Online calculator: size the secondary wiring of your current transformers

Anyone who has ever connected a meter or a power analyser through current transformers knows the problem. The transformer nameplate says a rated burden of, say, 5 VA, and IEC 61869-2 states that the accuracy class is only guaranteed between 25 and 100 percent of that value. But how much of those 5 VA does twenty metres of 2.5 mm² cable consume? And is there still room for a terminal block and a second instrument in series?

That is exactly what our new calculator answers. You enter the secondary current, rated burden and accuracy class of the transformer, the burden of the measuring instrument and the wiring parameters: length, cross-section and conductor material. The calculator instantly shows the wiring resistance and burden, the total circuit burden and the reserve up to the rated value. A colour bar tells you at a glance whether the circuit stays within the 25 to 100 percent band in which the accuracy class is guaranteed.

The calculation covers the full loop, that is twice the entered length, and uses a resistivity of 0.0175 for copper and 0.0278 ohm times square millimetre per metre for aluminium at 20 °C. You can split the circuit with a terminal block into two sections with different cross-sections, add a second measurement in series and watch everything reflected in a live wiring diagram.

The most useful feature for design work is the maximum cable length calculation. For both sections the calculator continuously shows how far the instrument may sit from the transformer for the circuit to still comply. The Apply to input button puts the limit value straight into the form so you can see the result.

You will find the calculator at the address below. It is free, requires no registration and suits designers, inspection technicians and fitters right at the switchboard, because it works on a phone.

And if you are still choosing the transformers themselves, you are in the right place. We supply split-core, solid-core, summation and billing CTs, along with Janitza measuring instruments and KVARBOX reactive power compensation. Send us the required ratios and your switchboard layout to kbh@kbh.cz or call +420 777 730 002 and we will prepare a quotation right away.