CT and RMS

When working with AC, usually, intensities are specified in RMS. For instance, the breakers at home are specified in RMS (I think).

Checking the CT documentation (very sparse), I see 30A/1V. Ok, 30 amperes input, 1 volt output. The 30A are RMS or peak?. If they are RMS, is 1V also RMS?. I am seeing the designs consider 1V peak, I think, if I am not mistaken. I am talking about the input to the ADC pin.

So, yes, I am confused about this mix of RMS and peak.


We always work in rms values unless it says otherwise. As far as the CT ratio is concerned, it makes no difference as long as both are the same. But the primary maximum current is always an rms value. So a 30 A : 50 mA CT is expecting a maximum primary current of 30 A rms, and you will get 50 mA rms out at that current. Equally a 30 A : 1 V CT (with an internal burden) will give you 1 V rms.

The peak-peak value is only of concern when the signal (now a voltage, converted by the burden resistor) gets to the ADC, which can only handle voltages within a limited range, hence we need to know that the peaks will not exceed those limits.

Peak, peak-peak and rms values are mathematically related provided you know the shape of the waveform, and normally we assume that is a sinusoid. But for currents, that is often not the case, and you must assume the peak value can be much higher than rms × √2.

I use 1.1 V rms for the emonTx, and 1.6 V rms for the Arduino, as the design value for the ADC input voltage. That also takes component tolerances into account and usually gives a conservative but safe value - but not one that is too conservative, because large loads tend to have fairly well-behaved waveforms, so little allowance is needed for peaks.