In electrical supply practice, it may become necessary to obtain sum of currents in a number of feeders. To achieve this, Summation C.Ts. are used. Summation C.Ts. are used with feeder C.ts. which may or may not have the same reaos. Each feeder is  provided with its own C.T. and the secondary windings of these are connected to the appropriate primary windings of the summation C.T. the summation C.T. has a single secondary winding which is connected to the burden.  It is essential that summation C.Ts. are used on currents of same frequency and phase.

A typical two circuit summation C.T. is illustrated in figure 10. If individual feeder C.T. ratios are the same, number of turns of each primary windings shall be identical. If, however, the feeder C.T. ratios are different, it shall be necessary for the manufacturer to alter the number of turns of each primary with an ultimate objective of obtaining current on the secondary side which is proportional to the summated load current. In such cases the individual feeder C.T. ratios must be specified. In the absence of this specification, the manufacturer shall assume same ratio of each feeder C.T.