Current Transformer Primary Turns Explained

June 16th, 2020:
The Current Transformer (or CT) is a type of “instrument transformer” which produces an alternating current in its secondary winding that is proportional to the current being measured in its primary. Current transformers can reduce or “step-down” current levels from thousands of amperes down to a standard output of either 5 Amps or 1 Amp for normal operation. Therefore, current transformers reduce high voltage currents to a much lower value and provide a convenient and safe way of monitoring the actual electrical current using a device such as a standard ammeter. Current transformers and ammeters are typically used together as a matched pair in which the design of the current transformer is such as to provide a maximum secondary current corresponding to a full-scale deflection on the ammeter. Most current transformers have a standard secondary rating of 5 amps with the primary and secondary currents being expressed as a ratio such as 100:5. This means that when 100 amps is flowing in the primary conductor it will result in 5 amps flowing in the secondary winding.

With all of that being said, did you know that current transformer ratio's can actually be modified?

Primary Turn Ratio Modification

The nameplate, or ratio, of the current transformer is based on the condition that the primary conductor will be passed once through the current transformer opening. So by using a current transformer with a ratio of 100:5 and passing a 100 amp conductor through the opening one time, we will get a 5 ampere output. However, this ratio can be reduced in even multiples by looping the conductor through the current transformer opening 2 or more times. So continuing with our 100:5 ratio example, we can reduce that ratio to 50:5 with 2 loops or "turns". By using 4 loops or "turns", we could reduce that even more to be 25:5. Check out our charts below for additional examples.

A "primary turn" is the number of times the primary conductor passes through the CT's window. The main advantage of this ratio modification is that you maintain the accuracy and burden capabilities of the higher ratio. The higher the primary rating, the better the accuracy and burden rating.

At Ram Meter Inc., we carry a wide selection of Current Transformers in both solid-core and split-core styles. Not sure which style or ratio is right for your application? CONTACT US today with your needs and one of our highly qualified salespeople will assist you.