When the current in a number of feeders need not be individually metered but summated to a single meter or instrument, a summation current transformer can be used. In consumer installation, where there are more than one feeder, it is more economical to use summation metering and for this purpose, summation Current Transformer (CT) is required.

The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding which feeds a current proportional to the summated primary current.

A typical ratio would be 5 + 5 + 5 / 5A, which means that three primary feeders of 5 Amps are to be summated to a single 5 Amps meter.