whole primary current will equal to the magnetization current when the secondary circuit is broken and lead to saturation. The high rate-of-change of the flux in the region of the primary current zero will produce a high secondary voltage.

  3. Rabie A mohamed

    could you please submit the calculation for open circuit instrument CT’s of control unit of capacitor bank
    Please help …. Thanks

  4. Scott Vaughn

    Excellent detailed explanation of the Current Transformer! Very descriptive and informative. If you are in need of Current Transformers, Power Transformers, Switchgear or Circuit Breakers – Please visit us at

  5. steve m

    This tutorial is so far off base re voltage of the open circuit secondary!!! There is no 480V in the primary. It is just the primary current times the wire resistance (ie voltage drop) of about an inch of wire passing through the core. Yes, the secondary voltage will be very high because CT transformer design tries to keep the correct secondary current (based on the turns ratio) times the secondary load resistance. Of course with an an open circuit you can not draw any real current but the voltage will try to go as high as possible within normal constraints.

    • Wayne Storr

      Hello Steve, thank you for this. I know from experience of a colleague who was hospitalised while changing a faulty ammeter on a board. The open circuit secondaries of current transformers can and do generate very high no-load voltages.

  6. eswar kpndapalli

    If input voltage is suppose 220KV ; wont the voltage in secondary more then primary voltage?? If yes then how much insulation have to provide to ct?? Then what about the size?? But what is the actual size of ct???

    • Wayne Storr

      Current Transformers measure current (hence their name) and not voltage. Also 220KV is a very dangerous voltage to be playing with. Please do not touch it.


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