Last updated on June 15th, 2026 at 08:55 am
Current transformers (CTs) are used in many applications that require current, power or energy monitoring. Examples include power generation, submetering, protective relaying, motor control, and machine fault detection. Selecting a current transformer involves picking the style, ratio, burden, accuracy and insulation to
safely measure an alternating electrical current. Making the wrong choice can lead to inaccurate readings, equipment damage and electrical hazards.
A current transformer (CT) operates on the principle of magnetic induction. An alternating current flowing in the primary winding generates an alternating magnetic flux in the core. This induces an alternating voltage across the secondary winding. When the secondary circuit is closed, the current flow is proportional to the primary/secondary turns ratio. In an instrumentation CT, the primary winding is often just the primary conductor routed through the center of the transformer core (1 turn). The secondary winding is wound around the same core with significantly more turns.
Core Styles
Instrument transformers are designed for use with measurement and control equipment. Metering CTs come in two basic core styles.
Solid-core CTs are typically used in new installations. The primary conductor is threaded through the center hole. These CTs offer excellent accuracy and are budget-friendly. However, installation requires a de-energized circuit, which can be an obstacle in maintenance and repair situations. A variety of window shapes and mounting methods are available.
Split-core CTs are easier to retrofit. They are also convenient for new installations, particularly when working with heavy conductors or busbars. Their hinged or clamshell construction can be snapped around an existing conductor without breaking the primary connection. While the split-core design may cost more than a solid-core equivalent, the savings in installation time often make it a better solution. Flexible CTs are split-core units which can be quickly twisted around large or irregularly shaped conductors. They may also be preferable in restricted spaces or crowded enclosures.
Key CT Parameters
For metering applications, CT selection and sizing involves these key parameters:
Current Ratio defines how the CT scales down the primary current to the desired secondary current (typically 1 A or 5 A). A 100:5 CT reduces 100 A to 5A full scale. A primary current somewhat higher than the normal operating level is often chosen to allow for surges and overloads. Applications with high inrush currents, such as motor loads, may also need a higher CT ratio. Units rated for a maximum current of 50 A to 6000 A are readily available.
Burden is the amount of power the CT can deliver to the secondary circuit while maintaining specified accuracy. The CT VA rating must be greater than the total I2R or VA of the secondary (wiring plus meter). The rating for off-the-shelf products vary from 1 VA to 40 VA.
Accuracy is the CT’s error limit at full burden. It is expressed as either a percentage or class (Class 5 = 5%, Class 0,5 = 0.5%). For a particular core size, higher current units generally have a better accuracy. Metering CTs commonly have an accuracy of 1-2%, with revenue-grade units offering 0.1-0.5%.
Frequency: Most metering CTs are specified for either 50 or 60Hz. Using this type of CT at the wrong frequency will affect performance and degrade accuracy. Occasionally a CT may state an accuracy for both frequencies or specify operation up to 400 Hz with a derating factor.
Insulation Level specifies the maximum safe operating voltage with a bare primary wire (e.g. bus bar). Low voltage CTs are 600 V insulation class. Above this level, insulation can be added or a medium voltage CT selected.
In addition to core style, other physical characteristics will affect CT selection. Most critical is picking a window size and shape to fit the primary conductor (or multiple passes of the conductor if the primary will be more than one turn). However, an excessively large window makes positioning the conductor difficult and can degrade accuracy. Mounting method is also a consideration. With small CTs or heavy primary conductors, the CT can sometimes be supported by the primary conductor. Larger CTs may require a more secure attachment. Secondary terminations are another variable. Captive wires, screw terminals or studs are frequently used. Each will affect the installation process.
Calculating Burden
A critical step in sizing a meter CT involves calculating the total load the CT must drive in the secondary circuit. This comes from two sources: meter impedance and lead resistance. An input VA rating may be listed in the meter specifications. The I2R of the leads is added to the meter VA to get the total secondary burden. If the meter VA is not known, for 50 or 60 Hz operation, the measurement of resistance at the meter terminals is sufficient to determine the meter load. This is added to the total resistance in both leads and multiplied by the secondary current to determine to total VA that the CT must drive. In many applications the lead resistance is the largest factor. Keeping the meter close to the CT and using heavy gauge wire reduces the burden. If multiple measuring devices are series connected in the secondary circuit, the burden from each is added to the calculation.
Safety Considerations
The most important safety rule when working with a current-output transformer: Never open the secondary circuit of an energized transformer. If the secondary is opened while the primary is energized, the CT will generate an extremely high and potentially lethal voltage across the secondary terminals, which can cause shock, arc flash, and insulation damage. A good practice is to short the secondary terminals at the CT before performing any work on the secondary circuit. For normal operation, grounding the low point in the secondary circuit prevents hazardous voltage buildup.
Other Metering CT Options
Voltage output current transformers convert the ac current input into an ac voltage output (typically 0.3 to 2 V full scale). This eliminates the need for heavy leads or a high VA rating. The voltage output also allows a greater distance between the CT and the meter. Another advantage – the output is clamped to a safe voltage level. Disadvantages can be incompatibility with existing instrumentation, higher noise susceptibility and spurious readings if the secondary circuit opens. These transformers may look identical to current output models, so careful attention to model number or nameplate data is important.
Current transducers produce a dc process output that is proportional to the ac current input. The output can be either a voltage (typically 0-5 V or 0-10 V) or a current (typically 0-1 mA or 4-20 mA). Some current transducers require an external power supply.
Rogowski coils are flexible, air-core sensors that wrap around a conductor. They measure the rate of change of current and require an integrator to produce a level output. Their key advantages are high linearity and an inability to saturate, making them ideal for measuring high frequency or transient currents that are beyond the limits of conventional CTs.
Frequently Asked Questions
What are the consequences of an incorrect CT ratio?
A CT with a primary rating that is too high for the normal operating current degrades measurement accuracy. Both the ‘of range’ accuracy component and magnetizing current become more significant. The meter accuracy may also be degraded if it is matched to the CT full scale output but operates significantly below that level. If the CT primary rating is too low, the core will approach or enter saturation, resulting in a non-linear output that also degrades accuracy. This could mask a fault condition or cause the CT to overheat and fail prematurely.
Is a 1 amp or 5 amp secondary better?
The 5 amp secondary is standard in the US for metering applications. Many CT models are offered and stock is readily available for both new and replacement situations. The 1 amp secondary is popular internationally since resistive losses in the secondary circuit are significantly reduced. However, availability of 1 amp CTs in the US is limited.
What is the correct CT ratio for a motor load?
For a single-phase induction motor, a general guide is to multiply the motor’s Full Load Amperes (FLA) by 1.25. This places the full load current above 2/3 full scale on the CT, while allowing sufficient headroom to also read overload conditions. The next higher standard CT primary ratio is then selected. For example, FLA of 330 A x 1.25 = 412 A and the next higher size is 500:5. For guidance on three-phase motors see Sizing a Motor CT.
How do protection CTs differ from metering CTs?
Metering CTs are designed for high accuracy under normal operating conditions (typically 5 to 120 percent of rated current). They saturate quickly above this range, which protects the meter from damage during high-current fault conditions. Metering CTs commonly have an accuracy of 1-2%, with revenue-grade units offering 0.5% or better. Metering CTs are specified at a 0.9 power factor.
Protection CTs are designed with larger cores that provide a wide operating range before saturation. They remain accurate during fault conditions when currents can be 10 to 30 times the normal operating limit. This allows protection relays to correctly identify and respond to faults. Protection CTs often have a specified accuracy of 5-10% at 0.5 PF.
Conclusion
Proper current transformer sizing is an important step to ensure accurate, reliable, and safe ac current measurements. It requires balancing the CT ratio for both normal loads and fault conditions, calculating the total burden of meter and wiring, and selecting the appropriate frequency, accuracy class, physical CT type, and insulation level.
At Weschler Instruments, we offer a wide selection of metering CTs. Our application specialists have decades of experience helping customers select the right measurement and control equipment, whether for a new installation or retrofit. Call us for assistance with your application.
More information: Turns ratio adjustment and polarity requirements
Converting a current transformer’s ac output into a dc process output
Chart of maximum lead length vs wire gauge and CT burden
