CT Size vs Breaker

[morphus]

All my 220v breaker (Heater, Boiler, Dryer) seem to be Red/Black cable, so I have to monitor both to get the proper reading according to docs.

To do so I can either

1. Use 2 CT and 2 Input
2. Use 2 CT, a Y and 1 Input
3. Use 1 CT and 1 input, swapping cable direction inside the CT

Q1. If I do #2, and my breaker, like the photo show, is a 40 amp, does I need 2x 50 CT, and setup my Input to ECS1050 (that’s my CT50 model), or I need to setup my input to CT 100?

Q2. If I do #3, and my breaker show 40 amp, is a CT50 enough, or I will need a CT100?

Q3. The Y cable is a normal 3.5mm stereo cable that you use for headphone/microphone?

Q4. If I do #2 and my breaker show 100 AMP, does I need a CT100 per cable, or a CT50 per cable is enough?

Thank you

[dstutz]

Red/black wires and the double-pole breaker just means 240V. See if any of the appliances ALSO have a white/neutral line. Only those need 2 CTs to accurately monitor. If it’s just the 2 hot lines, you can use one CT and check the box to “double” the reading in the setup.

Heater and Boiler are probably plain 240V, Dryers normally are “4 wire” and have a neutral because the tumbler motor (and maybe controls) are 120V while the heating elements are 240V.

As for sizing, the CT needs to have the same capacity as the breaker unless you are passing multiple through one in which case you need to do that math.

[morphus]

Thank for the info, all my appliance “seem” to have a white cable also, at least on the panel side, have not looked at the appliance side. I suppose those like water heater, or heat pump that are directly wired don’t have the neutral, but would have to validate.

So if I do 2 cables, reverse direction, that are both connected to the same 40 AMP, I need to use a 100 AMP CT?

What about the cable merging with Y cable, if I have 2 cable connected to different 40 AMP breaker, reading the doc, it seem I would need to use 2x 100CT, is that right ?

And since I have a 200AMP main breaker, can I assume that I can merge my 2 200CT using a Y cable, and that my reading will be good for 99% of the case, unless I use my panel to it’s maximum. And what will happen if I consume 205Amp, will it just show 200 in IotaWatt or will it cause issue ?

Sorry, I know it sound redundant, but that’s the part that is not clear, in my head at least

[dstutz]

@morphus

So if I do 2 cables, reverse direction, that are both connected to the same 40 AMP, I need to use a 100 AMP CT?

I haven’t personally done any of the splitting/merging/combining, but the way I understand it is that if they are separate phases, they need to go through different directions or usage on one will cancel the other out, you can combine the same phase the same direction. And yes, 2 40s would be 80, so a 100A CT.

I know I’ve read stuff on here about pushing more current through a CT then it’s rated for but I can’t remember, so I’ll wait for others that are sure to comment.

Some info here:

Yet another, how can I combine multiple CTs into one question

Either should work provided you orient the CTs appropriately for phase. I recommend before combining that you plug each CT sans the splitter and insure you are getting positive power. As far as the capacity issue, the CTs should be fine, even in your worst case scenario. The reason for oversizing is to insure the combined input doesn’t exceed the ADC capacity. The unit is conservatively rated at 50mA. There is about a 15% overcurrent tolerance so the IoTaWatt can actually handle about 57A bef…

Planning Loads vs CT Rating

As far as I know, of the many thousands of units worldwide, nobody has cooked the ADCs. The TVS diodes in the CTs are there to protect the CTs. Without them, if disconnected from the IoTaWatt while clamped to a current source, they can develop very high voltages internally and damage themselves. The IoTaWatt has its own protection. First in line are the burden resistors. They are 20 ohm 1/8 Watt. 1/8 Watt would be 158mA input - about three times the 50mA design limit. Somewhere above tha…

and

@overeasy

The SCT019 CTs are rated at 200A. They have a ratio of 6000:1 which means they would be at full scale at 250A when used with an IoTaWatt. That’s what I have had in my main panel for more than a year and my accuracy is better than .5% when compared to the meter.

The ECS24-200 (and ECS25200-C2) are 4000:1 ratio. They are full scale in IoTaWatt at 167A. By all accounts that’s more than enough and in the unlikely event the current were to exceed that, the grace leeway would get you to 175, and the protection diodes on the input would prevent any damage after that.

[morphus]

thank you

[overeasy]

Just to try and clarify some of this advice by cleaning up the terminology:

@morphus

So if I do 2 cables, reverse direction, that are both connected to the same 40 AMP, I need to use a 100 AMP CT?

Correct.

@morphus

What about the cable merging with Y cable, if I have 2 cable connected to different 40 AMP breaker, reading the doc, it seem I would need to use 2x 100CT, is that right ?

I take that to mean a 40A double breaker, which is the same as two single 40A breakers. You would put a 100A CT on each wire (one reversed), and configure the combined input as a 100A CT.

Note: The reason the CTs are oversized is so that their combined output will not overdrive the IoTaWatt input rated for 50mA. Other solutions to allow using nominal size CTs would be:

• Use CTs with 10-25mA output if available (probably available from Echun)
• Change the burden resistor from 20 Ohm to 10 Ohm by adding a second 20 Ohm resistor across the tip and sleeve of the splitter output.
• Measure each line during full load of the appliance and if the sum of the legs doesn’t exceed 50A you can use 2x50A.

@morphus

And since I have a 200AMP main breaker, can I assume that I can merge my 2 200CT using a Y cable, and that my reading will be good for 99% of the case, unless I use my panel to it’s maximum. And what will happen if I consume 205Amp, will it just show 200 in IotaWatt or will it cause issue ?

Like the 40A double breaker carrying 40A for each of the 120V legs, your 200A mains allow 200A per leg for a combined total of 400A. So if combining both into one IoTaWatt input with a splitter, you would need to put 400A CTs on each wire and configure as a 400A CT. I would not recommend that. Use two inputs for the mains.

If you have any doubts about whether the white neutral wire is used for any of your 240V loads, just put a CT on it for a day and see if it remains zero.