Proper way to connect the CTs cable shielding to a IotaWatt


Hi. I purchased a IotaWatt recently for my house. Due to space restrictions, it’s much simpler for me to fit everything near my breaker box using Jack plugs with 90º angles. Also since I’ve seen solid cores are more accurate, i decided to make my own. From this thread:

I followed @overeasy’s recommendation and got the HWCT-004.

Then I investigated the best way and best cable to connect them and got some shielded cable with a twisted pair inside, following the recommendations here:

Quoting from the last link “Decide at which end of the cable you have a good earth connection. You must connect the cable screen to earth at one end only. If you have a choice, connect the earth at the emonTx/emonPi end. If you do not have an earth connection, then you may connect to the emonTx/emonPi’s GND.”

I’m assuming the same thing is valid for the IotaWatt right? It would be much simpler for me to connect to the ground. Can I do this with IotaWatt… Is the ring of the jack connected to ground? Can I connect it to the ground some other way? Or I have to connect all 14 jacks to the IotaWatt and then connect all the shielding together to the house earth wire? Is there any advantage in connecting to the house earth instead of the ground?

Thanks in advance for any help on this topic.


I wouldn’t connect the shield to ground at either end. Neither of the conductors in the IoTaWatt are grown. One is bias voltage and the other is the signal. Don’t overthink this. You would be amazed at the rigs that work fine.


Thank you for the quick reply.
So should I connect the shield at the house earth wire or don’t connect it at all? I don’t know much about signal interference, but I had the idea that if you have a cable with shielding and don’t connect it to anything it can be worse that not have the shield at all.


My recommendation is to just try it without grounding and see what happens. Again, don’t overthink this. I have had zero reports of problems caused by induced noise in the CT cables.


I agree with @overeasy on this. Unless you are going many feet, you probably won’t have any real issues. All of my CTs have two conductor untwisted wire. But, the wire in your breaker box needs to be rated for the voltages and temperatures that are present, and likely needs to be UL listed in the US. UL1007 is a decent standard, 300V and 80C. The HWCT004 has very short leads typically, so you should use proper high voltage insulation wire for the extension. They do make microphone type cable with one or two conductors and a shield, if you really want that


Yes microphone cable is what I was counting on using. :slight_smile: The distances are short (under 1 meter). I’m putting the protection zener diodes just outside the CT, and encase everything in a 3d printed shell. The cable that comes out of that shell is already a thick shielded one.


You need to check the codes of the area your panel is in. Typical microphone cable is not rated for that, but there is some shielded cable that looks like microphone cable that might work, as it has 300V @80C rating. A 3d printed case in an electric box is probably not the best idea. Everything needs to be rated for 60C or higher. PLA would be a bad choice at that temperature.


Thank you for the tips @frogmore. For the 3d printed parts I was thinking of going for PETG, which take higher temperatures, should not melt at all at 60C.
The cable is rated for 70C, but i can’t find the information about the voltage… Is it related to this:
Insulation resist. per 1 km >1 GΩ
I assumed it should be ok because the outer layer of plastic is so much thicker than the ones I see on the other sensorsother people are using.


It is related to what is called the AHJ (Authority Having Jurisdiction). It depends on which part of the world you are in. In the USA, most AHJs have adopted the NEC. This gets updated every few years. It specifies what is approved and some AHJs go beyond that. The HWCT04 is not UL listed or even ETL listed. So many AHJs would have a problem with them. Attaching them to what most consider low voltage cable inside a main panel with other stuff inside a 3D-printed box is likely to raise questions for any AHJ. Now, if you don’t get it inspected, the AHJ will never know. At least unless there is a fire. Then things get really messy. Your home owners insurance could refuse to pay for damage, since stuff was done incorrectly. I have never heard of that happening to anyone, but many people raise that issue.

The wire you have is probably safe enough, but it is not too hard to find live high voltages in a panel. If the insulation on the CT wires gets damaged, you could get high voltage down in the box that the IoTaWatt is in. That would be bad. Is it likely, probably not if you do everything right, but it could happen. This is why @overeasy say you should have an electrician install the CTs. If the person doing the work doesn’t take appropriate precautions, it can be deadly, either immediately or perhaps later when this degrade/move enough.

I have been known to work on live power panels, as do many electricians, but many of them have been shocked a few times. I have but not since high school and only by shoddy work by others (and lack of caution on my part). The lack of caution is something I can control/change. So I have been and will continue to be very careful when working with high voltage stuff even if it isn’t powered right now. I looked at the HWCT04, but decided it was too much work/time to do it right and I might just as well get the ones from the shop. Since the wires are much thicker I now need more conduit to my low voltage box. I have seen UL1007 wire readily available and that is what I will use when I need to extend a CT. I also will use solder, with a western union connection, and heat shrink tubing or a high voltage rated connector for the splicing. I will put the TVS or zeners in near the connection in the low voltage box. All of those will make it safer. But what you are planning will probably work too. You have to decide if you want to take the risk.


Thank you for all your input. Last week was a bit hectic for me, but I managed to get it all working yesterday. I’m not a professional electrician but sometimes work with 220V installations and breaker boxes at my job, so am used to it. All the wires and well insulated and don’t move around. Each sensor is directly on top of each breaker and stays in place. Now I have everything running and will test if the measurements check out. :slight_smile: