Whether a current transformer can actually deliver what the protection scheme needs. An undersized CT saturates during a fault, and a saturated CT can make a differential scheme mis-operate on an external fault.
Standard method — stated scope limits apply
CT & Circuit
Burden
Stability (differential / REF)
Enter CT and burden data.
About This Calculation
A protection-class current transformer (CT) has to do two contradictory jobs: reproduce normal load current accurately for metering and relaying, and NOT saturate during a fault current many times larger, since a saturated CT distorts the secondary waveform and can cause a relay to misoperate or fail to trip at all. CT sizing checks that the CT's rated accuracy limit factor (ALF) and connected burden (relay coils, leads, meters) together keep it working correctly right up to the maximum fault current the protection scheme actually needs to see — not just at rated load.
Frequently Asked Questions
What is CT burden and why does it matter?
Burden is the total impedance the CT secondary has to drive — relay coil impedance, connected meters, and the resistance of the lead wire between the CT and the panel. A higher burden makes the CT more prone to saturating at a given fault current, because more of the CT’s limited output voltage capability is consumed just driving current through that burden.
What does the accuracy limit factor (ALF) actually mean?
ALF is the multiple of rated primary current up to which the CT is guaranteed to stay within its stated accuracy (composite error), for a given rated burden. A 5P20 CT is guaranteed 5% composite error up to 20 times rated current — exceed that current-times-burden combination and the CT can saturate.
Why do long CT lead runs matter so much?
Lead resistance adds directly to burden, and it does so twice — once out to the relay, once back — so a long run in small-gauge wire can dominate total burden even with a low-impedance relay, pushing an otherwise adequately-sized CT toward saturation at fault current.
Should protection and metering ever share the same CT core?
Not ideally — a metering-class CT core is optimized for accuracy at NORMAL load current and is designed to saturate early to protect connected meters from fault current, which is exactly the wrong behavior for a protection relay that needs to see the fault clearly. Multi-ratio, multi-core CTs with separate protection and metering cores are the standard solution.