OpenEnergyMonitor Community

Question regarding precision of CTs (SCT006)

Hi everyone,

I am quite new to the topic of energy monitoring. I bought an IotaWatt and some SCT013 and SCT006. My plan is to use the SCT013 to measure the main leads coming into the distribution board and use the SCT006 for additional per-room measurements. I was now playing around with the stuff I got and was little confused about the readings I get.

I hooked up a hot air gun into a power meter, and measured the current on the lead using one SCT013 and two SCT006. I noticed, that the power readings of the two different CTs differ significantly as you can see in the following screenshot:


The measurement of the SCT013 seems to be spot on (less than 1% difference compared to the power meter), but the readings of both SCT006 are really bad. Additionally they differ significantly each to another. The readings you see in the screenshots where made by additionally pressing them shut, without that, the gap between them is even worse.

The power factor reported by the SCT013 is spot on to that measured by the power meter, but again the SCT006 show significant differences.

What could be the problem here? Is this the expected accuracy of the SCT006? (I hope not, I ordered a bunch of them xD…). Is there anything I can do to improve the accuracy of these CTs?

Thanks and Regards,

There are test reports in the ‘Learn’ section for both the SCT-013-000 and SCT006.

If that doesn’t help, I suggest The IoTaWatt forum is a better place to ask a question that is specifically about the operation and use of IoTaWatt. That is managed by Bob Lemaire, and he and the other users there are more likely to have the specific knowledge to help you.

The IoTaWatt forum uses the same software as this, but you must register a new account there. If the user name you have here is not taken, it might be helpful to have the same user name in both places.

If you post your question there I’m sure someone will get back to you quickly.


thank you for your answer. I already had a look at this page here which suggests that the CTs should have a much better precision than what I see. I am not a fan of cross posting, but I followed your suggestion and copied my post into the IoTaWatt forum. You can find it here.

All split core transformers suffer from sometimes significant phase shifts and also from the split core not being the same as a solid core. They are much easier to install, but I would not expect great accuracy from them.

There is an estimate of phase offset that is used to compensate. It looks like it being wrong for your parts would explain both the phase offset and difference in real power measured.

There is a difference between absolute accuracy, repeatability and resolution (which sometimes means percision) which did you mean?

The transformers you purchased are definitely NOT revenue grade. I prefer the solid core ones, since they typically have better accuracy, are smaller and cheaper. But, you do have to remove the wire from the breaker to use them.