How to select a metering CT ratio
Revenue meters cannot take full service current directly, so an instrument current transformer steps it down to a 5 A secondary, the standard for metering in the U.S. under ANSI C12.11 and C57.13. Enter your connected load, voltage, and power factor, and this tool returns the load current, the standard CT ratio that covers it, and the meter multiplier you will program.
Load current is I = kW x 1,000 / (1.732 x V x PF) for three-phase service and I = kW x 1,000 / (V x PF) for single-phase. Metering practice is to pick the first standard ratio at or above about 125% of that current, choosing from the ANSI ratio list of 10, 15, 25, 40, 50, 75, 100, 200, 300, 400, 600, 800, 1200, 1500, 2000, 3000, and 4000 to 5. The meter multiplier is the primary rating divided by 5, so a 200:5 CT gives a multiplier of 40.
Oversizing hurts accuracy. If normal load falls below roughly 10% of the CT primary rating, the secondary current is so small that metering error grows, so step down one size when the load is light. Undersizing is worse: saturation during overload makes the meter read low. Confirm the accuracy class, burden rating, and any utility tariff requirements with your serving utility before ordering, and have a licensed engineer review the final metering design.
Frequently Asked Questions
Meters and protective relays in North America are standardized around a 5 A input, which keeps equipment interchangeable. A 1 A secondary is sometimes used on long secondary runs to cut burden and voltage drop.
About 125% of calculated load current suits most revenue metering. Use 150% to 200% only where expansion is already planned, since a larger ratio pushes normal operation into the low-accuracy end of the CT range.