Suggestion: It depends. Normally, the CT is sized for rated breakers in amps upstream. CT dimensioning is often related to the physical CT dimensions. A CT in the neutral can be sized on the ground fault current in the neutral divided by at most 20 since the CT meeting industry standards can withstand 20xIrated.
You need to know the burden of the relay and the wiring. This is usually given in ohms. Electronic relays such as the SR750 have minimal burden. The resistance of the wiring may be more significant.
ANSI rated CT's for protection applications are specified in terms of the saturation voltage with a standard burden. The standard burden is (CT class) / (20* Isec) ohms at 0.5pf. So a class C100 with 5A secondary can provide roughly 100V secondary into a standard burden of 1.0ohm, 0.5pf at 20x rated secondary current. For multi-tap CT's be aware that the sauration voltage is less on lower taps.
For a single SR750 with short leads, C100 or better will be adequate for most applications. Suggest consulting a good protective relaying book for details, such as Blackburn, "Protective Relaying".
IEC rated CT's are sized based on VA burdens and use different accuracy classifications.
Suggestion: The CT design phase should cover all CT parameters, i.e. VA rating, Frequency Rating, Accuracy Class, BIL, etc. It may be a good idea to follow applicable industry standards for CTs and project specs.
dear alpha75,
Quote from Multilin relay instruction book-
GE Multilin also provides phase and ground fault CTs
if required. Dimensions for these are shown in Figure
2.2a, Figure 2.2b, Figure 2.2c, and Figure 2.2d. Note:
Dimensions of a are for 100:5 to 1000:5 phase CT's;
for the dimensions of 50:5 and 75:5 CT's, consult factory.
Unquote
Details of the CT that can be seen in their relay instruction book are:
Rct, C***, Current Tranformation Ratio, Physical Dimension, typ. Magnetizing characteristics, etc along with the Multilin code for the CT. You may find an equivalent one in the market.
Best of luck!
Addendum to the last post:
Also available are the CT Burden in both VA and milli-Ohms due to the connection of the relay, CT Thermal Withstand, Relay Overload Withstand, etc
This might be all that you look for while sizing the CT.
Best regards,
