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I can't see how the upper two pictures relate to the lower one, so I'm a bit lost. If you want to measure whole-house consumption, clearly the easiest place to clip current transformers on is at the meter, and if you want to measure real power, then you need access to a socket where you can measure the voltage. It is convenient if it is close, otherwise you have to extend some cables.

Otherwise, you can measure whole-house consumption at any point up to where the distribution to the final sub-circuits takes place - either before or after the main incoming isolator or before the main distribution board.

One thing you ought to do, if you haven't already, is read up the forum comments about how other users in the USA have got on with their OEM kit. Basically, our system was designed for UK domestic premises where we have a single phase 240 V supply, with the neutral effectively at earth potential, whereas I understand yours is 120-0-120 V with the centre tap being neutral and effectively earthed. That means that for best accuracy, you should measure the two line voltages and the two line currents, however most US users seem to find the balance between the two voltages is good and you can get away with reading only one voltage.

Robert,

The service shown in tofurobot's picture is a 3 phase, commercial - i.e. not residential - installation. The meter is a 200 Amp unit, but the disconnect is a 400, or possibly, 600 Amp switch, which means the meter is driven by 5 Amp CTs rather than direct feed.

If he can manage to locate the meter CTs, would that be a good place to attach (if it's even possible) the new CTs?

Regards,

Bill

Bill: It looked as if that could be the case but there wasn't anything I could see to get a good idea of scale. The switch certainly looked around the 500A sort of size. It's the usual problem of having to guess, when there is not enough information given with the question. You never see cylindrical trunking, nor flexible conduit used in that manner, in the UK, and no factory electrician I've ever known would want to be seen dead with that mess of spaghetti!

It doesn't matter where the CTs go - they measure the current they see at that point, pure and simple. He could even put his own split-core CTs on the secondary side of the metering CTs! - it's not unheard of. Use 5A:50 mA ideally, or whatever is available with the monitor burden chosen to suit. Just don't cut the wires, else if on-load, the metering CTs will flash over and the electricity supplier will not be impressed. Obviously, the calibration would be for the compound CT (meter + his own), so if the metering CT is 500:5 and he has a 5:50 mA, then that is the equivalent of a single 500:50mA one, but with the non-linearities and errors of both.

The electrician I showed the photo to made the same comment (more or less) you did about the flexible conduit!

Hi guys! Thanks for the input so far. I have a little more information to share now. I hope they are useful pieces of the puzzle.

I poked around outside by the meter again and can confirm Bill Thompson's observation about the meter being CT driven. I now see how the number (and gague) of the wires feeding into it are a fairly clear giveaway. There is a large mounted box near the meter that houses the three CTs - and they are huge. They have multiple (2 or 3 large) cables running through each one, and I would guess the diameter of the opening to be 3 or 4 inches. Heavy-duty, mounted to the back plate on the inside of the box. I don't have a good photo as we had to peek through a crack in the door - this is why some of my CT intel is a bit sketchy thus far. The utility company used tamper-evident security clips on the door panel (I will find a way around this if we need the information.

I see how using my own CTs would technically work, but I'm thinking that acquiring CTs of this size might blow the budget 10-fold.

I also didn't know before that we had 3 phase installation. This seems to complicate things. Here's an updated photo highlighting what the important bits are (as far as I can tell).

Basically, the main disconnect feeds into 3 panels, one of them retaining all 3 phases (this is the panel that the air conditioning uses, so maybe this makes sense).

The large disconnect is rated for 600A. Here's a (bad) photo of the info plate:

(It reads: AMPS 600, VOLTS AC 240, 3 PHASE 240V, HP STD 75 MAX 200)

We wanted to open the panel to look for good CT placement candidates, but this is the type that must be turned off before the front panel can be removed (for good reason - no arguments here). We did the next best thing and looked at the output from this main disconnect (a nearby box that we could open). Take a look:

Good grief...

So, we're talking about a few giant CTs or several smaller ones (even though not all of these cables will need to be clamped from what I understand). I'm pretty sure that we'd still surpass the input limit of the emonTx V3 if we relied on several smaller CTs (or so it seems).

The meter does have an optical output that might be activated. I don't want to abandon the CT method, but as something to fall back on in the mean time, can the emonTx V3 accept an optical sensor for summing counts without too much modification?

One last note - agreed on the spaghetti! haha. Sometimes we just have to play with the cards we're dealt. I must say, however, that having you guys on my side provides a great deal of relief. I'm confident this project can remain a fun one :)