Adding CT clamps

I have two solar production sites within 100m of each other, can I use put two ct clamps, on the same phase but on different Inverters to add the outputs of the two?

Welcome @Lmm_Cams to the OEM forum.

I presume you mean current transformers, and using an emonTx or emonPi and taking the voltage reference from or near to one of the inverters? What exactly do you mean by “add the inputs of the two”, because you can either use the two inputs of an emonPi, or two of the four inputs of an emonTx, and add the powers in software in emonCMS, or (especially if you have only one current input available) wire the two c.t’s in parallel and connect to one input, physically adding the secondary currents of the two c.t’s. In this case, the c.t’s must be identical, and the total current must not exceed the 100 A that the input is rated for (using our standard c.t., that’s 50 mA of secondary current in total).

Your principal problem might be pickup and interference on a 100 m long cable, but otherwise you can have as many c.t’s as you have inputs for, as long as they are all on the same phase.
A second-order problem might be the voltage drop between the inverter and the point of use, arising from that 100 m cable run, meaning that the power you measure won’t be the power out of the inverter. I’m referring to the inverter output, not the c.t’s extension cable.

Thanks for the reply, in this case I am not using emoncs, I am actually asking ask i fitting at my house a Zappi electric vehicle charger, and it has 6 CT clamps. 3 monitor what is going out to the grid from my solar production, and 3 monitor the solar production. My problem is I have 2 solar production facilities onsite, about 50m away from each other. Both are 3 phase. I presume other than interference issues I may have, it’s possible to add the two together as one input. I maybe able to buy some shielded twisted 2 core. So they need to be in parallel not series, and on the same phases? Any other recommendations welcomed:-)
Thanks again for your help.

Yes, because a true current transformer is a current source, so the currents add when wired in parallel. If your c.t’s have internal burdens, i.e. a voltage output, they are voltage sources and you wire them in series and the voltages add.

Yes - otherwise, what are you measuring? Look at a 3-phase waveform and add two of the waves together - the answer is not the sum of the individual waves, because of the time difference between them. Or think of it this way: the voltage line-neutral is 230 V. The voltage line-line is 398 V, not 460 V.

Where do the cables from the two inverters come together? Or do they feed into separate distribution boards?

Hi mate, another question kind of on the same track. I have a solar iBoost, like an ImmerSun, im sure you know what i mean, it detects excess solar being produced going out to waste and redirects it in to an immersion heater.
I have a specific issue though, i have 3 of these iBoosts but in order to make the best use of surplus energy id need 3 immersions and unless i have one made (4k!) i have to buy 3 tanks with an immersion in each one and a iBoost on each one… so i was thinking, could i use one iBoost with one tank with a 3kw immersion, but with three CT clamps in parallel one on each phase, together? Would this summation detect how much energy is going out to the grid on all the phases, and then presumably the iboost will then load the one phase it is on to use that excess? In my mind it sounds like it would work?

My brain, belonging to a professional electrical engineer, says it won’t.

Look at what I wrote above - the clue is in there. If you’re wiring 3 c.t’s in parallel, it’s the same as putting all three wires through the same c.t. Draw out the three sine waves of the three currents, and add up the heights at a few points. You’ll soon see something that you might find surprising but in fact is fundamental to how 3-phase systems work and why they are used for transmitting energy over long distances. The sum at any point on the wave is zero. Always, provided the amplitude of all three is the same. And that means the vector sum of the currents - and this is what your one c.t. around all 3 wires, or your 3 c.t’s in parallel, will measure - is always zero. If the currents in all 3 phases are not equal, then what you measure will be the imbalance current that’s flowing in the neutral wire.

What you really need is a 3 kW 3-phase immersion heater element - e.g. Industrial single or 3 phase Immersion Heaters with Thermostat for Water connected to your three phases, then you load all three phases equally.

Or you could build a 3-phase diverter measuring and adding the powers on all three phases, driving a 3 kW ‘switch’ feeding your single 3 kW immersion element. This will only work if you have “nett metering”, which is when the energy across all three phases is added (algebraically) to determine the bill. If you get charged different rates for import and export, and each phase is considered separately my your meter, then this won’t help you very much.

Ah yes, i see now why it won’t work. Bugger. I have been looking for a water tank with a 3 phase immersion, but with a neutral…. I am wondering if I could use 3 iBoosts, one monitoring each phase via a ct, and wire the outputs to the three separate lives and neutral, if you see what I mean? So that if a phase is in surplus, that will be directed to the one phase of the three phase element. I was thinking, if the outputs of the iBoosts are isolated, that shouldn’t interact with each other……?

If you have a 4-wire system (3 phases & neutral, brown, black, grey and blue) then it should be no problem to wire 3 separate iBoosts as 3 single-phase systems, BUT if you buy a 3-phase immersion heater, you must make certain that the separate elements are connected in “star”, so you have 4 terminals: the 3 phases plus neutral. If the elements are connected in “delta”, so just 3 terminals (L1, L2, L3), it won’t work. (I am ignoring the protective earth each time, but you must of course wire it for safety.)