# A/c to A/c multiple units

Hi,
I would like to know if I can obtain real voltage readings from the following setup?:
3 switch boards all connected by WiFi or wireless.

1. House has EmonTx WiFi with 4 CT clamps and both DC power and Ac/Ac
2. Solar array has EmonPi with Dc power and Ac/Ac - may use 0 or 1 CT clamps
3. Main switch at transformer EmonTX batteries with 1 or 2 CT clamps.
Ideally I would like to have 2 clamps at the main switch, one for consumption and one for solar production. Will have to have at least a consumption clamp there.
My query is: Can I get real voltage readings for the whole system (can the consumption clamp on the battery EmonTX utilise the readings from other units)

I think youâ€™re a little confused there. A current transformer (please, a clamp is something holds a piece of wood or metal while Iâ€™m machining it) measures current - the name rather gives it away. For voltage, we use a small plug-in transformer. For you it would be this 9VAC 1A Unregulated Power Supply 7DC Plugs | Jaycar Electronics.

Now what I think you might be alluding to is that to measure real power (as distinct from apparent power), you do need a voltage reference close to the point where you measure the current. This is because voltage and current are measured on a â€śper sampleâ€ť basis about 2500 times per second, and itâ€™s impractical to send every sample by radio.

So I think you will need:
At switchboard 2: EmonPi, d.c power, a.c voltage (Jaycar adapter) and the c.t. Thereâ€™s no point in having the Jaycar adapter there if you donâ€™t measure power there.
At switchboard 1: Agreed - I presume you need WiFi because of the distance or obstructions in the radio path?
At switchboard 3: What do you mean by â€śEmonTX batteriesâ€ť - I think some punctuation is missing, emonTx powered by battery because thereâ€™s no socket to plug in the a.c. adapter? Battery plus inverter for off-grid backup or peak lopping? If youâ€™re measuring Solar production at the emonPi, you donâ€™t need to measure it here. And vice versa.

Thanks for your reply although I think you may expand your horizon if you â€śgoogleâ€ť CT clamp and note the results including Wikipedia.

I am sorry I referred to real voltage rather than real power - my bad.

To hopefully clarify:
The ground mount solar array has power outlets with an EmonPi. Inverer real power can be read here. 50m away is the main property power pole with provider meter. Grid power in, solar power in, total property power out takes place here.
Between this meter and the house are other minor buildings using power, so the main meter is the only place to read total site power consumption, This pole does not have a 230V power outlet, thus the need for a battery powered EmonTX.

Per your reply â€śyou do need a voltage reference close to the point where you measure the currentâ€ť answers one part of the question.
Can I somehow utilise the voltage reference at the EmonPi to convert the apparent power at the main meter (which is connected via wireless) is the second part of the question.

Thanks again.

A couple of years ago, the IET, the professional body that represents electrical engineers in the UK (and thereâ€™s a branch in your country too) sent me an enamelled badge denoting 50 yearsâ€™ of membership. That body thinks I know a bit about the subject, and the device in question had been called a current transformer for a long time before a mis-translation - in all likelihood emanating from China - turned it into â€śclampâ€ť. A rider to that: what would you call a ring-core current transformer?

Once the emonPi has the voltage - it matters not where it comes from (neglecting cable voltage drops), you can use it there to calculate apparent power from current. N.B. As the default emonTx output is real power if it has a voltage, and apparent power using an assumed voltage if it hasnâ€™t, then you actually need to divide power by 230 and multiply by the measured voltage. You appreciate that real power is what you pay for, so youâ€™ll see lower bills than the apparent power indicates.

Well a clamp meter measures current and theyâ€™ve been around a long time. Some even use a CT to achieve that. The CA-100 in this collection of AVO meters looks fairly historic and folkloric.

A clamp ammeter is a little different to a c.t.

What really worries me about the term â€śclampâ€ť when applied to ferrite cored split-core c.tâ€™s is the unknowing will try to clamp it onto its cable with wedges or the like â€“ and crack the core, which comprehensively and permanently alters its characteristics, and not for the better.

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