Relatively large changes in a Current Transformer ratio may be achieved through the use of “Primary Turns”.

One Primary Turn is equal to one pass through the Current Transformer window.

More than one pass through the window results in a current ratio modification according to the formula:

Modified Ratio = (Original Ratio) / (Number of Primary Turns)


Original Ratio
(One Primary Turn)
Number of Primary TurnsModified RatioOriginal Ratio
(One Primary Turn)
Number of Primary TurnsModified Ratio
 100:5A 2 50:5A 100:1A 2 50:1A
 200:5A 2 100:5A 200:1A 2 100:1A
 300:5A 2 150:5A 300:1A 2 150:1A
 300:5A 3 100:5A 300:1A 3 100:1A
 100:5A 4 25:5A 100:1A 4 25:1A
 200:5A 4 50:5A 200:1A 4 50:1A
 300:5A 4 75:5A 300:1A 4 75:1A

The main advantage of this ratio modification is that the accuracy and burden capabilities of the higher ratio are maintained.

In general, for the same secondary current rating, the higher the primary current rating the easier it is to achieve higher accuracy and burden ratings.

To achieve high accuracy and burden requirements with low primary current ratings use a Current Transformer with a higher primary current rating and the required accuracy and burden and apply multiple primary turns.