ct ratio

Since it's the ratio of primary, input, current to secondary, output, current, then you would need to know how many amps are going in and how many are output. For example a CT ratio of 1000:10 would be 10 amps of output current when the primary current was 1000 amps.

The primary current of the CT should be as close as possible but not less than the load current or the rated current of the feeder. The secondary current may be either 1A or 5A depending upon the relay or meter that you use on the secondary.

Hi, The other partcipents answered very well. But if you want to calculate the undesignated CT, then please do the following steps to calculate the installed unknown CT ratio;

1. Install a ampere mater on CT. An ampere meter always gives full dial reading on 5 Ampere. Note the reading of the ampere meter Say X=?

2. Check the ampere of power cable (which is passed through CT) by Ampere Clamp meter. Its reading will be Say Y= ?

3. Use this Formula

CT Ratio = ((Y/X)*5):5

Regards