How often do CTs fail?


First i’d like to say how much i’ve enjoyed this amazing product over the last year. The software and online documentation are great and IotaWatt has taught me a ton about energy use patterns in my home. Kudos to the creator of this.

Recently, i noticed that there was an inconsistency between my current transformer (ECS25200-C2) and my main utility meter. After snooping, i noticed that a 120V / 20A circuit i monitor w/ iotawatt was reading higher energy use than each leg of the subpanel containing the circuit. The attached graph displays the measured energy use of both legs of the 200A subpanel (measured with 2 CT ECS25200-C2 s) along with the measured energy use of the 20A circuit on this subpanel (the energy reading on the 20A circuit was measured with an ECS1050 and this reading is completely consistent with the energy use i expect from the space heater using that circuit).

After finding this out, i went back and looked at the measured power use over a longer time period (2nd plot attached). My energy use patterns in the house didn’t change dramatically over the year, but it looks like in mid summer the CT ECS25200-C2 on one leg of my 200A subpanel drastically reduced in measured power despite the energy use patterns not really changing.

Is it common for these CTs to fail after ~ 6 months? Any advice on troubleshooting if this might be fixable since the CT does still seem to be registering a fraction of the energy use?


No, it’s not, and the failure mode would most likely just not working at all. So let’s look for something else. Your reasoning diagnosing the CT appears sound.

Those clamp type CTs are easy to install but require a little more care. They come with little squares of paper between the faces of the iron cores that usually fall out when you open them, but sometimes remain. Given that it was working better, that’s probably not it, but they are easy enough to remove and examine, so you should do that.

Another common problem is that they get disturbed when the panel cover is installed or other activity disturbes them such that they open a little. The above recommended inspection should address that as well if you make sure it closes fully.

Thanks for the quick reply. The CT clamps looked closed, but i noticed some corrosion on one terminal that i attempted to clean off (see pic). Readings now seem to obey the laws of physics after cleaning/re-attaching.

Just curious what the expected lifetime of the 200A clamp CTs are. Again, amazing product and support.


Probably wasn’t closed all the way as a gap would be needed for that corrosion to form. The surfaces are supposed to be mirror smooth and are protected prior to installation by that piece of treated paper. Once mated, no corrosion should be possible.

Weird, the clamps looked closed when i checked, but clearly the spring mechanism on these is far from air tight. After crudely cleaning, i pushed the clamps together manually and things still seem ok.

Any tips for properly cleaning these surfaces? Also, how long should i expect these clamp CTs to last before they no longer work (assuming the contact is properly sealed)? Guessing the snap and permanently closed CTs last for quite a while (didn’t realize the “clamp convenience” would cause hiccups down the road).

Thanks again for the quick support.