Setting a Different CT Ratio for Each Meter on the MCM



It is possible to set a separate current transformer (CT) ratio for each load on a PowerLogic Multi Circuit Meter (MCM). It is a common misconception that only one CT ratio can be set for all loads on the MCM. This is possibly because the Modbus register list for the MCM only shows one register location for the CT Scale. However, it is important to note that this register exists for all 8 meters on the MCM, each with an independent slave address.
The default CT Scale is 100:5 for each meter. This must be changed for any circuit using CTs other than 100:5 to get accurate readings from the MCM. The CT ratios can be configured using MNode or through Modbus register writes. This knowledge base entry addresses the latter.

To configure each CT ratio in SMS, the user should create a separate device for each metered load on the MCM. For example, if the base address of the MCM is set to 3, the user should add 8 different devices in SMS, each with a different Modbus address from 3 to 10. Then each device can be selected separately in Control > Diagnostics > Read/Write Registers and the appropriate CT primary value can be written directly to register 30 (with an assumed CT secondary of 5A).

Alternatively, another software package that is able to write to Modbus registers can be used to write the appropriate CT primary value to register 30 of each Modbus slave address.

Legacy KB System (APS) Data: RESL198175 V1.0, Originally authored by AnVa on 04/01/2009, Last Edited by AnVa on 04/01/2009
Related ranges: MCM (Multi-Circuit Meter)