Why CT 100A?

First of all I am super fun of OpenEnergyMonitor project I was developing how to get the power factor and I found this answer and more.

Second, I dont understand why the suggestion for a installation is a CT 100A (In the shop you just can find this version and 200A version). In a standard installation (4.5kW) we are going to see currents peaks of ±4500W/220V = 21A before to the electrical company meter actuation, so if we choose a 30A/5V CT the resolutions is higher because you have 5V to represent 30A which is 5/30= 0.16Volts per Amperes resolution and not 5/100= 0.005Volt per Amperes which is 32 times smaller.
So I think that I am missing something.

With this thought I chose 50A/5V CT (SCT-013-050) and I dont finger out if it is fully compatible with emonTX V3 board.


Hello Stepan, welcome to the OEM forum.

The answer to “Why 100A?” is very simple: the normal supply to a house in the UK is a single phase, rated at 60, 80 or 100 A. Ask any c.t. manufacturer and they will say that you should always buy a c.t. that greater than, but as close to, the maximum current that you will ever want to measure.

Therefore, our maximum power could be 24 kW continuously for ever, and it could be much higher than that even for a short period of time.

If your 50A/5V CT (SCT-013-050) gives you 5 V rms at a current of 50 A rms, then I am sorry but it is not suitable. The maximum voltage that your emonTx can accept (and you must remove the burden resistor inside the emonTx for any voltage-output type c.t. to be compatible), is approx 1.1 V rms. You will need to remove the burden resistor inside the emonTx and replace it with one of the correct value. I can calculate that for you if that is what you want.

If you must use a 50 A c.t. (and our standard 100 A : 50 mA SCT-013-000 is good to about 200 mA, and will show a current smaller than that even), then you want one with a current output and a ratio of 50 A : 50 mA, that is capable of giving 1.1 V with a 22 Ω burden.

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Thank you Robert.Wall everything clear, I will use 50A/5V CT for other purposes then.