Question about CT's for a North American installation

I received my kit the other day and am looking forward to the build and seeing how this will all work out.

I have a few questions about CT installations...

Ideally I'd like to monitor the total power consumption for the house AND several individual circuits, but with only 3 CT's available out of the box, I need to be more selective/creative.

With North American Residential Power being Split-Phase (3-Wire Single Phase) I think I should be able to have both hot legs feeding my main panel running through a single CT to capture my total power consumption for the house. If correct, then I would have "left over" 2 CT's for things like Hot Water Tank circuits, etc. As well, any 240v circuits in the house (Stove, Hot Water, etc) could be monitored the same way, both hot legs through a single CT. All 120v circuits would obviously just have the hot leg run through the CT.

Am I missing something obvious, or totally misunderstanding the way this works? I'm embarrassed to admit I work for a company that makes this sort of equipment, and yet my understanding of it's operation is quite shaky. I'll try to confirm with some of the engineers here and perhaps answer my own questions.

Thanks in advance,


MartinR's picture

Re: Question about CT's for a North American installation

I don't think it works like that. The two "hot" leads are in anti-phase so that there is 240V between them. If you put both wires through the same CT the currents will subtract.

It may work if you swap one wire around so that the currents will add and then 240V appliances will show double the current at 120V which will give the correct power.

Cliffy's picture

Re: Question about CT's for a North American installation

I believe you are right Martin, I think the 2 hot legs are 180 degrees out of phase. Which would mean I can monitor my total consumption, plus 1 120v circuit before I have to modify the eMon or buy additional units.

Too bad.

Robert Wall's picture

Re: Question about CT's for a North American installation

This question comes up from time to time and I think if you search the forums, there's some good information.

So yes, Martin, you are generally right, but it depends on the two voltages being equal, and the reports are, that is indeed a valid assumption in most cases. 

But the problem might be getting two cables through one c.t., so you probably do need to use 2.

[I need to do some deep thinking here and work out what happens if you parallel two c.t's into one input...  I'm not suggesting it until I've thought it through].

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