CT Burden

What is CT Burden?

CT burden is the total resistance of the secondary load of a current transformer or, in other words, the maximum load that can be applied to a CT’s secondary. A CT’s maximum burden will vary depending on the CT’s turns ratio, the desired CT output, and the sensor’s current rating.

CT burden is commonly expressed in one of two ways:

1. The total impedance of the circuit in ohms (Ω)
2. The total VA (volt-amperes) and PF (power factor) at a specified current/voltage and frequency

A CT’s total impedance is a combination of three factors:

1. The sum of all resistance present in the CT’s secondary winding
2. The resistance in the CT’s lead wires
3. The resistance present in the meter, relay, or other device connected to the CT

Why is it important to know a CT’s burden?

Depending on a facility’s layout, it is possible that a power meter may need to be installed some distance away from the load to be measured. In these instances, it can be helpful to extend the leads of a CT to accommodate longer distances. However, it is also important to note that there is a maximum distance that the leads can be lengthened to beyond which the accuracy will decline. This is because the CT’s maximum burden is being exceeded by the added resistance of the CT leads. The lead’s burden can be calculated by using the following formula to determine how far the leads can be extended without impacting accuracy.

Output Current² x Resistance of Cable x 2(Length of Cable)

For example, if a 600/5A CT is selected with a resistance value of 0.02Ω and has a 2.5-meter lead, the VA of the cable will be:

5²×0.02×2(2.5)=2.5VA

It is important to note that the resistance value of the wire used to extend CT leads will vary depending on gage and other factors. Verify all wire specifications with the manufacturer before extending a CT’s leads.

What is the impedance of my CT?

To see the CT impedance rating for an Accuenergy CT, view the corresponding datasheet for your sensor and check under the “Electrical” subheading. There, CT impedance values will be listed for each CT model (in ohms). An example CT burden ratings for AcuCT-4161R models are listed below:

How to Use Impedance Values to Calculate CT Burden

If you know a CT’s impedance value, you can easily calculate the burden (VA) by using the following formula:

Output Current² x Impedance = VA

For example, if a 600/5A CT is selected with an impedance value of 4.16Ω, the VA of the CT will be:

5²×4.16=104VA