Instrument Transformers – Application Guide

The primary purpose of this guide is to give the reader a basic understanding of how to apply instrument transformers in a practical way while observing good engineering practice. It is not intended to make the reader an instrument transformer designer. A special effort will be made to keep to a minimum technical terms and language.

Purpose of Instrument Transformers

Instrument transformers provide either a current or voltage at a useable level to monitor the voltage or current in a given circuit. For example, it would not be economical to have an ammeter to measure 600 amps in a conductor directly. It is economical to have an ammeter to measure current over a range of 0–5 amps. By inserting a current transformer in the circuit, it will produce a current which is precisely proportionate to the current in the conductor you wish to monitor over a range of 0–5 amps which corresponds to 0–600 amps. The ammeter will have a scale of 0–600 amps. Likewise, it would not be economical to measure a voltage of 14,400 volts directly. By inserting a voltage transformer in the circuit a directly proportionate voltage over a range of 0–120 volts will be present which corresponds to 0–14,400 volts. Current and voltage transformers are also used to provide the energy to operate various protective relays. The degree to which an instrument transformer produces a current or voltage that is proportionate to the one to be monitored is referred to as its accuracy. This subject will be covered in much greater detail later in this guide.

Current Transformers

As the name implies current transformers are generally used to step down current in a very predictable fashion with respect to current and phase. For example, you wish to measure the current being drawn by a motor to determine if the motor is lightly loaded or being overloaded. First, you must know the voltage level of the motor circuit. From this you can determine what voltage class insulation the current transformer should have. The following is a list of the various voltage classes and some of the most common voltages for each class:

Voltage Class (KV)Voltage Range (KV)Common Voltages (V)
0.6 0–0.6 120, 208, 240, 277, 380, 480, 600
1.2 0.601–1.2 840, 1200
2.5 1.201–2.5 2400
5.0 2.501–5.0 3300, 4200, 4800
8.7 5.001–8.7 6600, 7200
15.0 8.701–15.0 11000, 12000, 14400
25.0 15.001–25.0 18000, 24000
34.5 25.001–34.5 27600, 34500

The motor we wish to monitor is 480V, referring to the above would indicate a 0.6KV (600V) voltage class. Next you should know the motor's Full Load Amps (F.L.A.). Our motor's F.L.A. is 96 amps. A CT with a ratio of 100:5A would work, but you would be pushing the upper limit of your meter if you had a meter with a 0-5A movement and a scale of 0-100A. It would be better if you selected a CT with a ratio of 150:5A with a meter movement of 0-5A and a scale of 0-150A. A CT with a 150:5A current ratio has a turns ratio of 30:1 (150A/5A = 30/1). Therefore, if you have 96A in the primary, you should have 3.2A in the secondary (96A/30 = 3.2A). This is perfect transformation which is not possible. In any device there is always some degree of losses and distortion that takes place. These losses and distortions will cause, in our example, the secondary to produce a current somewhat different than 3.2A. Also there will be some difference in the wave form of the secondary from the wave form of the primary. These differences are known as the accuracy, or lack of, in the CT. The American National Standards Institute has published standards which are generally accepted as guidelines for performance. In the case of instrument transformers the applicable ANSI standard is C57.13. This publication serves as a guideline for instrument transformers manufactured in the United States. According to ANSI C57.13 there are two accuracy categories, metering and relay accuracies. For metering there are 0.3, 0.6 and 1.2 which represent percent maximum ratio error when 100% of the rated primary current is flowing. It is not a complete statement to say a current transformer has a maximum ratio error of plus or minus 0.3%. It is only a complete statement when an accuracy is stated at a given burden. ANSI C57.13 also defines burdens with respect to OHMS and phase angle shift. Standard ANSI burdens are BO.1, BO.2, BO.5, BO.9 and B1.8. These standard ANSI burdens are ohmic values of burden respectively, 0.1, 0.2, 0.5, 0.9 and 1.8 ohms of load.

You can convert the ohmic value of burdens to VA (volt-amps) by simply multiplying the ohmic value by 25 for a current transformer with a 5 amp secondary. For example, the standard ANSI burden of BO.2 is equal to 5 VA (0.2 X 25). Therefore, an ANSI statement of metering accuracy such as 0.3BO.2 is saying that the current transformer's ratio error shall not exceed 0.3% plus or minus at a burden not exceeding 0.2 ohms of burden. The ANSI standard metering accuracy class also defines the maximum phase shift plus or minus as well through the use of parallelograms which requires a technical background beyond the scope of the intended reader.

ANSI relay accuracy class defines the performance of a current transformer under certain conditions with respect to the burden the secondary of the current transformer will support at a given voltage, as well as the maximum ratio error. All ANSI relay accuracy classes require that the ratio error not exceed 10% plus or minus. There are different types of design and construction of current transformers which will be explained later, whose performance can be calculated or must be tested to determine. This is the reason for the letter "C" or letter "T" prefix on the ANSI relay accuracy classes. The standard ANSI relay accuracy classes are C10, C20, C50, C100, C200, C400 and C800, or T10, T20, T50, T100, T200, T400 and T800. The numerical suffix is the voltage that the current transformer's secondary must develop when the current in the secondary is 20 times nominal. For example, if the nominal secondary current is 5A, the designated voltage must be developed when the secondary current is 100A (20 X 5A = 100A). Once again the accuracy is not a complete statement without a specified burden. In this case the burden can be determined by dividing the numeric suffix by 100. For example C100, 100 divided by 100 equals a 1 OHM burden. In the case of a ANSI relay accuracy class of C400 means that the maximum ratio error shall not exceed 10% plus or minus when 20 times nominal current flows in the secondary and the secondary voltage will be 400V with a burden of 4 OHMS. Also the letter "C" says that the design and construction is such that the performance may be calculated.

Toroidal Type Current Transformers

In general there are three types of design and construction for current transformers. They all have a magnetic core or cores and one or more windings. The most common type is the toroidal or donut type which has no internal primary winding as such. The primary is the conductor in which the current is to be monitored. The primary conductor is simply placed through the window of the current transformer. The core in this type is a band of magnetic grade steel concentricly wound upon itself. This is an extremely efficient core design which has no breaks or gaps in the magnetic path. While other types of core designs may produce relative high levels of audible noise, (approximately 30-70 DB's) the toroidal core rarely produces an audible noise level. The toroidal or donut type current transformer is very popular because it is non intrusive to the circuit being monitored. There is no direct physical or electrical connection in the circuit. The only link between the circuit being monitored and the current transformer is the magnetic field which is developed around the primary conductor as current flows through it. The toroidal type in general also has the lowest cost associated with it. While a few manufacturers offer toroidal current transformers of the 2.5 and 5.0 KV voltage class, the vast majority of toroidal type current transformers are 600V class. It is possible to use a 600V class toroidal type current transformer in higher voltage class circuits if the primary conductor is fully insulated and shielded or the current transformer is placed on a bushing that is designed and rated for the circuit voltage class. Most of the switchgear circuit breaker manufactures design the bushings so they will accept one or more 600V class current transformers even though the breaker is rated 15KV class. This approach is much more cost effective as the cost of a 600V class compared to a 15KV class current transformer is relatively much less.

Ratio Adjustment

Another reason for the popularity of the toroidal type of design and construction is that with relative ease the transformers ratio may be adjusted. You can make course adjustments by adding primary turns. For example, let's say you need a ratio of 50:5A, but you only have a 100:5A current transformer available. You can simply take two primary turns (100/2 = 50) and adjust the 100:5A to a 50:5A current ratio current transformer. This is accomplished by looping the primary conductor so it passes through the window two times. This technique is often used to achieve improved accuracy and burden capability at the lower current ratios. In general, the greater the current ratio the better the accuracy and burden capability is with current transformers. Therefore, you can take, for example, a 100:5A current transformer, take four primary turns and make it a 25:5A ratio (100/4 = 25), and enjoy the better 100:5A performance characteristics at the 25:5A current ratio. It is possible to make fine adjustments to the current by applying secondary turns to the current transformer. For example, you may have a 100:5A current ratio current transformer and need a 90:5A current ratio. Dependent upon how the secondary turns are applied (additive or subtractive) you can adjust the primary rating by 5 amps for each secondary turn. To take the 100:5A current ratio and adjust it to 90:5A you only need to apply two subtractive secondary turns. These course and fine adjustments can be made in the field only with the toroidal type current transformers.

Wound Primary Current Transformers

The main difference between the toroidal and wound primary type current transformers is the wound primary type has an internal primary winding and has no window for a primary conductor to pass through. The wound primary type has an advantage in that the designer can make the units with low current ratios have better accuracy and burden capabilities. The wound primary type's primary is actually inserted in series with the conductor which is to be monitored. This then is intrusive in the circuit to be monitored. For this reason there is some hesitation to use the wound primary even though it may be the most effective way to achieve the desired performance, especially for the low current ratio requirements. The wound primary types may be designed using the toroidal type core because of its excellent efficiency. In this case a window is not provided as it is not needed. The wound primary type is more common in the higher (greater than 600V class) voltage class current transformers as it makes the problems of the higher voltages easier for the design engineer to cope with while keeping the design as cost effective as possible.

Bar Type Current Transformers

A true bar type current transformer is a toroidal type transformer with a bus bar as an integral part of the current transformer which is permanently inserted through the window of the torroid. The bus bar serves as the primary conductor. The bar type is inserted in the circuit to be monitored. It is a common error to refer to a wound primary type as a bar type because the primary connections are made to bus bars. In the wound type the bus bars are a means of connection and are not a continuous bar.


While the toroidal wound primary and bar type are the three major types of current transformers, there are many possible variations of thee types such units with tapped windings, multiple windings and multiple cores. Multi ratio current transformers are common. This is, in fact, a tapped secondary which through reconnection can have numerous different ratios. ANSI standard C57.13 defines multi ratios as follows:

50:5, 100:5, 150:5, 200:5, 250:5, 300:5, 400:5, 500:5 and 600:5.
100:5, 200:5, 300:5, 400:5, 500:5, 600:5, 800:5, 900:5, 1000:5 and 1200:5.
300:5, 400:5, 500:5, 800:5, 1100:5, 1200:5, 1500:5, 1600:5 and 2000:5.
300:5, 500:5, 800:5, 1000:5, 1200:5, 1500:5, 2000:5, 2200:5, 500:5 and 3000:5.
500:5, 1000:5, 1500:5, 2000:5, 2500:5, 3000:5, 3500:5 and 4000:5.
500:5, 1000:5, 1500:5, 2000:5, 2500:5, 3000:5, 3500:5, 4000:5 and 5000:5.

The above is the standard ANSI multi ratios with their respective taps. It is possible to get other multi ratio ratings with different taps as the need requires.

Another common variation is the split core or take apart current transformer. This variation is used to install monitoring of a circuit when it is not desirable to open the circuit to install the toroidal or wound primary type. The split core is commonly rectangular in shape. It should be noted another variation is a rectangular shaped current transformer (non-split core or non-take apart).

Another variation is the three phase current transformer which is simply nothing more than three single phase current transformers in a common case.

The ground fault sensor is a current transformer that is designed to work with a specific ground fault relay. The ground fault sensor is designed to provide an adequate current to cause the ground fault relay to be tripped at a predetermined level.


The following are considerations which need to be made in order to properly select a current transformer.

    Determine if the transformer is going to be subjected to the elements or not. Indoor transformers are usually less costly than outdoor transformers. Obviously, if the current transformer is going to be enclosed in an outdoor enclosure, it need not be rated for outdoor use. This is a common costly error in judgement when selecting current transformers.
    If you want an indication, the first thing you need to know is what degree of accuracy is required. For example, if you simply want to know if a motor is lightly or overloaded, a panel meter with a 2 to 3% accuracy will likely suit your needs. In that case the current transformer need to be only 0.6 to 1.2% accurate. On the other hand, if you are going to drive a switchboard type instrument with a 1% accuracy, you will want a current transformer with a 0.3 to 0.6 accuracy. You must keep in mind that the accuracy ratings are based on rated primary current flowing and per ANSI standards may be doubled (0.3 becomes 0.6%) when 10% primary current flows. As mentioned earlier, the rated accuracies are at stated burdens. You must take into consideration not only the burden of the load (instrument) but you must consider the total burden. The total burden includes the burden of the current transformers secondary winding, the burden of the leads connecting the secondary to the load, and of course, the burden of the load itself. The current transformer must be able to support the total burden and to provide the accuracy required at that burden.

    If you are going to drive a relay you must know what relay accuracy the relay will require.
    You must know what the voltage is in the circuit to be monitored. This will determine what the voltage class of the current transformer must be as explained earlier.
    If you have selected a current transformer with a window you must know the number, type and size of the primary conductor(s) in order to select a window size which will accommodate the primary conductors.

Applications of Current Transformers

The variety of applications of current transformers seem to be limited only by ones imagination. As new electronic equipment evolves and plays a greater role in the generation, control and application of electrical energy, new demands will be placed upon current transformer manufacturers and designers to provide new products to meet these needs.