How to use the CT calculator
Enter the rated ratio from the CT nameplate—not the maximum current you happen to expect. A 400:5 CT has a nominal ratio of 80:1, so 240 A primary ideally produces 3 A secondary. The inverse calculation is useful during commissioning: 3 A measured secondary implies 240 A primary before ratio and phase errors.
Burden is the complete secondary load
CT burden includes meters, relays, transducers, terminal resistance and the complete outgoing-and-return conductor path. Because burden power is I²R, a traditional 5 A secondary creates 25 times the cable VA of a 1 A secondary at the same loop resistance. That does not automatically make one selection correct; protection performance, existing standards and device compatibility control the design.
Accuracy and saturation are separate checks
A ratio result alone cannot verify metering class, phase error, protection accuracy, knee-point voltage or transient saturation. Protection CT selection must use fault current, relay requirements, secondary circuit impedance and the applicable study/specification.
Never open a live CT secondary
A CT secondary must not be left open while primary current flows. Dangerous voltage can develop. Use approved shorting/test terminals, procedures, PPE and qualified personnel. Do not treat the calculator as switching authorization.