Confused about required CTs

Hello, I’m planning on purchasing an Iotawatt in the near future and I’m a little confused about what CTs are required. We have multiple double pole/240v breakers of varying amps 30/50/60. I see the recommended way of connecting 240v is a single CT with both conductors passing through in opposite directions which I understand is due to the opposite phases. My confusion is regarding the sizing of the CTs. If we have a 240v double breaker (not sure if that’s the correct term, it takes up two slots and lever to throw it on/off is connected) that is 60A would a 100A CT be sufficient for passing both wires through in opposite directions or would I be required to get a 200A CT?

What about combining CTs with a 3.5 splitter. I understand they have to be the same model CT and if they are on opposite phases one needs to be turned the opposite direction but again am confused about the total amperage. If I have three 120v 20a breakers, can I use a 50a CT on each one and combine them with spitters into a single input or no because all three at full draw would be 60a?

I suppose the same question then for a 240v 60a, if I use two 100a CTs (one on each leg) and combine them with a splitter would that be ok or no because the total load could actually be 120a?

Welcome. It appears you have read a few of the others here with simular questions, and I was were you were about a year ago. I can say that you will not find a better product, support and community than you will here with the IoTaWatt.

A circuit is rated on the breaker for a given value. If you have a 60A 240v breaker, this means that the breaker will trip if more than a 60A load is being used. So, even if you have the wires from a 240v connector go through in opposite directions, there is no more than the load written on the breaker itself (60 amps) in your example. For your CTs, you need them to be equal or higher than the rating on the breaker itself. You can use 100A CTs on a 60A circuit, but not the other way around.

Bob has made it easy in the software portion of the product to simply have one CT on such loads and tell the device to double the energy usage via a checkbox. So you could have a single CT and use the opposite cable configuration, use a single CT and enable doubling the energy, or as you also stated, you can also use a splitter to put two CTs in opposite directions and tie them together into the device via a splitter.

You have options and it comes down to the number of circuits you want to monitor, open ports on the IoTaWatt, and your budget. Again, the breaker it rated for amps total (both directions) so you only need a CT that is equal or greater than the circuit you want to measure. Hope that helps.

@quella has given good advice and has quite a bit of practical experience with IoTaWatt. Relative to CT sizing, you might consider that the electric code requires that a circuit is sized for 125% of the expected load. So the maximum expected Amps is 80% of the breaker rating. On the other hand, the CTs work fine right up to their rating, and will handle about 15% more in most cases. So combining 3 20A branch circuits into a 50A breaker is fine.

Applying the above 80%, each leg should be no more than 48A, so passing both through a 100A should be fine.

All this said, once you have the circuits monitored, you can look at the typical and peak amperage and make sure the designed safety cushion is there.

That “practical experience” is only because I have been sitting at the feet of the one and only energy monitoring master; @overeasy. I am but a student who desired to learn and collect knowledge. :slight_smile:

Ok that all makes sense thank you very much. And thank you for such a great product @overeasy.

How do people normally monitor a furnace/ac/air handler? My thought is to get three 100A CTs (due to the furnace having a 60A breaker) and combine them with a three way splitter into a single input so that way the Iotawatt is always getting whatever the total ac or furnace load is or do most people get a 100A for the furnace and two 50A for the AC and Air Handler, use three inputs on the Iota and then use the outputs function to add all three together?

And what about the mains? Would you combine the two 200A CTs with a splitter to get the full reading or use two inputs and add them together? Is there a benefit to monitoring each leg independently other than seeing if the load is unbalanced?

I can only tell you my thoughts on your questions. I would think you may want to break out the AC/Furnace/Air handler so that data could be analyzed on their own. To me that would be important data to know but not if you combine it. Maybe a Heater and AC as one is used on the summer and the other winter so you could see the difference, I personally would like to see it separated; sorry I’m a data guy.

I know of people that do both options with mains, again this comes down to a personal preference. Most I speak with have two CTs on the mains and just add them together in the IoTaWatt to get a Total. Not sure how helpful it would be to have info on one or the other, but I followed the others who have gone before me and did the two CTs and two inputs in the IoTaWatt.

Happy to hear from others or the master creator himself for his added wisdom.

If you want to combine the two mains with a splitter, the rule is that the two must be the same and must have a capacity to handle the combined load. I know that sounds wrong, but it has to more with the iotawatt being configured to handle the whole load than the individual CTs being able to handle the load it’s measuring.

So you would need two 400A CTs to use a splitter.

@quella that makes sense on wanting to differentiate the data.

@overeasy that also makes sense it’s not a limitation of the CT but that the Iota doesn’t expect that amount of current from a 200A under the assumption it is a single CT? I’m sure it’s a little more complicated then that but is that the gist of it?

you have it just fine. It’s not complicated at all. IoTaWatt accepts a maximum 50mA at the input. Each 200A CT is capable of producing that, and two can produce 100mA.

That said, it would require an aggregate mains current in excess of 200A to be a problem. That is rare. My home tops out at about 60A total.

Also, there are 400A CTs available in the store.