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See here for my response to a very similar question.

Hi Robert,

thanks - that is what I expected. I thought there is maybe a way to somehow avoid those 3 or more Power Plugs. I understand the need for it but it has an impact on costs (Wall Plugs, Power Plugs). Is there any chance to Use only a single Transformer - similar to the Transformers for the Doorbell? Here in CH we have them often located at the main switch box.

Any further thoughts?

Thanks and Best Regards,

Axel

There should be no problem with that - in principle. The encapsulated adapter is preferred principally for safety reasons, but the fidelity of the output waveform is also important. It has been wrongly suggested by some that this is unimportant, however for best accuracy you do need a transformer that shows minimal phase shift and distortion at the highest voltage you expect from your supply. You will of course need a 3-phase transformer (or 3 single-phase) if you wish to measure true power accurately across all 3 phases. There is a sketch that uses only a single voltage reference and synthesises the voltage wave for the other 2 phases, but this can never be as accurate as measuring the true voltages. You will also need to make some changes to the input circuit of the emonTx's to accept the common voltage reference. If you decide to do this, post again and I'll tell you exactly what needs to be done. I assume you know the regulations in your country regarding electrical installations.

You might want to read the report on the Mascot adapter, and calypso-rae's comments about transformers, phase shift etc.

Hi Robert,

thanks for your explanation. As far as I understood the measurement with a CT it is a kind of comparison against an "known" source and the unknown target. The consumption modifies the target. The "known" source is the transformer. We skip the rest of explanations - I can accept the fact "one transformer per phase".... . 

Normally the three main phases entering the household, cross the mail fuse per phase, cross the counter and go into the main switch board. Here it splits up to different rails and from the different rails to the automatic fuses per room or area in the house.

The Question is now - Can I use one transformer per rail? Should go - as far it sits on the same phase I want to measure?

And yes I understand the safety rules. That is the reason for this post. I want to order the switchboard change the right way and I want to be able to explain what I want and that is a power Plug per "Main Phase" on the beginning of each rail (= Phase). The guy from the installation will think I am crazy but who cares :-)

Thanks for further comments!

Best Regards,

Axel

"the measurement with a CT it is a kind of comparison against an "known" source and the unknown target. The consumption modifies the target. The "known" source is the transformer."
No, not quite correct. A c.t. is very simply a means of detecting and measuring the current in a wire.

You can use as few or as many c.t's as you wish, but each one must feed into one channel of an emonTx or an Arduino.  You can also measure the total current in a number of circuits if you pass the respective cables through one c.t., but all must be on the same phase. All the c.t's on one phase can use the same v.t. (voltage reference) to calculate power.

If you want to know only the total power taken, you should have 3 c.t's and 3 v.t's. The c.t. will go on the 3 main cables between the meter (Zähler) and main switchboard, this is the least expensive option. The next step is to split the individual circuits into groups and use one c.t. around all the cables of the group - the c.t. measures the total current taken by the group. Finally, you can use one on each final circuit, this is the most costly option.