60A 240v not showing voltage? - resolved

Started the process of hooking my IoTaWatt tonight. Got my 4 200A CTs tracking the mains, added a few other CTs added and tracking a few circuits, but when I went to add 100A CT to a 240v (double pole, 60A) circuit that I thought was the primary circuit for the geothermal furnace (pump for closed loop horizontal piping) but I get readings of 0 watts. I moved the plug from port 14 to 12, configured the same way, same thing. Moved the CT to track two 15A 120V circuits, and I at least got a reading of wattage when a compressor turned on.

There are two other circuits (each 60A, 240V) for the furnace, which I thought were just for the backup heat strips - it’s been so relatively temperate I doubt they’re used. I moved the 100A CT to one of them and waited for the furnace blower to kick on (blower tends to run more due to the nature of geo) and still saw no measurements.

So I doubt my IoTaWatt has faulty ports, and I doubt the CT doesn’t work, I triple checked that it was closed - this feels like I’m missing something. Any ideas?

The bright red and matching black leads are the circuit in question. Copper conductor, may be oversized for the load, but wasn’t particularly hard to fit within the CT.

There may be some parallax at play, but it looks like you have both the red and black wires going through the CT in the same direction. That would read zero. You might read up on monitoring 240V splt-phase in the docs. If indeed both conductors are passing through the CT in the same direction, you should put the CT on only one of them and check “double” in the setup.

Aha! That’s precisely it then, interesting. I’ll try and do some reading on 240v. Thanks!

It’s not quite as simple when there is a neutral in addition to L1 and L2.

For the hell of it I grabbed a second 100A CT and placed it on the second leg, and it provides a different wattage while everything runs, but in the same ballpark. Though there’s a neutral wire, I’m not certain it’s in use (tried looking at the wiring diagram but couldn’t quite make it out). If both CTs report similar wattages, could it still be a pure 240 that I could double?

I don’t seem to have much slack in my legs within the panel, so I suppose I’m in a bit of a predicament now. Are there any better clues to suss out if it’s pure or not?

It feels odd to use 1CT and then double it for a “rough” estimate, but do some folks do that?

It’s not rough if there is no neutral. Put the second CT on the neutral wire. If it reads zero all the time, it’s window dressing and doubling is fine. If not, then you need to use one of the three-wire strategies.

Maybe something like this?

because of the gauge of the wires and lack of much slack, I opted to watch the neutral and see. I saw a few blips of a couple watts here or there (2W for a second, then back to 0, and maybe a few minutes later as something was spinning down, 7W, then back to 0). For now, I have two 100A CTs monitoring each leg. Every time I’ve checked the data though, they seem to be very close to one another, so as I move forward I may consider repurposing one of those CTs for something more interesting, and just doubling the remaining one.

I did use that sideways trick for my detached office subpanel (30A double pole breaker), it was towards the top of my list in terms of “interesting things to monitor”.

Again, thanks so much for all of the help, I like many others I think needed to update my mental model of how split phase electricity works - I was mostly working off of 120V and 240V operates a bit differently. I think I’ve got a much better understanding now - thank you!

I’m planning on doing a “share what you did” post here soon, but for now I have plenty of documentation on how to setup feeds in emoncms lol

OK, so the neutral isn’t being used for anything remotely significant. I’m curious just how different the two legs are. Rather than look at Watts in the status display, you might plot them over a day and look at the kWh in the statistics tab. If they are the same, then monitoring one and double is just as accurate as using two.

It doesn’t take much slack. If you need to do it, you could just move that breaker up to the two empty slots above to gain plenty of slack.