Emonpi for current peaks measurement?

Dear emonpi experts :),
How difficult it would be to use Emonpi for current peaks measurements?
I am planning to merge two grid connection spots and for that I need to check what is the maximum current at each of them for to be able to determine correct size of circuit breaker I will need for the new spot.
My idea was, to put emonPi with CT sensor on and try to start as many appliances as possible and check what was the maximum reading.
But than I realized that the nodes are updated just every 5s. Is the number I get the actual measurement at that moment (than I would probably miss the peak), or average in 5s (then it does not work either) or by a chance maximum in 5s?
Is there easy way how to modify emonpi so I will get the maximum current?
Would I need for some reason to measure the VRMS as well, or CT sensor is enough for the correct current measurement.

Thanks a lot in advance :slight_smile:

I’m afraid your thinking is wrong.

You must calculate the size of the circuit breaker based on the maximum size of the cables on the outgoing side that feed your loads. The circuit breaker is there to protect the cables from overheating should you, or somebody else in the future, connect too many large loads. It is the cables that need to be the correct size to support the load that is connected.

So all you need do is measure the size of the cables and knowing how the cables are installed, from the published tables obtain the maximum current of each. Then the circuit breaker should be the largest size that is less than the current-carrying capacity of the smallest cable that it feeds. The rating of the circuit breaker should also be less than the rating of the one that supplies it (otherwise the wrong one is liable to trip if there is a fault).

I can give you the UK’s cable ratings, but the rules might be different where you live.

In theory I of course agree and the number you suggesting is definitively the upper limit for the breaker, but the reality is, that the power supplier is charging us money according to the size of the breaker.

That is why I do not want to have the breaker as big (or strong?) as possible, but more like as small as possible to accommodate all the appliances I have :slight_smile: and for that reason I want to measure the maximum current all the appliances can take in the worst case scenario.

Does that make sense?

They don’t do that where I am. OK, but you still need to know the maximum size that the cables will support, then go below that.

Do you need to measure, or can you look at the ratings of the appliances and do the sums? Your breaker will in all probability have a characteristic that makes it immune to moderate amounts of inrush, so finding and measuring the short-term peak might not be the best for you anyway. Depending on the specific breaker you choose, it might pass up to 10 × rated current for 1 s and 2 × for 10 s without tripping.

Do you have the emonPi? If not, it will be easier to make the changes you need to the software using an emonTx.
What I think you will need to do is take either the ‘continuous’ sketch by calypso_rae (Robin Emley) or the PLL sketch by MartinR (Martin Roberts) and modify one of those so that you measure the rms current over a small number of cycles (maybe only 2 or 3, not more than 5 cycles - 100 ms) and report the maximum reading that you get from that. This will mean that you almost completely rewrite the sketch, and even though I know both sketches quite well, it is not something that would be easy nor quick.
Both sketches should run on an emonPi given the correct pins numbers and changing to serial output rather than RF, but the procedure for loading them is somewhat more complicated than it is on the emonTx.

But do you have a computer and a sound card? If you only want to make a few measurements while you switch things on and off, you could look at connecting a CT to a sound card as I did for testing the parameters (see my report in Resources > Building Blocks), recording the waveform in your laptop and taking the peak current measurement (after you’d calibrated everything of course) from that.