What circuits should I monitor with Current Transformers? Large or Small?

I have a good mix of large and small circuits in my panel.

This is my panel schedule:

Being that an EmonTX has six current transformer inputs and because putting 42 current transformers in this panel would be overkill, I want to be sure I’m selecting circuits that provide useful data.

Obviously, two inputs are going to be used for the Mains.

I can probably get HVAC runtime data from the Nest Thermostat, so do I really need to put CTs on the HVAC loads?

I can probably get away with one each on the stove and water heater since they’re 240 volt resistive loads; same current will be on both hot leads.

That leaves me with two left.

EV charging is handled by OpenEVSE, so I shouldn’t need to monitor those.

Maybe the refrigerator circuit and the laundry room circuit that has a deep freezer on it? Give me data about how long those units run and maybe alert me if they run too long if someone left a door open?

Maybe the dryers?

Not terribly concerned about small appliances, as those can be subtracted out of the total on the mains once big loads are known.

I keep seeing ads for commercial products (Sense) that seem to indicate that they have an algorithm to determine what’s running with CTs on the mains ONLY. Anything similar exist for Open Energy Monitor?

Edit - Modified post to include picture vice linking to it via a 3rd party. Moderator, BT

which you can extend to 12 by adding an expansion board.

It’s really your decision for what you need or want to monitor. A current transformer measures the total current in the wire or wires passing through it, so you can bunch a set of wires supplying different circuits and put them through one c.t. The technical constraints are: if the current in a wire flows in the wrong direction, that current is subtracted, not added: and the obvious one: the wires must have enough slack to reach. You do not need or want 2 c.t’s on the 240 V loads, the maths in emonLibDB takes care of that (emonLibCM does not, so make sure you use 2 phases of the 3-phase emonVs and the right sketch with the right library - mention this when you order). If you haven’t seen it, you probably should read this: Use in North America — OpenEnergyMonitor 0.0.1 documentation, Part 2 of which specifically deals with using the emonTx4 on your system (it’s different in some significant ways to the emonTx V3).

This seems to be the Holy Grail of energy monitoring. To the best of my knowledge, no-one has yet succeeded in doing it reliably and consistently. It’s a very difficult problem.


Will the Expansion Board be compatible with the emonPi v2 when it comes out?
I see preorders are open for it and it’s due to be available on the 13th of December.

I was planning on using the 3 phase power supply and just ignoring the 3rd phase.

Hi Theo,

Unlike other sites, we want you to attach your pictures to your posts.

If the linked picture becomes unavailable, your post can lose its meaning and/or
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I’m fairly sure that is a no, it is only for the emonTx4. You need an emonBase (i.e., a Raspberry Pi and you don’t need the RFM69Pi if you have a cable connection to the emonTx4) to give you 12 channels, or an emonPi as well would give you a possible 18 channels total, or a RPi and two emonTx4s would give you up to 24 channels. The emonVs could supply all that, but we’d have to think of a way to split the lead to feed two emonTx4s, if you went that way.