Let's begin by going over a very basic application. You have a current transformer with a ratio of 100:5 AC Amperes. You take your 100 AC Amp line and feed it through the hole/opening of the current transformer and it gives you a 5 AC Ampere output which you then run to a meter of some sort.

That's easy enough, right?! Current Transformers are pretty basic and straightforward. It is what it is, right? Or IS it? Did you know that current transformers are actually quite versatile? Through the use of primary turns and secondary turns, the current ratio can be altered to fit your application. A 500:5 ratio can be changed to be 100:5. A 200:5 ratio can be changed to be 50:5. A 60:5 ratio can be changed to be 30:5, and so on.

Maybe your application has changed and you need all new meters & current transformers. Perhaps, through the use of primary turns and secondary turns, you can still use those existing current transformers and save your company some money. Or maybe your favorite supplier (Ram Meter Inc. of course!) doesn't have the exact size in stock that you're looking for (unlikely, but we'll let it go for the sake of the argument!), maybe you can purchase a current transformer with a different current ratio and make it work by adding primary turns or secondary turns.

But how do these "Primary & Secondary Turns" work? Allow us to explain!

Primary Turns

The current ratio (nameplate ratio) of a current transformer is based upon the condition that the wire will be passed through the hole/opening once. However, this ratio can be reduced in even multiples by passing the wire through the hole/opening 2 or more times. This is called a "Primary Turn". To be more precise, a "Primary Turn" is the number of times the wire passes through the current transformers opening. Below are a few examples of Primary Turns.

Secondary Turns

It is also possible to make smaller ratio modification adjustments by using additive or subtractive "Secondary Turns". Additive Secondary Turns are achieved by placing the "X1" lead coming off of the current transformer back through the window from the side opposite the "H1" polarity mark. Subtractive Secondary Turns are achieved by placing the "X1" lead coming off of the current transformer back through the window from the same side as the "H1" polarity mark. See the diagram below for examples of both.

Here's an example: if you have a current transformer with a 100:5A ratio, 1 additive secondary turn will change the ratio to be 105:5A while 1 subtractive secondary turn will change the ratio to be 95:5A. When there is only 1 PRIMARY turn, each SECONDARY turn modifies the current ratio by 5 amperes. However, if there are more than 1 PRIMARY turns, each SECONDARY turn's value is changed. In these cases, take the 5 ampere output and divide it by the number of PRIMARY turns. For example, if you have 2 PRIMARY turns, you would take 5A and divide it by 2 for a value of 2.5A. So now each SECONDARY turn modifies the current rating by 2.5A. So using that same 100:5A current transformer with 1 primary turn, an additive secondary turn would change the ratio to be 102.5:5A and a subtractive secondary turn would change the ratio to be 97.5:5A. Confused yet? The following table will help to explain this.

As you can see, there is quite a bit more than meets the eye with current transformers!

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