OpenEnergyMonitor Community

Monitoring of two buildings with solar in North America

I have been researching how to utilize Open Energy Monitor technologies as the foundation for energy monitoring for our property in Minnesota, USA. We have an average size house and large shop, each with 200A, split phase 240V service fed from a single meter. In addition, we have a large solar plant that will feed both buildings. The following is a high level diagram of the existing electrical infrastucture:

The “GREEN” boxes in the above diagram indicate what I think should be the AC monitoring points of the system, and the cable sizes at those points. The physical distance and the metal shop building rule out wireless communication between the clusters of equipment in the two buildings. We have a robust Ethernet network between and throughout both buildings and an equipment closet in the house where all of the networking converges, and contains servers and NAS for my business.

I have received two emonTX Arduino shields that will be combined with an Ethernet shield or Ethernet Arduino to set up Voltage and Current monitoring at the two 200A main panels. An Ubuntu virtual machine will be set up for emonCMS and integrated with an existing OpenHAB installation for home automation. The communication will initially be Ethernet between all the data acquisition equipment and emonCMS. Once the initial monitoring for the two main panels is worked out, I will expand to cover the other “GREEN” monitoring points.

I will also attempt to collect and integrate Modbus data from our Schneider inverters and charge controllers to integrate Solar Plant monitoring into this system (See Connectivity to Schneider XW+ and XANBUS).

I have found several articles on the OEM forum about utilizing Ethernet/emonTX, Ubuntu emonCMS installation and North American sized CT’s. I will be piecing together the hardware in the next couple weeks, and hope to have a functioning OEM environment by the end of the month. I would appreciate and feedback or suggestions.

Is there a connection missing - or are the “critical loads” only fed via the inverters?

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Which should read 4/0 and 1/0. :wink:

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Good catch @Robert.Wall, they are actually 60A transfer switches wired to the main panels. They carry much of the lighting, receptacles, refrigerator/freezer, computer closet, network switches. I will update the diagram.

Thanks @Bill.Thomson, I was going to look that up, will also correct.

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Diagram updated for transfer switches and cable sizes.

I think I get the picture now. On the face of it, it seems a fairly straightforward scheme.

Firstly, you probably don’t need 200 A c.t’s for the two main incomers (service entrance wires).
Bill has written about this many times, basically you need to determine the kVA rating of the transformer feeding your property, and hence the maximum current that it can deliver. That will be less than the cable rating: More info here: US 200 Amp Service explained or Do I need a 200 Amp CT?
You should also check the maximum load you anticipate.

One problem I do see is where you monitor your voltage. For the main panels, it’s obviously the mains, but for the critical loads, you need to measure the inverter voltage.

Is it possible to move the c.t’s around?
If you move the House Critical Loads c.t’s to the other end of the 50 m cable, you can have those plus the Shop Critical Loads c.t’s working into one emonTx Shield and using the inverter voltage to compute real power.

If you move the Shop Main Panel c.t’s also to the other end of the cable, on the outgoing side of the Disconnect, you can have those plus the House Main Panel c.t’s working into one emonTx Shield and using the mains voltage to compute real power.

Two points remaining:
Would you want to measure independently the Critical Loads power when fed from the mains?
And if you can’t talk via Modbus to the Inverters and you want to know their individual outputs, then you need a third EmonTx Shield for that. Otherwise, you have the total inverter output already from the sum of the two infeeds.

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Yes @Robert.Wall, the CT’s can certainly be moved around. In the long term, I want to be able to collect data sufficiently granular enough to characterize the loads, sources and loses across the entire system independent of any vendor specific component. I want to be able to use this as a “research” environment to test add on products, and plan to expand the telemetry over time. Thanks for the link to the @Bill.Thomson post on 200A Service/CT’s, hadn’t seen that one yet.

I like your suggestion to move the feeder CT’s to supply side to compute Real Power, and have updated the diagram accordingly. I have left the feeder demand side CT’s on the diagram in RED for now. There may be value in having these to account for feeder loses in the future.

Regarding measurement of grid power to the critical loads, the connections between the main panels and transfer switches consist of 10 individual circuits (Reliance A510C Pro/Tran). I imagine that CT’s could be clipped around multiple conductors, but there are a mix of 120V and 240V circuits. Would need a second CT that would include just one of the 240V legs. It is something to consider for future expansion. Updated diagram to indicate the multiple conductors and CT’s in RED.

On the inverter outputs, do you mean that we should also be measuring voltage at the inverters? The inverter CT’s for current are already on the diagram. I have assumed that I would need more than the two shields I ordered. Those were just something to get me started and work out all the emonTX/Arduino/Ethernet chinks and start feeding something to emonCMS.

I did think that could be a long-term aim.

I don’t understand - you know the current, there’s almost no loss to capacitance, so it’s the same current at both ends of the cable (certainly to better than the accuracy of the c.t’s). The losses are almost all I²R and you can calculate that.

That - depending on how much spare wire you have - should certainly be possible with the 10 circuits handled by the transfer switches, one c.t. per leg (automatically accounts for 120 V & 240 V circuits if you use 120 V as the voltage reference.

Yes - or at a circuit fed by the inverters. When the mains disappears, you’ve got no voltage reference for those loads.

The main caveat there is to be aware that circuits on even numbered breakers are on one
hot leg and circuits on odd numbered breakers are on the other hot leg. So you’ll likely need two CTs to read all 10 circuits.

That said, with only 10 circuits, it is possible all 10 could be on one leg.

All Daniel needs to do there is imagine a pile sheets of this drawing, but each page having only one load, with their appropriate line conductors all passing through one or both c.t’s:

The important thing is not to get the A and B legs mixed up.

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It’s simpler than that.
All he’s got to do is pass wires attached to even numbered breakers through one CT and do the same for the wires attached to odd-numbered breakers.

Keeping track of even and odd numbered breakers takes care of that.

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Robert and Bill, thanks for the feedback, I really appreciate it. The lines from the transfer switch do alternate between the 2 phases, and any two adjacent lines can be used to create a 240V circuit. After reading your posts, it is obvious to me now that I will need 2 CT’s that each should contain the 5 lines from the same phase, Doah!

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Not a problem. I’m in Oklahoma, so am familiar with US residential electrical service.

If you need CTs that will fit the 4/0 mains, I have these 16mm ones in stock.

Thanks John, I will check them out.

I had to trim the plastic on the movable part of the SCT-016’s shell to get them to fit my SEWs.

I guess it depends how thick the insulation is on your mains. 4/0 is 11.68mm + insulation thickness. From what I can tell the most common thickness is 1.27mm with a jacket of 0.18mm in the US in accordance with UL-1083

It can vary quite a bit. e.g. anywhere from 0.624 to 0.635 inches for THHN AWG 4/0 wire.

Thanks for the information Bill and John. Sorry for the slow response, this week has been busy with my day job and everything else that is going on. Here is a photo of the Shop Main Panel and transfer switch (a work in progress):

The Service Entrance cable is 4/0 URD, the outside diameter is .66 inches/16.75 mm. So I will probably have to look around for something else. I have been working first on the emonCMS server and emonTX/Ethernet shields/Arduino integration. But will need to dive into the sensor sourcing in the coming week.

quite a bit loose wire that is neither in the wall , stabled down or in conduit- renovating ??