Calculating a current transformer: ratio, CT factor and sizing

The ratio of a current transformer is the quotient of rated primary and secondary current: Kn = Ipn / Isn. A 400/5 A transformer has a ratio of 80, a 400/1 A transformer 400. Everything else follows: the secondary current is the primary current divided by Kn, the primary current is the measured secondary current times Kn. With 320 A through a 400/5 A CT, it delivers 320 / 80 = 4 A to the instrument.

On an energy meter Kn is called the CT factor or multiplier. A CT-operated meter only counts what flows through the secondary side; consumption is the difference of the readings times the CT factor. Example: three 400/5 A CTs, reading from 1,250.0 to 1,312.5 kWh – that is 62.5 kWh × 80 = 5,000 kWh. Meters with a programmable ratio already display the primary value.

Choose the rated primary current just above the highest operating current. A transformer that is far too large works mostly at part load, where the error limits are wider (class 0.5: ±0.75 % at 20 %, ±1.5 % at 5 % of rated current). For strongly varying loads the S classes 0.5S or 0.2S, defined down to 1 % of rated current, help.

With plug-on CTs the ratio can be reduced by passing the conductor through several times: with n passes Ipn / n applies. A 100/5 A CT with two turns works like a 50/5 A CT. For permanently small currents a wound primary CT is the better choice. The burden in VA is calculated with the VA calculator.