Hello,
I liked my first IotaWatt so much it’s time for another!
My question is around the AC reference transformer.
Would it be better (more accurate?) for me to use a single reference transformer and split the output using CCTV barrel splitters?
Or would it not matter to have two separate AC reference transformers?
Regardless of one or two, voltage accuracy depends on the calibration. Splitting has no measurable impact on accuracy, even splitting 4 or 5 times. With a splitter, you should expect identical voltage readings in the two units so long as you use the same calibration value.
Practical considerations should govern here. If the two units will be colocated, the splitter makes a lot of sense. If they are far apart, it may be more convenient to use a separate VT.
Not to hijack the thread but I have almost the same question. Note I have read your document here on impact of measuring one leg of split phase.
I think I’m going to mount the two units in different boxes, and each with their own power outlet. I also noted in another thread here the statement to ensure if you split a reference source you maintain polarity. So I think I already know the answer but…
Is there any advantage to (and technique to allow) one to use the two reference sources to be more accurate by measuring each leg independently?
Or just stick with “use the same leg for each reference source” in which case I’ll run one branch circuit to both boxes.
In my case I’m going to have to references anyway (not split), just trying to see if I can glean any additional info from doing so.
Short answer to “is there any advantage to” is depends. Short answer is “and technique to allow” is yes.
This is a never ending question with no definitive resolution. I believe it all started on another site where there are academics who beat it into the ground with theory, but introduce no practical evidence.
Here’s a post that explains my thinking on this topic: 120V/240V split phase voltage reference
Where you have two VTs in close proximity, if you wanted go to the trouble of wiring them to opposite legs and splitting them with CCTV type voltage splitters that observe polarity, and sacrifice a power input on each device to bring in the additional voltage reference, you could assign each 120V circuit to the corresponding voltage reference and be more exacting with those. But the bulk of the power in a US service is typically consumed by 240V appliances like water-heaters, AC, and clothes-dryers. This does nothing to address that. When the voltage of either leg varies from median, the other does the same in the other direction. So the doubled 120V reference for a 240V circuit is off by +/- twice the voltage difference. That’s why my final recommendation in the above post is to use a 240V VT and calibrate to the median. It’s simpler and more effective than using two 120V VTs.
All that said, I have a 240V VT yet I haven’t bothered to switch from the 120V VT which has been giving me very accurate results for years - within 1% of my revenue meter.
Thank you. So to work, each Iotawatt device needs both voltages? I can’t take one into each device and have the calculations done digitally?
I had read your linked posting (I mentioned it in mine) showing a practical indication that there is not a lot of point. Though one could argue all the monitoring I am doing falls loosely into the same category. 
Though something else occurred to me: NEC – I think the voltage reference transformer falls into the class 2 category, so running it across the panel inside, I think, is not allowed. Right? Any NEC experts here? And getting conduit around the panel would be ugly (meaning literally ugly not hard). So unless I’m wrong about the restriction, maybe I’ll pass.
My plan is one box on each side of the panel, power into each for their respective transformers, and 1" conduit to each box for all the CT cables and the power to the outlet in the boxes.
That sounds fine, except you might consider separate conduit for the 120V and CT leads, and you might find it easier to run all those CT leads through 1.25" conduit.
Wow, I thought I was overkill with 1", one posting I noticed with the orbit they used 3/4". I’ve just bought the 1" stuff (before reading this), and the knockout on each side where I wanted to come out on the panel is for 1", so I think I’m going to give it a try. Drilling out the panel is not high on my list of happy thoughts.
I’m going to make it a straight run, just long enough for two male connectors to join, which will make it easier. I guess I’ll find out over the weekend, USPS permitting.
Sorry to revive this old thread, but I’m designing my setup and was hoping for clarification on a few points that were discussed here…
(Emphasis added)
Is this part true even if one is using a separate CT for each leg (L1 & L2) of the 240VAC appliances? Most of my 240VAC loads are wired as MWBC appliance circuits (Black/Red/White) so accurate metering requires a CT on both legs anyway.
Stated another way, is the referencing of a CT input to a VT input done before the calculated load (CT + CT) is synthesized?
Alternatively, if one does the single 240VAC VT (although, I must admit, that collecting data on the relative voltages on each leg does sound interesting to me), which VT should be used? I see you sell a EU version of the AC transformer, but it appears to be rated to a max of 230VAC and only at 50 Hz. Is there an appropriate VT to use for ~240VAC 60Hz? If the EU one is the correct transformer, do you know when it would be back in stock?
If using a 240VAC VT, does that make the configuration of the IoTaWatt any more difficult?
Thank you again for offering your guidance and expertise on these topics!
No, that should be fine.
If you are not comfortable with that, you can get a 240V:12V AC transformer as long as it doesn’t exceed 15V RMS into 13Kohms. Even if it does, you can add a resistor such that the current does not exceed about 1mA and calibrate.
There is the technique of two wires through the CT in opposite directions, which favors the 240V CT.
There are a lot of metrics but the primary colors are Volts, Watts and Amps. Volts and Amps are the VT and CT respectively. Watts is an integration of the product of hundreds of Voltage and Current samples.
No.
The bottom line is that I only get these questions pre-order. Virtually all users get satisfactory results with one 120V reference on split-phase service. It’s also pretty easy to change anytime.