A current transformer (CT) is used to reduce the current or voltage of an electric circuit so that the electric flow can be cost-effectively measured. CTs are commonly used on customer services requiring more current or amps than the class rating of standard meters. Many customers, especially medium and large commercial and industrial customers, require significantly more current to run their equipment than self-contained meters can safely handle. One solution to this problem would be to design and use a larger meter capable of handling the increased current level. It is more cost-effective, however, to use instrument transformers to meter this type of installation.
Here’s how the CT technology works. CTs are made in varying ratios that indicate how much reduction in current they can accomplish. For example, if a transformer rated installation uses CTs with a 200 to 5 amp ratio, for every 200 amps flowing through the service to the customer, 5 amps would be sent by the CT to the metering equipment. In this manner, the flow of current going to the metering equipment is reduced by a fixed ratio and a standard meter can be used.
Because the installation is measuring only a portion of the kWh actually used by the customer, a multiplier must be applied to the metered amount to determine actual usage. Using the 200 to 5 ratio as an example, the account multiplier would be 40 because the meter is now measuring 1/40 of the customer’s actual usage. Thus, at the end of the billing period, the customer’s metered consumption must be multiplied by a factor of 40 to determine its actual usage.
(photo credit: https://commons.wikimedia.org/wiki/File:CurrentTransformers.jpg)