How to choose the CT sensors

I’m planning to buy an emontx WiFi in order to monitor my house power consumption.
Before placing my order I would like to resolve a little doubt I have regarding the CT sensors.
The CT sensor Ampere ratings (i.e. 20A, 50A etc) ,we can select in the shop, refers to the maximum current RMS value or the peak value?

For instance I have the main house circuit with s breaker rated at 20A (RMS I believe). Is it ok to use a 20A CT sensor or I should use the 50A because I could have s peak current of sqrt(2)*20A?

Thank you Anne sorry for the silly question

Welcome, Alessandro, to the OEM forum.

For things like this - and your circuit breaker, switches, motors, etc, it is invariably the rms value. The only thing that I can think of immediately where you might see a peak voltage is on a capacitor - and one intended for mains electricity (power factor correction, or interference suppression) will normally be labelled with an rms value anyway.

But, you might want to be able to measure more than 20 A for another reason - “inrush”. When certain things are switched on - a refrigerator for example, it will draw a large current, which over a few seconds or maybe longer will fall away to the “normal” running current. this current might peak at 3 or 4 times the normal current. Most circuit breakers found in houses are designed to not trip under these conditions, and only trip if the abnormal load remains for some minutes.

It’s not a silly question at all - a silly question is where someone doesn’t tell us enough for us to be able to answer it. :smile:

Thank you for the quick answer.

The in rush phenomena is interesting, I didn’t know it could go to 3 4 times the stationary current. I will definitely consider it.

Even more for a tungsten filament lamp, it could be around 12 - 13 times!

Here’s my refrigerator starting:

Very interesting! I see it is quite s quick phenomena, probably not affecting the overall consumption, but useful to know in case of anomalies or the like.
I guess the choice is a trade-off between the capability to measure these in rush peaks and the accuracy of measurement for low loads.

In my case I would go for the latter, since I am not sure if the main consumption comes from low loads for long time or high loads for short time.

The start spike in my 0.1kW fridge in my house is subsecond and several kW. Averaging 60ms once per second, about six of sixty fridge start events in a typical day have a great big spike of random-looking size up to 2kW, and in most fridge start events, that is all over within the second before the next 60ms in which my nonstandard monitoring every second will be looking.

My small heat pump, which is 0.7kW(electrical) does not seem to do that. It could be due to better motor control electronics …

I’d choose your current sense coil to be capable of triple what you know you use or generate.