Why a Stereo and not a Mono Plug?

I’m trying to adapt one of the 200A CTs from the stuff.iotawatt.com site to use with a Sun G2 limiting grid tie inverter as the CT they provide is only 100A and not big enough for my mains wiring. They use a GX12 plug, which only has connections for 2 wires. I was trying to find a non-destructive way to use the 200A CT by buying a stereo plug breakout (Amazon.com) to wire to the GX12 plug.

However, in researching stereo (or TRS) plugs, it appears this is usually a 3 wire arrangement with a ground, left channel, and right channel. Since the GX12 only has 2 poles, I’m wondering how the CTs from the stuff site are actually wired to the stereo plug. I had assumed, before my research, that this was a 2 wire affair, and I don’t really want to just cut into it to find out, so wanted to ask here.

Perhaps you can enlighten, @overeasy ?



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The CT is wired to the tip and sleeve. The ring is unused. A mono plug could have been used, but they are all but obsolete and stereo wire/jack assemblies are more readily available and less costly due to their higher demand.


Afterthought. I think it’s unlikely that an IoTaWatt 200A CT would be a direct substitution for the Sun G@ 100A CT. Does it have any specifications printed on it or elsewhere?

Yes, it’s the YHDC SCT-013-000, so 100 A/50 mA. So, the ratio won’t quite be same, but I was thinking it would read approximately half the power than there actually is. Do you think I’m way off base with that assumption?


That’s right. So it should work. You can compensate for the half power by using two 200A CTs combined with a headphone splitter.

Thanks for the alternative idea.

RE: the headphone splitter
I’d seen the suggestion of a headphone splitter for combining legs of a 240V circuit that also has a neutral if you were concerned about only using one IoTaWatt input. I was wondering what happens if both CTs start reaching their max current. Would the IoTaWatt input see 100 mA (50 mA from each CT) instead of the 50 mA max it would be expecting? Or is it getting the parallel equivalent of the two CTs?


Yes to both. It would be the equivalent of a 100A/50mA CT. If your main exceeded 100A, you would be overdriving. I understood the problem to be that it was a 100A main but the SCT013-000 would not fit.

@overeasy , sorry, I’m conflating the issue now. I purchased a 200A CT for this inverter for two reasons:

  1. I have 200 A service. Having the IoTaWatt on now for 5 days, I see the likelihood of me even exceeding 100A on a leg is very low, but 200 A are what the main breakers are rated at and the wiring is sized for.
  2. The big boy wiring is too large for the 100A CT.

So, my plan was to make an adapter to convert the 200A CT I purchased from you to work with the plug type of the inverter without having to cut the cable and I needed to know the pinout. This is with the expectation that the inverter would read about half the power since it would be 200A / 50 mA instead of the one it expects at 100A / 50mA. Since this is just for preventing backfeeding, that seemed like a resonable tradeoff.

When you mentioned the dual CT with a headphone splitter, it reminded me of another post you had made for an entirely different question about ways you could monitor a 240V circuit with one IoTaWatt input, either by doubling the wires through a single CT or using 2 CTs joined by a splitter into a single input.

So, the question in my last post was regarding how 2 CTs joined by a splitter would act to the IoTaWatt, which isn’t really related to my original question at all. Just to confirm your response, if you have two 100A / 50 mA CTs joined by a splitter on the two legs of a 240 V circuit, would their currents combine at the IoTaWatt input for a max of 100 mA if you had 100 A flowing through each leg? And would that be OK for the IoTaWatt? If you would prefer that be answered in a different post since it has nothing to do with my original question, that’s fine too.


The answers are Yes and No.

Interesting… So, if you were to try and monitor a 240V circuit using 2 CTs (X A / 50 mA) combined by a splitter, you should ensure that the combined amps will be no more than half the CT’s specific maximum (X / 2) to ensure their combined output are less than 50 mA?


From the docs:

Two individual CTs can also be combined with a common headphone splitter and fed into a single IotaWatt input. When combining this way, both CTs must be the same model with an individual capacity sufficient to measure the combined capacity of the two circuit breakers.

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Thanks. I saw that, but the full implication of what that means was sort of lost on me. It’s more clear to me to state that the combined input of the two CTs must stay below the maximum input of 50 mA and that is accomplished by sizing the individual CTs to be able to handle the full load of both breakers.

Anyway, think I’m good to go on making up my new connector. Thanks for that information.