Current Transformers

Current Transformers: Split Core, Solid Core, and what you need to know



Some energy monitoring meters require current transformers (CTs) to operate. If you have ever questioned why they are used or how they work, here is the basics of the current transformers.

Current transformers reduce high voltage currents to a much lower value and provide a safe and convenient way of monitoring the actual electrical current flowing in an AC transmission. CT’s work by converting the primary current into secondary current through a magnetic medium. They make it viable for energy meters to measure current on circuits that, if they measured the current directly, would overpower the meters. Generally, most meters can measure up to 100A max current before needing CTs to read any higher.

Current Transformer Types

There are two main kinds of CTs:
  • Solid core CTs shape a completely closed core. Installing a stable core CT requires disconnecting the conductor to get it through the CT core (think of a string through the eye of a needle). The advantages of solid core CTs are that they are usually much less costly and can be more accurate. They are most regularly used in new installations.


  • Split core CTs have a "split" in the core that lets in the CTs to open and be positioned round the conductor without having to disconnect the conductor or disrupt the wiring. Split core CTs may be greater in price however they’re comfort usually outweighs their price when dealing with retrofit installations. Simply unclip the coil, wrap round the load you are measuring and reclip!



Single and Three phase

Depending on the meter you have purchased you will either need single or three-phase current transformers. A single-phase meter will pair nicely with a single-phase split/solid current transformer and a three-phase meter pairs nicely with 3 x single-phase split/solid current transformers or 1 x three-phase solid core current transformer. On our website we offer single-phase split or solid core and three-phase solid core.


Current rating

Most CTs are labelled in accordance to their nominal current rating. It's vital to use the right CT to gain the CT’s accuracy at the lowest feasible load.

  • Most CTs begin to turn out to be correct at 5-10% of their rated ability relying upon their accuracy classification. At decrease loads, CTs can turn out to be rather inaccurate.
  • Most CTs keep accuracy up to 120-130% of their rated capacity. Beyond the most rating, the CT will overload, and overall accuracy will fall rapidly. Overloading a CT can be dangerous and possibly damage any equipment or energy monitor that is linked to that CT.

Selecting the best current rating (CT Rate) for a CT ought to be primarily based on predicted minimum, common and maximum loads. We recommend to get the CT primary rating as close to your maximum current of the application as possible.


CTs are handy in a range of accuracy lessons ranging from 0.1% to 5% error. Typical CT's have an accuracy of 1% (referred to as Accuracy Class 1). Accuracy will be expressed over a unique load range. In the case of a 1% rated CT, accuracy is expressed over a size vary of 10 to 120% of the CT's primary current. So accuracy class 1 CT with a primary current of one hundred amps will have 1% accuracy from 10 to 120 amps of current.

Physical size and Aperture

You will need to pay attention to the inner diameter (Aperture) of the CT. This describes the dimension of the opening interior of the CT. If you attempt to use a CT with too small of an aperture, it may not fit round the conductor(obviously!).



CTs will most likely have a modern output such as 1A or 5A representing the output at the nominal ranking of the CT. Millivolt CTs use an inside resistor to create a voltage output such as 0.333mV versus a modern output. Modern CTs can produce enormously high and risky voltages when the leads are disconnected and the CT is mounted on a live conductor. So, although they are less common, millivolt CTs provide a security benefit and are favoured on bigger electricity circuits. 5A CTs being the most common, only allow you to extend the cable from CT to Meter a maximum of 5 metres before accuracy is affected whereas, with 1A or Millivolt CTs, this can be extended further before accuracy is affected.