Current Transformers (CT's)
Current transformers as the name implies are used to transform high currents used by a large customer to lower levels that can safely be measured by a standard watt-hour meter.
Current transformers or CT's are designed so that the service conductors going to the customers service entrance run through the center of the CT. The secondary winding terminals have reduced current and are connected to standard metering equipment. In this manner, only a portion of the electricity used by the customer is actually metered.
CT's have their current reduction ratio given on their nameplate. A CT with a current reduction ratio of 400:5 would reduce the current by a ratio of 400 divided by 5 or 80 times. For every 400 amps going to the customer, 5 amps would go to the metering equipment. The meter now only measures 1/80th of the amount of electricity actually used by the customer. In order to bill the customer for their actual usage, the meter reading must be multiplied by the CT ratio of 80. This value is called the account or meter multiplier and will be found on all customers accounts with CT metering.