It is often the case that a specific application will require a current transformer with an odd ratio which is not available commercially. In these instances it is possible to use additional wraps of either the primary line or the secondary lines to change the ratio of a standard transformer to have the desired ratio. The formula for computing the ratio of a CT is as follows:
Xnew / Ynew =
(Xstandard + (Ystandard * # of secondary wraps)) / Ystandard* # ofprimary wraps
Xnew = The desired numerator
Xstandard = The numerator of the ratio of the standard, stocktransformer
Ynew = The new denominator; for 99% of applications this will beconstrained to equal 5
Ystandard = The denominator of the ratio of the standard, stocktransformer
For example, if two primary wraps and negativethree secondary wraps were added to a 75:5 transformer, the endratio would be:
(75 + (5 * -3)) / (5 * 2) or 30:5
Primary wrap refers to wrapping the line with thecurrent being measured around the body of the CT. Each primary wrapbeyond the first decreases the ratio of the transformer; in fact,two wraps divides the ratio by two, three wraps divides the ratioby three, etc.
Secondary wrap refers to wrapping the lines runningfrom the CT to the meter through the body of the CT. A secondarywrap in the same direction as the primary loop is a positive wrap,while a secondary wrap in the opposite direction of the primaryloop is a negative wrap. Each secondary wrap changes the ratio ofthe CT by a value equal to the denominator of the CT (usually 5)and are either negative or positive, depending on direction.
All of the transformers retailed by Simpson have asecondary value of 5 Amps, and ratio tables for these tables areavailable here in PDFformat. The top row of the table shows the numerator of the stocktransformer ratio and the number of primary and secondary wraps areshown along the right. To use the chart, simply find the number inthe table which is closest to the desired value, then order thetransformer with the same numerator and add the requisite number ofwraps