OpenEnergyMonitor Community

emonTX - 3Phase Firmware - do I actually need CT4?

I just bought an emonTX with the 3Phase Firmware and am about to calibrate it. Currently i am only having 3 clamps attached but in the sketch there is the mention that you should use the 4th clamp on load/infeed connected line-neutral.

Does this mean I have CT1-3 on L1-3 and CT4 on N? How does this impact calibration?


You do NOT normally have a c.t. on a neutral. All it measures on a 3-phase circuit is the vector sum of the current, which in the ideal case, is zero. If the loads on the three phases are not in perfect balance (which is very likely), then you will measure the imbalance current. This might be interesting, but it is of no practical use.

Thanks for the answer, so I simply don’t connect CT4 and disable it in the sketch right?

Or can I even use it to measure a specific circuit with it independent of the 3 phase setup?

I used it on my EmonTx to track my controlled load, totally independently of the 3 phases that were metered separately to the controlled load, so yes you can :slight_smile:

Hi Matthias,
I am using CT4 for boiler, so I can see, when it is running and how much my hot water cost.


Both these are correct. If you wish to use it to measure a single phase load, up to approximately 18.3 A (4 kW @ 220 V, 4.4 kW @ 240 V) then you may include it, make sure you set it to the correct phase in the software.
If you do not use it, then no harm will come if you leave it enabled in the software, but it is better to remove it.

All the details are in the documentation that came in the Zip file where you downloaded the sketch.