Overview

For version 16 and later of the WattNode® Modbus firmware, we updated the functionality of the registers CtAmps, CtAmpsA, CtAmpsB, and CtAmpsC to reduce confusion.

The CtAmps, CtAmpsA, CtAmpsB, and CtAmpsC registers are used to configure the rated CT amps of the attached current transformers.

Users who are accustomed to using only the three phase registers (CtAmpsA, CtAmpsB and CtAmpsC) or only the CtAmps register to assign the ratings are unaffected by this change, but if you use a combination of all four registers to assign them, please read further to see how this change may affect how you do this.

If you have one model of CT connected to all active phases on the WattNode, then you would generally assign the rated CT amps with CtAmps, although you could also write the common value to CtAmpsA, CtAmpsB, and CtAmpsC in place of or in addition to writing CtAmps. Note: this is true for all WattNode Modbus firmware versions.

If you have CTs with different rated amps on different input phases of the WattNode (ØA, ØB, or ØC), then you would write the rated amps to CtAmpsA, CtAmpsB, and CtAmpsC. Note: this is true for all WattNode Modbus firmware versions.

New Behavior (V16+)

  • The global CtAmps register acts as a command register that assigns the specified value (amps) to all three phase registers (CtAmpsA, CtAmpsB, or CtAmpsC) whenever it is written. So if you write 100 to CtAmps, you will end up with:
    • CtAmps = 100
    • CtAmpsA = 100
    • CtAmpsB = 100
    • CtAmpsC = 100
  • The global CtAmps register reads back zero unless all three per-phase CtAmps registers are the same, in which case it read backs the common value. The zero value alerts the user to the fact that the per-phase registers must be read to determine the CT rating for each phase.
  • Internally, the WattNode Modbus meter just uses the three phase registers CtAmpsA, CtAmpsB, or CtAmpsC when computing power, energy, current, etc. CtAmps is not used except as a shortcut for setting the other three registers to the same value.

Old Behavior (V15 and older)

  • Writing to the global CtAmps register did not affect the three phase registers (CtAmpsA, CtAmpsB, or CtAmpsC), so it was possible for a confusing situation like the following:
    • CtAmps = 50 (overridden by the three phase registers)
    • CtAmpsA = 100
    • CtAmpsB = 100
    • CtAmpsC = 100
  • The global CtAmps register was independent of the three phase registers (CtAmpsA, CtAmpsB, or CtAmpsC), so reading CtAmps would provide no indication that the actual CT rated amps values might have been overridden by non-zero values written to CtAmpsA, CtAmpsB, or CtAmpsC.
  • Non-zero values in any of CtAmpsA, CtAmpsB, or CtAmpsC would override the global CtAmps register.

See Also