(Another) New user requesting CT assistance

Hi to all! Pulled the trigger a few weeks ago and ordered up an emonTx+WiFi for delivery to South Africa to commence my power monitoring journey. With delivery inching closer I’ve started diving into things a bit deeper, and find my mind swirling a bit with the information available… Eventual aim is to determine general power use patterns, and to aid in deciding on a suitable battery backup system for our recurring cyclical power cuts (hopefully eventually leading to a PV or wind charging system). (We do not (yet) locally have domestic power being fed into the grid, although the climate around this is changing.)

Domestic power supply in S. Afr. is ~230 Vac @ 50 Hz.
Main incoming circuit breaker (CB) to my house is rated 63 A. I also have:

  • 32 A CB - stove
  • 20 A CB - electric geyser
  • 20 A CB for each of three plug circuits
  • 10 A CB for each of two light circuits

To save on shipping charges I’m sourcing current transformers locally. I believe I have a similar/identical YHDC SCT-013 (100 A : 50 mA). This will be fitted to the main CB for total power measurement.

Although my thinking was to get a SCT013-050 ( 50 A : 1 V) for the stove circuit and and further two SCT013-020( 20 A : 1 V), one for the geyser and last to find my most power-hungry plug circuit, further reading on the forum has me confused.

I am new to working with Raspberry Pi based systems (this project is part of my learning).
I have basic soldering skills, and basic understanding of electrical power theory.
I do not know Arduino, nor am I (yet!) familiar with modifying and updating sketches (e.g. modifying calibration values).

Question 1:
I believe the YHDC SCT-013 I can source locally is identical to the one offered in the OEM shop. With a 50 mA output, is there a preferred port/input to plug this into (#1 I’d assume?)

Question 2:
For my described use case, would I be just as well getting three SCT-013 for mains, stove and geyser, and only one smaller CT for the higher-precision input 4?

Question 3:
With my remaining CTs being 1V output (compared to the 50 mA of the “main” CT) - do any modifications need to be done to plug these into the remaining input ports? I am aware of the power limitation (19A / 4.6KW (@240V)) for port 4 - to my mind this means I should use a lower rated CT for this input?

I can provide URL to the supplier data sheets for the CT I’m looking to purchase if these can be of use.

Edit 1: Formatting and numbering correction

Welcome, Jason, to the OEM forum. And congratulations on taking the plunge. I think you’ll find all the help you need here, one way or another.

No - the first 3 inputs (CT1-CT3) are identical, CT4 is “high sensitivity” for 16 A maximum, though that can be changed.

Probably - but maybe not even bother with the smaller c.t.

Indeed they do. The SCT-013-000 is a “proper” current transformer and its output is a current, so inside the emonTx is a “burden” resistor to convert the current into a voltage. The “1 V output” c.t’s aren’t really current transformers because they already have the burden inside the case, so you need to make two modifications - remove the burden resistor inside the emonTx for those c.t’s, then alter the calibration accordingly.

You can use any c.t. with the emonTx, but it needs to be capable of delivering enough power (its VA rating) to generate about 1.1 V across whatever burden resistor you need, given its secondary current. So be prepared to make modifications of one kind or another. And don’t look for a VA rating on the data sheets - very few manufacturers quote it directly, you need to infer it from the other data (often the burden or load resistance). So if you can’t figure it out, post the links and I’ll take a look.

Thanks for informative response Robert, looking forward to engaging the old squishy grey blob of a brain and learn some new stuff!

That’s what I thought I understood from some of your responses to other (new) users, and from the/your technical report on the SCT-013 (that the low-current accuracy, combined with the ADC bit-rate, results in similar enough error rate). I’ll revisit down the line once I’m up and running and getting more info.

Larger rated CT means I can also “bundle” some of the plug or light circuits (if enough free wire in the DB) for combined power readings and to aid in resolving power-hungry areas.

I think I’m best off keeping this simple for now, and avoiding unnecessary modifications. :grimacing: (With some pictures from other threads I’m confident enough doing the soldering, but not yet with playing around with sketches.)

I’ll have a go at seeing what I can figure out re power ratings. In case there are other South Africans investigating, here’s the URL/link to a locally available SCT013-000.

Same supplier also has the SCT013-050 and SCT013-020 I mentioned initially.

Those c.t’s should be OK, with the burden removed of the two smaller ones. The 100 A one looks identical to the “Shop” one.

The modern sketches allow you to adjust the calibration “on-line” - but you still need a programmer to connect to the FTDI (programming) port, and some way of seeing and sending the serial data, so it’s usually easier in the long run to install the Arduino IDE and libraries, then you can rewrite or change and reload the sketch, if you need or feel inclined to try!