3-phase - necessary to clamp incoming feed lines from the grid?

Hi everyone,

I just became aware of this project and it looks fantastic! I’m keen to buy a kit but have a couple of questions. I have read the manual and did search the forum but couldn’t find my answers so I do apologise if my questions have been answered in the past.

I’ve got 3-phase running to my home in Australia. The instructions in the manual mention to clamp the 3 incoming feed lines coming into the mains board from the grid. Is this necessary? I apologise if this is a stupid question but if it is necessary, why? I’m only interested in monitoring loads on my circuits, not the totality of what I’m drawing from the grid (I’ve got a separate Fronius power meter for that).

The reason I ask is that I’ve got 13x single phase circuits and 2x 3-phase circuits so I’m a little short on capacity in the 14 inputs.

Thanks in advance for any help!



If you are not interested in what you are drawing from the grid, you don’t need CTs on the mains. The docs however explain how to determine the phase assignments relative to the voltage reference using the mains. So would either need to put some on the mains temporarily to follow that procedure, or be more creative on your own.

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The shortage of inputs can be solved by combining some circuits (on the same phase) and doing some maths on the outputs.

Two (or more) wires (they have to be the same phase and the same direction) can go though the same CT and the current will be summed.

You will need to work out why you want an Iotawatt and physically install accordingly. If it is a combination of ‘monitoring’ and ‘investigating’ you probably want to put CT’s on your power hogs first (usually not lighting) and once you understand the behaviour of those devices move items around.

So, how to consolidate?

I always recommend monitoring the incoming supply not only to determine the iotawatt accuracy relative to the suppliers meter but also to make the maths easier.

Option 1

3 CTs on the 3-phase incomer and 3 CTs on one of your other 3 phase device will consume 6 inputs leaving you to consolidate the 13 single phases into 8. If that is do-able (it often depends on the space inside your board) then the final 3 phase can be derived (per phase) as incomer minus everything else on that phase = that phase of the unmonitored circuit.


Allocate 9 channels to your incomer and 3 phase devices (they are probably power hogs or they wouldn’t be 3-phase) and then allocate the remaining 5 to the circuits of most concern. You can still use the incomer minus circuit 1 minus circuit 2 minus circuit 3 = everything else approach (per phase).

There are other Options - but the above are the most probable.

As an aside, if you decide to allocate one input/CT per phase to ‘moveable monitoring’ it is well worth physically labelling that CT inside the distribution board with the iotawatt input number and phase (once you have derived it) and then setting up the inputs on the iotawatt for those circuits as phase A monitor, Phase B monitor, Phase C monitor etc. Then if you want to investigate a specific circuit you just move (respecting phase) the CT to the circuit of interest.

If monitor A, B and C are excluded from your generic ‘everything else’ output calculation they can be moved (on the same phase) with no changes to any calculated output.

If ‘everything else’ on phase A is 4 circuits and you put a monitor on one of those 4 - you could (if you wanted) setup an output that subtracts the monitor from ‘everything else’.

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The possibilities are just about endless. I don’t think I’ve ever explained this, but if a three-phase load is three-wire (no neutral), you can measure it with just two CTs. Anyone interested in trying that should start a thread and I’ll walk you through it.

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Thanks so much for your help guys, I’ll take it on board when designing the solution.