Practical CT extensions

This has been a recurrent topic both here and in the OEM website. Sourcing CTs with 1.5M cords went a long way toward reducing the demand, but there are still folks who need to mount the IoTaWatt further away or monitor multiple load-centers.

There is guidance on this topic in the OEM website that is academic. It looks at the theory of electrical interference and takes a best-practices approach in their recommendations. In my experience, while the advice is based on sound theory, I’ve not seen any specific account of where something simpler doesn’t work fine.

In particular, I have looked at using some decent quality 3.5mm headphone extensions in various lengths up to 25’ and found no discernible difference with and without. I’m talking about within one watt in several hundred.

I had thought to design and offer converters using cat6 wire with RJ45 connectors to extend up to 4 CTs for the length of the cat6 cable. I tried it out and it worked great in a 25’ test, but the cost to build and enclose something like that would be high and the market isn’t ripe for it.

The CT circuit is a current loop. So like the ubiquitous 4-20mA instrument standard (IoTaWatt uses 0-50mA CTs), it is immune to added resistance to the extent that any added resistance doesn’t cause the VA requirement to increase out of range for the CT. Noise could be a problem but we are talking about extensions outside of the load-center where the routing can avoid obvious noise sources.

So I recently sourced these cheap extensions from a company called a 12’ extension was $2.93. Six foot was $1.77. These seem to be supple, decent quality, with molded connectors. I’m sure you can get the same from many vendors on Ebay.

I would recommend that if you have a need to extend CTs indoors, you give something like this a try. I wouldn’t hesitate.


I paid (in beer) the company next door that does precision soldering on CNC machine circuit boards to build me a couple using 1/8" RCA ends I cut off of old headphones from the thrift store. I’d say your suggested solution is far superior, @overeasy.