Packet Power Blog

Split core, solid core, and why you should care

Posted on Wednesday, May 18, 2016 by Alan Katz
At the heart of every power monitoring device is the current transducer (CT). If you have ever wondered why they are used or how they work, here's a quick review of AC CT basics.

CTs measure current on AC circuits. They operate by magnetically inducing current from the conductor they are placed on into a proportional electric current that flows through the CT conductors.  They make it possible for power meters to measure current on circuits that, if they measured the current directly, would overpower the meters.


There are two primary types of CTs.

  • Solid core CTs form a permanently closed core. Installing a solid core CT requires disconnecting the conductor to get it through the CT core. The benefits of solid core CTs are that they are generally less expensive and can be more accurate.
  • Split core CTs have a "split" in the core that allows the CTs to be placed around the conductor without having to disconnect the conductor or disrupt the wiring. Split core CTs may be more expensive but their convenience generally outweighs their cost when dealing with retrofit installations.


Current rating
Most CTs are labeled according to their nominal current rating. It's important to use a CT sized as close to the actual current as possible to attain its rated accuracy at the lowest possible load. 

  • Most CTs start to become accurate at 5-10% of their rated capacity. At lower loads, some CTs will become highly inaccurate.
  • Most CTs retain accuracy up to 120-130% of their rated capacity. Beyond the maximum rating, the CT will "saturate" and measurement accuracy will fall rapidly.

CTs will natively have a current output such as 1A or 5A representing the output value at the nominal rating of the CT. Shunted CTs use an internal resistor (shunt) to create a voltage output such as 0.33V versus a current output. Current output CTs can produce exceptionally high and dangerous voltages when the leads are disconnected and the CT is installed on a live conductor. So shunted CTs offer a safety advantage and are preferred on higher power circuits. 

CTs are available in a variety of accuracy classes ranging from 0.1% to 5% error.  Typical CT's have an accuracy of 1% (referred to as Class 1.0). Accuracy will be expressed over a specific load range. In the case of a 1% rated CT, accuracy is expressed over a measurement range of 10 to 120% of the CT's nominal current rating. So a class 1.0 CT with a nominal rating of 100 amps will provide 1% accuracy from 10 to 120 amps of current.

Physical size
Pay attention to the inside diameter of the CT. This describes the size of the opening inside the CT. If you try to use a CT with too small of an inside diameter, it won't fit around the conductor.

One of the most common mistakes in CT installation is misorientation. CTs need to be oriented with a specific side facing towards the power source (away from the load). CTs will generally have indicators to help orient correctly. A misoriented CT will result in negative power readings. While Packet Power uses a smart sensing algorithm that changes the CT vector to always keep it positive, it's still a good practice to correctly orient CTs. 

Packet Power provides a large variety of CTs designed to meet your specific requirements. Most monitoring kits can be pre-wired with the selected CTs using polarized quick connectors to avoid cumbersome control wiring work in the field.

Please email  if you have questions.

Topics: power monitoring, Power basics

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