Use in North America

The emonPi / emonTx hardware units are designed for use on single-phase systems. However, the they can be used on a North American Split-Phase system with the following considerations:

1. Ensure clip-on CT sensor will fit your conductors

Typical residential Service Entrance Wires are AWG 2/0 Copper or AWG 4/0 Aluminium. The opening in the standard clip-on CT sensor sold in the OpenEnergyMonitor shop is 13mm, and hence can accommodate a maximum wire size of AWG 1/0. Therefore, we recommend you measure the diameter of your feeders before purchasing a CT sensor.

If the standard CT sensor does not fit your conductors there are two options:

  1. Use an Optical Pulse Sensor instead of CT sensor to interface directly with a utility meter. This will give an accurate reading of energy consumed (kWh) but will not provide a real-time power reading (W).

  2. Use an alternative larger CT sensor: It’s possible to obtain larger CT sensors. However, using an alternative CT will usually require re-calibration and possibly, hardware modification (burden resistor may need replacement). See Learn > emonTx in North America for more info. Please search / post on the community forums for further assistance.


Installing clip-on CT sensors on USA electrical systems should only by undertaken by a professional electrician. In a typical residential installation, even if the main breaker is switched off, the load center Service Entrance Wires and bus bars are always live.

2. Use USA AC-AC Adapter

Use the USA AC-AC voltage sensor adapter sold via the OpenEnergyMonitor Shop. The system has been calibrated to work with this adapter. Using any other adapter will usually require re-calibration.

3. Software Calibration

a) emonPi: Set EmonHub USA Calibration in the [[RFM2Pi]] interfacer section of emonhub.conf set: calibration=110v.

emonhub.conf can be edited directly in local Emoncms as described in EmonHub > Overview. See [[RFM2Pi]] section of emonHub configuration guide for advanced emonhub config info.

b) emonTx: Enable USA calibration using on-board DIP switch

4. Sum power values

If monitoring a split-phase system the power values from each leg can be summed in Emoncms to calculate the total power.

The emonPi by default reports power1_plus_power2, this input will need to be logged to a feed. See Setup > Logging Locally.

If using emonTx then the power values from multiple CT inputs can be summed in Emoncms using + feed Input Processor.

See Learn > emonTx in North America for further technical info about the USA system.