Depending on their accuracy class, current transformers are divided into two groups: Metering and Protection (Relay). A CT can have burden ratings for both groups.

Metering CT's are typically specified as 0.2 B 0.5

The last number specifies the Burden in ohms. For a CT with secondary current of 5 A the VA burden rating can be calculated as:

VA = Voltage * Current = (Current) 2 * Burden = (5)2 * 0.5 = 12.5 VA

Relaying CT's are typically specified as 10 C 400

The last number specifies the max. Secondary voltage at 20 times the rated secondary current without exceeding the 10 % ratio error. For a CT with secondary current rated at 5 A, 20 times rated current secondary current would give a burden of 4 ohms.

Burden = 400 / (20*5) = 4 ohms

Burden in VA can be specified as:

VA = Voltage * Current = (Current) 2 * Burden = (5)2 * 4 = 100 VA