OpenEnergyMonitor Community

CT sensor for USA mains

Tags: #<Tag:0x00007fc0ac9dfcc8> #<Tag:0x00007fc0ac9dfbd8> #<Tag:0x00007fc0ac9dfa48>

Problem: I am planning to monitor whole house power consumption using the setup below. The CT sensors I have are too small for the incoming mains (200 amp USA service, 120-N-120).

Setup: emonTX3.4, RPI3 + emonBase, 9 volt AC-AC power supply. Latest firmware.

Question: Not being an electrical engineer, I need help in finding two CT split core sensors that will fit around my incoming service wires (4/0 ?). I would like some that would require minimal reconfiguration/calibration of the existing system (removing/replacing resistors, recompiling software, etc.). Can anyone suggest a close replacement for the SCT-013-000 but with a larger diameter opening?
I am less interested in cost than ease of installation. I have seen the SCT023R mentioned but don’t know what it may involve to set it up.

As no good deed goes unpunished, I would also ask for help with how to calibrate and install the sensors.

Many thanks in advance for any help from the group.

Bob Grattan
Ashland, Virginia USA

Look at the page “Use in North America” in the ‘Learn’ section. There are many listed there, complete with their calibration coefficients. The YHDC series doesn’t require hardware changes.

Many thanks, Robert. I have ordered a couple of the SCT023R CTs, coming early next month.
When these are connected to inputs 1 and 2 on the emonTX3.4, will the results be added together automatically or do I need to change power1 and power2 to power1+power2 in the config?
Your help is much appreciated as I am very new to this.

What else do you have in the way of emon hardware, i.e. are you using an emonPi or to view the data?

I have an RPI3 with emonBase installed. Latest firmware.

You can edit the sketch in your emonTx and add the two powers there, sending the combined value; or you can send the values separately and add them in the input processing of emonHub; or you can add the currents in the wiring before it gets digitised in the emonTx.

If you’re concerned about the balance between the two legs of your supply, it’s probably best to send the two powers separately. If you need the other 3 inputs of your emonTx, then you need to wire the two c.t’s together to use just one input.

It sounds like the easiest way for me to do this is to send the two current readings from the emonTX and add them on the emonhub. I’m not certain how to do this. I’m not planning to use the other emonTX inputs ( 3 and 4 ) at this time. Sorry to ask so many questions here…

Sorry, I meant emonCMS, not emonHub. How to set up an input and connect it to a feed (storage) and which database to use for storage is described in the guide. If you follow that for each input, you’ll have two separate records, one for each input.

You can do that and add another feed with the sum, or you can do that - modified - to just one input if you want only the sum.

The processes you want will be ‘Reset to ZERO’ and ‘+ input’. If you want just the sum (the easiest), then the first process will be ‘+ input’. Do this on one input, selecting and adding the second, then do ‘Log to feed’ and that will store the sum of the two.
After you select the process, you’ll see what it does written below. “Passed back” actually means ‘passed onwards to the next process’. Not every process passes onwards a changed value, e.g. ‘Log to feed’ passes on the value unchanged. You can have more than one ‘Log to feed’.

If you want the sum as well as the individual data, then do as the guide says to each input separately, then to one of the inputs, at the end, add the process ‘Reset to ZERO’. This does what it says. You now have nothing in the data flow, so do ‘+ input’ twice nominating each input in turn (to give you 0 + input 1 + input 2 = the sum), then carry on as per the guide, adding a third feed.

Not trying to toss a spanner in the works, but from what I’ve read, using a feed vice an input to sum two values is more robust according to PB.

Ref: (2nd post in the thread)

Yes, I’d forgotten that. I knew there was a small complication there, but couldn’t remember what it was.

Just to clarify then, you use ‘+ feed’ instead of ‘+ input’. It does of course mean that you need to create at least two feeds - for one input and the sum, or maybe three feeds: for each input and the sum.

Thanks Robert and Bill. I’ll try this with a couple of inputs on 1 and 2 while waiting for the larger CTs. Since I’m not real sure of myself, I’ll practice before putting my hands in the breaker panel to connect to the mains.
Of course, I may have more question before all is finished.
Much obliged for the help.


1 Like

Hi Robert,
I have my larger CTs installed and they seem to be working and producing data correctly. The mains (120-0-120) are connected to power1 and power2.
I need to produce a feed which sums power1 and power2 to give me a whole house reading on a single feed.
Earlier, you kindly sent me instructions on how to set this up, however, I can’t seem to get it working. I’ve tried various configurations but never get power1 + power2. Maybe I’m getting things in the wrong order. Sorry to be so dense here but I’m not real sure what I’m doing with the feeds and inputs.
Thanks in advance for a few more details on the process.

Presumably you have two inputs, that are set up the same, logging to two feeds, power1 and power2.

In the power2 input page, after your ‘log to feed’, add “+ feed” and use the drop-down to select power1 feed. Now do another ‘log to feed’ to a third feed called ‘totalPower’ [or whatever].

Thanks again, Robert, that’s exactly what I needed to know. It works fine now. I was confused by what you meant in using “Reset to ZERO” in your earlier answer but I don’t guess I need it here, at least to get me started. Sorry for the simplistic questions.

“Reset to ZERO” wipes the slate clean. You didn’t need it here, as you are using the value carried forward in the input processing and adding the second value (actually the one called power1) to it.

Why be sorry? We all had to start somewhere, it was all a bit overwhelming for me to begin with.

carried forward

That ought to be the term used vice “passed back.”

I can see - from a programmer’s POV (although I’m not a programmer) - why they used "passed back."
Similar to the way a function passes a value back to the routine that called it.
carried forward is less confusing, especially for non-coders.

I don’t buy that as a reason. The inputs page, where it appears is USER FACING. What are the words used on multi-page bank statements etc? “Carried forward” and “Brought forward”.
“Passed back” to me is something discarded (and that fits with the function return value concept when you think of it - the function has finished with processing and it’s about to die), “Passed forward” implies it’s going to be used ahead of where you are now.

Here’s an example that uses the idea of control vice an actual value:

When a lower-level program, which has been accessed by a LINK command, issues a RETURN command, control passes back one logical level higher than the program returning control.

and one that references a function returning a value:
By returns a value, we mean that the function creates some sort of results, which is passed back to the calling function.

It’s programmer-speak to be sure, which is why I like “carried forward” much better

Just saying that I can see where they might have gotten it from, since they are both coders.