emonTx 3 phase setup (Apparent power)

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I hope somebody can help me, I am a bit lost.
I want to measure apparent power at home. Most of my loads are resistive, and I can live with a 10% error on the measurement.
I have a 220V phase to phase voltage, and a 3-phase input without neutral.

My setup that is currently installed is:

  • 1 Raspberry Pi, with a RFM69Pi, and the last emonSD downloaded from the site.
  • 1 emonTx v3, with 3 CT sensors connected to the three phases, and 1 AC-AC connected to 2 phases for voltage input.

All is configured as stock, the emonCMS is receiving data from the emonTx, but of course the data I am receiving has nothing to do with the real power. Even the Vrms data is around 110v, not 220v.

So, what should be my next steps? How can I calibrate and measure 3-phase apparent power?
I would love to avoid changing emonTx firmware, as I don’t have the USB cable for that, and I would have to wait for it.

Can I do the maths on emonCMS? If I can, where and how should I do that?

I really tried looking on the site and the forum, but I can´t find anything that helps me with this task.

Consider that I am really new on this.
Thanks a lot for your help!



Ok I´ve been looking at emoncms, and thinking about the problem.

I think this can be solved by doing the math on a virtual feed that equals to the apparent power.

Anyway, I would like to confirm if I am right, and that all my assumptions are correct.

I have 4 usable inputs; power1, power2, power3, and vrms

power 1, 2 and 3 are calculated by emonTx as “vrms” * “current”

So, I can get the current on each phase: I1 = power1/vrms

Then I can calculate the apparent power by doing: kVA = (I1+I2+I3)*vrms/squareroot(3)

Does this make sense?


Unfortunately, that won’t work. The reason is that [quote=“Juliofitipaldo, post:2, topic:4413”]
power 1, 2 and 3 are calculated by emonTx as “vrms” * “current”
is not correct. Each sample pair of voltage and current are multiplied and then averaged to give you the true real power - and for a 3-wire system, dividing by the rms voltage won’t give you the line current on any line.

The rms current is available in the sketch, but it’s not transmitted by default. I’m afraid the only way is to get a programmer and edit the sketch - and you might as well send apparent power added together for all 3 phases, unless you particularly want to know the line currents.

Ok, I understand. Thanks for your fast reply!
What do I need to program on emonTx, and where should I program that? Can
you give me an idea?


Hardware: You need a programmer from the shop. And a USB cable if you do not have one.

The official OEM software is Platformio, however I have tried to use it and I have to warn you that I found it unusable. The Arduino IDE is widely supported (except here) and works fine. If you prefer to use the Arduino IDE, I have instructions for installing it and the necessary libraries. You’ll find the source code for your sketch in the OEM section of GitHub.

I have a 3-phase, 3-wire sketch that will give you real power measurements - subject to the same limitations as the 3-phase, 4-wire sketch. (it’s actually a 2-phase sketch because it uses one phase as the reference (the same as the neutral in a 4-wire system). This sketch is not on GitHub.