How ANSI CT burden and accuracy class are checked
US relaying current transformers are rated by accuracy class rather than by an IEC accuracy limit factor. A C400 CT guarantees that ratio error stays within 10% up to 20 times rated secondary current, which is 100 A, while delivering up to 400 V at its terminals. The C number is simply that terminal voltage: C100, C200, C400 and C800 match standard relay burdens of 1, 2, 4 and 8 ohms at 100 A. An IEC 5P20 CT states a VA burden and a factor instead, so the two systems are not interchangeable and a datasheet conversion is never exact.
The check is straightforward. Add the round trip lead resistance to the relay burden to get the external burden in ohms, find the secondary fault current by dividing primary fault current by the CT ratio, then multiply the two to get the terminal voltage the CT must deliver. If that voltage stays under the class rating and the secondary current stays under 20 times rating, the CT will not saturate for a symmetrical fault. Lead resistance uses NEC Chapter 9 Table 8 values for copper at 75°C, doubled for single-phase or residual runs.
DC offset in a real fault, terminal block contact resistance and test switch resistance are not included here, so leave margin beyond a marginal pass. Have a licensed protection engineer confirm relay settings and CT selection before energizing.
Frequently Asked Questions
Only roughly. C400 means 400 V at 100 A through the external burden, so it supports about 100 VA of burden at 20 times rating, which is far above a typical 5P20 15 VA CT. Compare actual burden and saturation voltage rather than swapping labels.
Increase the lead size. Going from 12 AWG to 8 AWG cuts lead resistance by about 60%, which is usually cheaper than moving to a higher accuracy class CT or relocating the relay panel.