Accuracy of 100A CT


what accuracy may I expect from the recommended 100A CT?

As an example: would it be possible to detect whether a 10W consumer is running or not?

Thanks for your help,


If you’re only using the CT to provide ON/OFF status the accuracy is not very important. What is important is that you pick up a signal that reliably senses status. If needed, you can pass (wind) the wire through the CT twice to double the ‘perceived’ signal, thus increasing sensitivity for ON/OFF status.

Good luck

You’ll have a big problem measuring 10 W, or 43 mA, with a 100 A c.t. In practice, you might not even see the change from 0 to 10 W.

Every c.t. ever made is inaccurate at low currents - it is inherent because of the way the magnetic fields inside a c.t. work. What “low” means depends very much on how much money you pay for your c.t. The SCT-013-000 is only specified down to 10 A by the manufacturer.

A “revenue” grade one costs many times more than the one from our shop, but I still wouldn’t like to say that it would read 43 mA with any accuracy.

As Neil says, you can improve the sensitivity by making a multi-turn primary winding. If you use 20 turns, your 10 W load will look like 200 W, and that’s about the lowest power that an emonTx or emonPi can measure if you can accept a moderate error. If you do that, the maximum current that you can measure will be 5 A.
(And that is in fact how I test the SCT-013-000 - using a 20-turn coil and a current of 5 A.)

Thanks @Neil and @Robert.Wall for your quick and helpful replies!

To be honest, the 10W were a rough number I had in my mind when thinking about monitoring the power supply of a single-family home. I think I will go with the 100 A c.t. for now and see how far it will take me.

Thanks also for mentioning the “multi-turn primary winding” trick. Didn’t think about that.

For your reference: in the meantime I stumbled upon the “CT sensors” chapter in the documentation of this project. It also contained lots of helpful information:

You will indeed find a lot of basic theory and practice in the ‘Learn’ section.