How Does CT Burden Affect Current Transformer Accuracy?

CT burden is an important factor in current transformer performance, but it is often overlooked when selecting or installing a CT. The secondary circuit does not only include the connected meter or relay. Cable resistance, terminals, connections, and other loads also contribute to the total burden.

When the actual burden becomes too high, the CT may no longer maintain its specified accuracy. This can increase ratio and phase errors and, under demanding conditions, contribute to saturation. As a result, the quality of the secondary current signal can affect both measurement and protection performance.

This article explains what CT burden means, how it is created and calculated, and how it affects CT accuracy. It also covers common sizing mistakes and key burden considerations for outdoor and renewable power applications.

1. What Is CT Burden?

CT burden describes the total load connected to a current transformer’s secondary circuit. It determines how much secondary-side demand the CT must supply and is an important part of matching a CT to its connected equipment.

CT Burden in VA and Ω

The burden can be expressed in VA or Ω, depending on how the secondary circuit is specified or calculated. For a mainly resistive circuit, the relationship is:

VA = I²×R

Because the current is squared, a 5A circuit produces 25 times the resistive loss of a 1A circuit with the same resistance.

Rated and actual burden should be distinguished during selection. Some CTs specify an accuracy range such as 25%-100% of rated burden. The applicable range depends on the CT specification.

2. What Creates CT Burden?

CT burden is the total secondary-circuit load, not simply the burden of the connected meter or relay. Several elements contribute to the load seen by the CT, so each part of the secondary circuit should be considered during selection and calculation.

The actual burden depends on the complete secondary circuit. In practice, cable length and conductor size can become especially important when the CT is installed far from the connected equipment. The manufacturer’s specified calculation method should be followed when determining which internal CT losses are included.

3. How to Calculate Total CT Burden

The total burden should be calculated from the complete secondary circuit rather than one connected device.

Calculate the Actual Burden

List meter, relay, and other device burdens.

• Calculate secondary cable resistance.
• Convert cable resistance to VA using VA = I²×R
• Add relevant terminal, connection, and winding contributions according to the manufacturer’s method.
• Compare the total with the CT’s rated burden.

Check 1A vs 5A

A 5A secondary produces higher line losses than a 1A secondary. With the same resistance, its I²R loss is 25 times higher, making 1A suitable for many long secondary circuits. The final choice should match the connected equipment and CT specification.

Worked Example: A 5A CT with 1 Ω resistance has a 25 VA resistive burden. If rated at 15 VA, the actual burden is too high. Reducing resistance or using a suitable 1A secondary can lower the burden.

4. How CT Burden Affects Accuracy

CT burden affects how accurately a transformer reproduces primary current on its secondary side. As the secondary load increases, the CT requires more excitation current, which can increase measurement errors and reduce protection performance.

Burden and Saturation: For protection applications, burden should be evaluated together with fault current and CT excitation characteristics. ALF, knee point, and accuracy class help determine whether the CT can reproduce current accurately under demanding conditions.

Too High or Too Low: For CTs specified with a 25%-100% rated-burden accuracy range, the actual burden should remain within that range unless the manufacturer specifies otherwise.

5. Common CT Burden Sizing Mistakes

Small errors in burden estimation can affect CT accuracy, especially in long secondary circuits or demanding protection applications.

Before selecting a CT, review the secondary circuit, accuracy class, burden, and operating conditions as one system rather than evaluating burden alone.

6. CT Burden in Outdoor and Renewable Power Projects

Outdoor transformer projects require more than a burden calculation. Environmental conditions can affect secondary-circuit resistance and connection reliability over time.

When CTs support metering or protection systems, consider burden, accuracy, installation conditions, and long-term operating reliability together.

Conclusion

CT burden is the total load of the secondary circuit, so accurate selection requires more than checking meter or relay burden. Secondary cable resistance, connections, CT specifications, and actual operating conditions all contribute to the final burden. A proper assessment helps control measurement errors and reduce the risk of saturation, supporting reliable metering and protection performance.

For outdoor transformer applications, CT selection should also account for temperature, humidity, installation conditions, and other environmental factors. CNMIBO supports outdoor transformer applications with attention to both electrical requirements and operating conditions. Need help with an outdoor transformer or CT-related application? Contact CNMIBO with your CT requirements, secondary-circuit details, installation conditions, and application requirements.