# Determing Turn Count on a Current Transformer

I have a current transformer that I bought on EBAY as part of a small voltage+current meter.

It does not specify what the CT is rated at.

From another post I found a formula and would like to know if my calculations are correct.

You could also calculate it if you had the input current and output current. If the output current at 20A was 10 mA (0.01 A) you would divide 20 by 0.01 to get 2000.

So I live in New Zealand and I am using a FAN heater as a test bed at 1/2 heat. I believe the fan heater is 2000w at full power, and 1000w at 1/2 power.

Current AC Voltage : 233v AC Fan Heat set to 1000w which is 1000 / 233 = 4.2918 AMPS

Now cabling one of the AC cables though the CT it is producing 13.84 milliamps of AC current on my digital multimeter.

So from what I believe from the above formula :

4.2918 / 0.01384 = 310.10 turns.

So it is probably a 300:1 CT ?

Chris

Always connect a shunt resistor securely to the terminals of the CT if it doesn't have one built in. Operating one without can generate high voltages and breakdown the insulation on the windings. In particular you may need to use screw terminals to multimeter probes to ensure they cannot slip while measuring the current.

With a shunt resistor in place you can measure the current with a voltmeter instead which is easier.

MarkT: With a shunt resistor in place you can measure the current with a voltmeter instead which is easier.

I take it a shunt resister could be considered a burden resister as well?

Chris

I have put the formula I am using into a google spreadsheet.

It calculates the Ideal Burden resister and also allows you to use another value and then compute the current and power. Simple but it was what I was looking for.

May be useful for someone else in the future.