To determine the current transformer ratio you need, start by calculating the maximum continuous primary current. This is the highest current the circuit is expected to carry under normal operating conditions, including any planned future load. Do not simply use the breaker rating or transformer rating unless the actual load is known to reach that level. A CT should be chosen for the real load, not for the largest possible conductor or device.
For metering applications, the normal operating current should generally be between 60% and 80% of the CT primary rating. If the normal load is 80 A, a 100/5 A CT is usually suitable. If the normal load is 300 A, a 400/5 A CT may be appropriate. This range provides good accuracy while leaving room for small overloads. A CT with a much higher ratio will produce a very small secondary current at normal load, which can reduce accuracy and resolution.
For protection applications, the ratio must allow the relay to see fault current clearly. The CT must not saturate before the relay operates. The accuracy limit factor and knee point voltage are important here. A protection CT may need a lower ratio than a metering CT to provide enough secondary current during low-level faults, but it must also withstand high fault currents without damage. The relay settings and coordination study should guide the final choice.
The secondary current must match the connected equipment. Most meters and relays accept 5 A, while 1 A is common in some systems. A 1 A secondary reduces burden and cable losses, which is useful for long cable runs. The ratio must be programmed correctly into the meter or relay. For example, a 200/5 A CT has a ratio of 40:1. If the meter receives 2.5 A, the primary current is 100 A. If the ratio is entered incorrectly, all readings will be wrong.
Standard ratios should be used whenever possible. Common examples include 50/5, 75/5, 100/5, 150/5, 200/5, 300/5, 400/5, 600/5, 800/5, and 1000/5. Standard ratios are readily available, cost less, and are easier to replace. Custom ratios may be needed in special cases, but they should be avoided unless necessary.
Physical factors may also affect the ratio. A high-ratio CT is often larger and may not fit the available space. Busbar-type CTs are common for high currents, while split-core CTs are useful for retrofits. The CT window must accommodate the cable or busbar, and the insulation level must match the system voltage.
In summary, determine the maximum continuous current, apply the 60–80% rule for metering, check protection requirements, match the secondary output, use standard ratios, and verify physical fit. A correctly chosen ratio ensures accurate measurement, reliable protection, and safe operation. A poorly chosen ratio can cause billing errors, false trips, or failure to detect faults.