The size of a current transformer refers to both its physical dimensions and its electrical ratings. Physical size determines whether the CT will fit around the cable or busbar and inside the enclosure. Electrical size determines whether it can handle the current, voltage, burden, and accuracy requirements. Both aspects must be considered together.
The first physical consideration is the window size. The CT must fit around the primary conductor. For cable-type CTs, the window must be large enough for the cable insulation and any bending radius. For busbar-type CTs, the window must match the busbar dimensions. If the window is too small, the CT cannot be installed. If it is too large, the CT may be unnecessarily bulky and expensive.
The second physical consideration is mounting space. The CT must fit inside the panel, switchgear compartment, or enclosure. Height, width, depth, and mounting hole spacing must be checked. Cable routing and access for maintenance should also be considered. Split-core CTs are useful where space is limited or cables cannot be disconnected.
Electrical size begins with the primary current rating. The CT must handle the maximum continuous current and expected overloads. The ratio should be chosen so that normal load falls within a suitable range. For metering, 60–80% of the primary rating is often recommended. For protection, the CT must withstand fault current without saturation or damage.
Voltage rating and insulation level must match the system. The CT must be rated for the system voltage and environmental conditions. Burden rating must be equal to or greater than the total burden of the secondary circuit. Accuracy class must meet the application requirements.
Thermal and dynamic ratings are important for high-current applications. The CT must withstand short-time thermal current and peak dynamic current. If these ratings are too low, the CT may be damaged during a fault.
In summary, determine the CT size by checking window dimensions, mounting space, primary current, voltage, burden, accuracy, and thermal ratings. Select a standard size whenever possible. A correctly sized CT will fit the installation, handle the load, and provide accurate and reliable performance.