Tips for a more efficient use of CT channels?

Just setup my IotaWatt yesterday, and I love it already. It was a fair bit of work to put in an orbit box and a new outlet as my panel is outdoors, so I’ve initially just setup my mains, solar breaker, and monitoring for my two AC systems and my car charger.

Read through CT basics, and erred on the side of caution, so I used two CTs for each of those 240v circuits. Problem is, I’ve now taken up 10 channels, and I’ve got two sub panels and a bunch of minor circuits I’d like to monitor.

I know ideally I’d use the loop trick to run both wires in a 240 circuit through a single CT, but it’s just not possible with the slack in my panel. There seem to be two likely solutions:

A: Just monitor a single wire on each 240v breaker and double it. Even though these are all 120/240 circuits with neutral wires, the watts readings I’m getting on them are very close but not identical. Since I have watt accurate data from the mains consumption reading, perhaps this is good enough?

B. Use splitters to merge the dual CTs into single channels. Confused about the safety issues this poses. Can I safely put 3.5mm splitters inside my panel to save conduit space?

Also confused about CT sizing if I use splitters. Say I have a 60 amp EV charger. Right now I have two 50amp rated CTs, one on each wire. Can I just attach them to a splitter, or do I need to use two 100amp CTs instead?

On sizing, two 50A CTs in parallel will get you the equivalent of100A going into the Iotawatt, which is too much unless they are on a 20A circuit. So, for a car charger I would just use one CT on the line that is using power when the car is not charging. If both read close to zero when NOT charging, just use one and double it.

Depending on what you are trying to learn, doubling the single line that feeds the motor of a dryer might be good enough. It will show you double power when on tumble air only, but that is likely a small percentage of the total power.

What is your goal?
Why are you measuring power/energy use?
What do you hope to learn?
What decisions do you expect to make?

Splitters inside the panel is likely a violation of “code”. The splitters are not UL listed and not rated for high voltage. Having said that lots of people do it.

My goals are somewhat open ended. We just expanded our solar system, so we’re trying not to pay a power bill anymore and want to keep an eye on our grid balance. Also want to explore the usage from our larger appliances as a baseline for future efficiency upgrades. Plus I’m just curious and having fun.

Let me see if I have this right. The reason you monitor both legs for 120/240 loads is because part of the device in question only pulls 120v (ie the leds and computer on our EV charger is probably just taking 4w from a single leg). So if I only care about the actual power used charging the car, I can just double one leg. Similarly, if I’ve got a condenser for HVAC on its own circuit, I can probably just double that as well? In other words, if I want a reasonably accurate setup, I can try monitoring both legs of a device, and see if they’re within a few watts when idle?


Even easier is look at the device and see if it actually has a neutral connection. Most of my 240V loads don’t have a neutral, so measuring both legs is mostly a waste. Since you only have one Iotawatt, measuring both legs is most likely a waste. Most AC compressors only use 240V, no neutral. If you have a sub-panel and it has 120V loads on it, measuring both legs is a good idea. That is the case for my barnyard. The 120V loads are rarely balanced but the 240V component is about 80% of the total energy use. So, how accurate a picture do you need/want?

Yeah, it’s frustrating because I’m pretty sure most of mine do have a neutral, but my panels got a ton of wiring coming in so it’s not easy to trace all the neutrals back for every 240v circuit. I’ll definitely use two CT’s for the two sub panels, probably fed into splitters.

Out of curiosity, I graphed an output of the difference between the legs on the EV charger, and it came out to a single wh for the day, so that seems pretty insignificant!

Thanks for your help.

I would say that is evidence that the neutral is not in play and you can use one CT with double checked. You can make the same observations with any other 240 circuit where you have two CTs. I would not bother with an output, just plot the two over a period of a day or more and observe if the two plots lie exactly over one another and see if the Wh integration shown in the statistics tab are the same.

You can see some other variations on using splitters in this post:

Awesome. Thanks!

I had the same issue - was new to CTs a few years ago and so here are a couple of things I noticed:

  1. the resistance heater for supplementary baseboard heat has a 3-conductor cable but only red and black are used in the device, white is not connected. Easy - one current path, one CT.
  2. the kitchen stove (about 15kW max) sometimes shows a mismatch between red and black so I do keep monitoring that one on both sides.

As I’m reading the comments it occurs to me you could measure the neutral on these 240V loads for a while, and let IotaWatt do some of this work, without us doing any math! If you see near zero on the neutral in “all” operation modes, you know that one CT will be enough. Iotawatt even lets you configure CT inputs as “double” for these situations. [edit: ‘double’ for when one of the two 240V hots is monitored]