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Another newbie question of scalability and CT sizes

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I’m trying to find a some neat solution to monitor multiple phases at home. So far I have made several single phase fully DIY wattage monitors based on ACS712 and arduino/ESP8266. The grid here is quite stable so after calibration I usually manage to get values in 10-20W error margin without extra voltage calibrations or similar. Also I use good quality 230V->5V transformers in my project so they tend to have stable outputs. So now I have soon all the important single phase household machines monitored (washing machine, dishwasher, fridge/freezer, always on computers, air heatpump etc). The whole home power usage is monitored by blink/pulse monitor. Now however, I would like monitor the three phase bigger appliances with some solution which wouldn’t require that much of DIY (yes, I’m getting old). These appliances are:

  • Main heat pump (3x16A)
  • Sauna stove (3x10A)
  • Stove/oven (3x16A)

Main fuses are 3x25A. So what I’m trying to find is:

  • Solution which would be on hardware level more like a plug and play. I want to avoid calculating resistors/components, calibrating every sensor separately, soldering tons of cables/connectors and designing & 3D printing cases and stuff. Shortly in this project I’m more interested of the information than building it from scratch.
  • Solution which is somewhat open source that I can connect it to my Domoticz which is currently acting as hub/collector for my home automation. Some nodes connect directly to Domoticz, some via MQTT or MySensors.

So what would be your pro suggestions? I have used emon sketches and algorithms as a base in some of the above projects so I’m somewhat familiar with the concept. However, the biggest drawback with currently purchasable emon lineup is that they use 100A CTs. As I noted above my biggest single phase amperage is 16A so 20-30A would be a better choice or even 50A. Moreover when Arduino has 10bit analog to digital converts (and if I’m understood right only 0-1V is acceptable) it sounds to me that the precision wouldn’t be very good.

Have you considered using a Shelly PM or a Shelly EM +50amp clamp? They should be small enough to fit inside the switch box (or in the wall behind).

I know nothing about Domoticz, MQTT and MySensors, so I will only write about c.t’s and the Arduino.

It’s true that everything here mentions mainly the 100 A SCT-013-000 current transformer, but you do not have to use that. There is a fairly serious limitation if you use the 5 V Arduino - you need to have a current transformer that has enough power to generate a voltage that approaches 5 V peak-to-peak (that’s about 1.6 V rms). So I’m afraid your understanding of the Arduino (Uno in particular) is wrong there. The SCT-013 series does include variants with an internal burden resistor that is set to give a 1 V rms output at the rated current, but it’s possible to use the ‘native’ -000 version with a burden chosen to give 1.6 V rms at the maximum current you require. So a 100 Ω burden would give you a 30 A “rating” using a 5 V Arduino.

If you want something that is very close to “plug & play” - but it would still involve a small hardware modification, you could use the emonTx. It has 4 channels, comes in a nice aluminium case and is fully assembled. That should give slightly better performance than an Arduino because it runs off 3.3 V (and c.t’s are happier when they don’t have to generate a big voltage). The modification would involve removing the surface-mount burden resistor from the circuit board and soldering a wire-ended one in its place, a 75 Ω would give you 29.33 A maximum on each channel, of which there are 4. There is only one voltage input, from a “wall wart” style transformer, but given that your supply is stable and using the 3-phase sketch, you can use the emonTx as a 3-phase monitor and still have one channel spare.

A question for the record: is your supply 3-wire or 4-wire, i.e. is it 230 V line-line or 230 V line-neutral?

At the moment, I’m monitoring my house current with a standard emonTx and 100 A c.t. It’s not properly calibrated (I’m actually testing for a different problem) but it reads 229 W now and when I turn my HiFi amplifier on (no sound!) it reads 235 W. I don’t claim those numbers are totally accurate, but it can see the difference (and that difference is not very far wrong) bearing in mind the maximum is 24 kW.

One point: is your heat pump a balanced 3-phase load? If it is, then you only need to monitor the current of one phase, the other two should be the same, so need not be measured. The same might be true of the sauna stove. It won’t be true of an oven with hot plates.

Thanks for the quick answers! @Slartibartfast thanks for the tip, I need to check out the Shelly product lineup if there would be some neat solution. :slight_smile:

@Robert.Wall. Domoticz is basically an home automation controller kind of an umbrella like HomeAssistan and OpenHAB. I tested all these but ended with Domo as it tends to need less maintenance and setup work than the others. MQTT is a light-weight message queuing protocol suitable for small IoT devices to send data in reliable manner. MySensors is more like a framework for DIY guys to build all kind of microcontroller/sensor/actuator/etc stuff and wrap the information and communication under one format. One could build also the transmission protocols, error checking, message formats etc. by itself, but MySensors makes things much simpler. Currently MySensors support NRF24, RFM69 and ESP8266 WiFi transmission layers. If using Wifi you can also wrap the MySensors data in MQTT messages. MySensors needs always some controller where to send the data like OpenHAB or Domoticz.

So that’s the short tutorial of those. Point of all this is that if there is some point where you can access the output of the sensor data in readable format like kwh, W, pulse count or whatever, it can be wrapped in MySensors or some other format Domo/OpenHAB understands. That’s why the solution should be open source or at least have good APIs to access it…and if it has a good API the chances are that Domoticz/HA/OpenHAB already have a direct support for it. There are several dozens of supported interface in Domo like Philips Hue, Netatmo and Evohome.

Thanks for the correction of my Arduino 5V statement. In some project I remember when I switched from Arduino to ESP8266 the analog input tolerance dropped to 1V but luckily the precision stayed the same but just in this smaller voltage window. And yes, I just thought this from code point of view. But anyway it’s good to know that the 000-versio can be “modded” to act like a smaller one. I have looked closely the emonTx and before committing to buy those, I decided to check here if it’s suitable for my requirements. It seems that at least you have been quite happy with it. Moreover if the precision is really couple of watts, then the modding shouldn’t be necessary.

Here in Finland we have 230V between phase and neutral and 400V between phases. Normal 230V appliance gets neutral-ground-phase and three-phase appliances gets neutral-ground-three phases. The main heat pump (geothermal) gets the five poles mentioned above, but to my understanding it will alternate the load between the phases. It has a it’s own clamps connected to the three mains of the house so that it can direct the load to less loaded phases. The sauna stove however most probably could be monitored with single phase. I think the logic with it’s thermostat is just hard cutting the phases with relays based on temperature. However, I need to cross-check this with the new sauna stove when it comes. And yes, with oven and hot plates this is definitely not the case. I remember when the electrician just jump wired the oven to one the three phases of the stove. :slight_smile: