Current transformer accuracy class: which one do I need?

The class states the maximum deviation from the ideal ratio at rated current: class 0.5 means ±0.5 %. At part load the limits widen – except for the S classes (0.2S, 0.5S). For billing a calibrated CT (usually 0.5 or 0.5S) is needed, for monitoring class 1, for display class 3, for protection relays 5P10.

Bar chart of error limits to IEC 61869-2: class 0.5 at 5, 20 and 100 % rated current ±1.5, ±0.75 and ±0.5 %, class 0.5S at 1, 5, 20 and 100 % ±1.5, ±0.75, ±0.5 and ±0.5 % — Class 0.5 and 0.5S compared: in the rated range ±0.5 % applies to both. At partial load the S class allows only half the deviation and is the only one specified down to 1 % of rated current.

Which class for which purpose?

Class	Use
0.2S / 0.5S	billing with varying load
0.5	billing, accurate metering
1	energy monitoring
3	display
5P10	protection relays

Calculators and related pages

Class comparison

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FAQ

Which class for billing?

A calibrated CT together with a MID meter, usually class 0.5 or 0.5S as specified by the grid or metering operator.