Why do the WattNode integer energy registers sometimes read negative values?

If the WattNode phase LEDs never flash red when power is being consumed from the utility grid (red usually means a CT is connected backwards which results in negative power) then your Modbus Master is misinterpreting the energy registers which are 32-bit signed integers. If your Master reads them as 16-bit integers, you may see negative values come and go. You need to reconfigure your Master to interpret both 16-bit registers that comprise the 32-bit energy value as a single 32-bit integer. Also, be sure to configure your Modbus Master to interpret the first register as the least significant one and the second register of the pair as the most significant (see Large Jumps in Energy on the Modbus WattNode). Also see: 32-bit Energy Register Details for the Modbus WattNode.