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Hopefully a blindingly obvious point - the meters are meters and DON’T include isolation and protection, so you cannot replace your circuit breakers with meters, you need both. Of course, your electrician wouldn’t have allowed you to do otherwise.

You’ll either need a very big consumer unit, or a separate but adjacent box to house the meters. The nice way would be to put the meter adjacent to its breaker, but depending on how many other circuits you have that aren’t metered, you could need a very big consumer unit - 2 banks of 20-way is the biggest I know of.

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FWIW, we have two adjacent consumer units because of the number of circuits we have. So it would be possible to put meters adjacent to MCBs (or RCBOs) by addding additional CUs if necessary. They’re simply fed from a Henley block to split the supply.

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I would say that is good practice, I rewired some of our circuits eg the freezers have their own circuit so that I don’t come home to defrosted freezers because (say) the dishwasher has had a moment and tripped the kitchen ring main, plus I can monitor them separately and also control them, forcing them to run more during off-peak tariff periods by disconnecting the power during peak times if the temp remains low(ish).

I did say

There isn’t a simple dedicated “bus” for this, you would need to use a USB/rs485 adapter and either expand on the current emonhub “modbus” offering or write a custom script that polls your devices and posts the data, either via an emonhub socket, MQTT or HTTP. Can sound daunting but there’s a lot of help out there, I would certainly chip in. I have my own modbus interfacer in development and whilst I’m not at the stage to share just yet I have no issue with sharing concept’s or code snippets etc. My interfacer is aiming to be universal to all modbus devices, by either simple custom config options or by templates, I have a dozen or so devices templated and have had them all running at once on one bus, but other stuff has had priority lately, but this is a pet project that I would love to get back into.

No no jumpers, all the devices I have require the Id to be set via modbus so I have a little script that I use on a one to one basis with each new device to set a unique id before adding it to the bus.

I would say they are around the same size but as the meters are rail mounted they are much neater and since they are mounted they usually have a box, rather than leaving them hanging on exposed cables or cramming them into the CU. They are (IMO) much better for the same space used in that they are hardwired, revenue grade, neater etc for the same money/space, not because they are smaller/cheaper.

You can’t really mix breakers and meters easily as the busbar fingers get in the way of the meter conns. Yes you can cut them off but it’s still tight getting a cable into the meter, especially when it’s a short fat cable from the adjacent breaker, any chaffing of cable on sharp busbar finger remains, could short (bypass) the breaker if not careful.

You can use an unpopulated CU if you want the meters on display, I tend to prefer a simple box with top hat din rail at a depth that puts all the meter faces just below the lid face and allows any wiring to run behind/under it. Since you are reading the meters via modbus they needn’t be visible. The plain box lid is tidier and perhaps provides somewhere to put a schematic or legend or even some indicators or buttons etc should your system grow. Not to mention being cheaper to boot, so a big box to house all the gubbings is cheaper than a CU for just the meters.

“gubbings” = meters, din mounted Pi, din mounted 5v supply, din mounted relays etc?

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The specific question was, would they go in the C.U. But I’d go with the separate box - it will also make segregation of the low voltage data bus wiring and the mains that much easier as you have more control over the internal layout.

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Very helpful thread and wondered if you had been able to progress this, eliotstock?

I’m in a very similar position, about to embark on a complete rewire of a house and envisage 3-5 DIN-mounted modbus units in my CU to submeter an EV, Heat Pump, Immersion Heater etc. to be read back to my emonPI. I had a couple of questions:

  1. I’ve read up on Trystan’s interface for the SDM120 but understand this only supports one modbus device currently. Has anybody had any success with a multi-device handler yet?

  2. As eliotstock mentioned, I’m also having trouble imagining the physical installation. Assuming the modbus meters would need to be daisy-chained inside the CU, could this be handled by way of a single ~10m Cat5 run back to the emonPi? Excuse my ignorance on modbus wiring standards…

Terrific community here - really appreciate any guidance out there.

Thanks.

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Welcome, Ben, to the OEM forum.

I believe my colleague @Bill.Thomson uses Modbus to obtain data from his many inverters, but he’s having his house re-roofed (16 solar panels to be moved and reinstated), so he’s unlikely to be around very much for at least a couple of weeks.

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Many thanks @Robert.Wall

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This might be helpful.

Simon

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Extremely helpful, thanks @Bramco. I’ve been looking for an install guide like this for a while. Much obliged.

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We’re building a new house, so I’m going through the same thoughts. I built one of the diverters which has been doing great service for a few years but I’m thinking of replacing that with a commercial unit and using SDM 120s to monitor usage, PV generation and EV charging point usage and eventually a battery system.

However, one of the EV points and the solar will be on a separate CU in the garage, the rest in the house. So I can’t hang all the SDM 120s on the same bus.

I’m now looking at RS 485 to wifi units - that way I can pick up the readings from wherever I site the server with emoncms on it. These are DIN rail mounted. Might buy one and a meter and have a play to see if it would work. USRIOT Din-Rail Serial port RS485 to TCP/IP WiFi Ethernet Serial Device Server | eBay

Simon

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Very interested to know how you get on with those 485-to-Wifi units. I may have a similar challenge in the nr. future with 2 CUs. Thanks!

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Indirectly, as it’s an Elkor WattsOn and PZEM-016 the data is coming from vice the inverters themselves.

I’ve had sucess with reading data from more than one modbus device, but the data isn’t collected/visualized with emonCMS. So, unfortunately, I can’t help the you WRT an emonHub interfacer.

Here’s the modbus spec document.

The article Bramco linked to has some info that differs from what’s in the spec doc.
e.g. the author mentions a 700 meter max bus length. The spec doc says 1000 meters.

He also says

The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible.

The branches must be no longer than 1200 m! Longer branches could cause signal reflections and
generate disturbances and consequent errors in the reception of data.

Below that is this drawing

image

Where you can see the max stub length is depicted as 1 meter.

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wondered if you had been able to progress this, eliotstock?

Nope, I’m still a few months away from the new build getting to the point where I’m ready for this. I’m still eagerly following this thread though.

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Good catch Bill.

If the meters are all in the same CU any stub lengths would be measured in CMS in any case.

I guess it’s a bit like setting up the wiring for a one-wire bus for temperature sensors. Keep things as short as possible.

Simon

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Good luck with it. Interested to hear how you get on.

Thank you Bill, well noted and appreciate the link and advice there

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That’s true.
Seems like stubs would be more hassle than not, especially in a cramped area like a Consumer Unit.

YVW, S! thumbs_up

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  1. Trystan’s sdm120 interfacer is a very early release, it is hardcoded for just one sdm120 and 2 other users are having issues with using it with that same device. So don’t rely on that interfacer too much at this point as it already serves it’s original purpose, to read the one device Trystan has, so further progress may be slow. I hope I’m wrong though and I hope to help debug it but that hasn’t happened just yet.

    I do modbus a different way and yes I’ve had a lot of success with multiple devices, I have a test bus currently with around 12 different models of modbus meters, some duplicates like 2x sdm120’s and 3x pzem016’s. That bus also has 2 temp and humidity sensors (1x rail mounted and 1 on a fly lead). That collection I can read once every 10s (haven’t actually tried any faster) with 100% success, through a coil of around 30m of Cat5e (just to add some distance).

    So I do have quite a bit of experience with coding for these devices and I’m willing to assist anyone giving it a go. I’m not ready to share my code just yet as it isn’t complete and once these things are out there, they can be taken out of your hands and end up being being something entirely different. My aim is for a generic interacer that works with a mix of many meters and provides user definable data frames, that’s a litle harder, but coding for one or 2 specific models and/or a handful of the same model on one bus would be pretty straight forward.

  2. As I mentioned earlier in this thread I wouldn’t install the meters in the same CU unless (for example) you were using a 2 gang CU for one circuit, ie 1x breaker + 1x meter (you would need to cut the busbar) or (for example 2) you got a double row CU and removed the busbar to one row.

    You need to think of the meters as another row/layer/level to the breakers as intermingling them it physically difficult due to the busbar and not as aesthetically pleasing as you would expect just because they are the same shape. Far better to have a section/row/CU or breakers and then a section/row/CU of meters, you of course install them in the same order and/or in the case of a seperate CU/box, align those so each meter is in line with the relevant breaker, but it’s not important as long as they are well labelled, unlike the breakers that you may need to access, if you are remote reading the meters and that works well, you may never need to cast your eyes on them again.

    I use 1 twisted pair in a Cat5e cable and yes they should be simply daisy chained.

Be wary of following any single guide, just as Bill has highlighted some discrepancies in that guide, there are so many “experts” out there with differing info, based on differing experiences. I find rs485 (the physical bus) quite forgiving to work with. I read the linked guide too and also some of the comments below it, when questioned “why 32 devices” the author replies that it’s fixed in the protocol. That may have once been true but you can now get what are refered to as “1/2 nodes” and “1/4 nodes” which present just a proportion of the load on the bus of an original spec’d device (not sure of the exact “load” in this respect, capacitance, resistance etc) so the number of devices is also determined by the device types eg you could have up to 64 x “1/2 nodes”. I do have a couple of half nodes on my test bus.

As long as your garage isn’t over a Kilometre away from the house you can run a cable and have them on the same bus. I understand the appeal of separate busses and if a cable is not possible (undesirable) then yes 2 busses will do the job. Personally I would avoid any wireless connection where I can get wires to, it makes a permanent connection and keeps the wifi congestion down. I know many prefer the opposite and prefer the “invisible” wifi connections for everything, each to their own :smile:

The rules are slightly different in that the distances between the master and each device is not important in rs485/modbus but with 1-wire you should always ensure that each device is a unique electrical path’s length from the master, prefrablly no 2 will be within 30cm of each other (overall master to slave distances) as the one wire master can put out a single call and rely on the time taken to respond to collect all the replies, so in some places adding an extra foot of cable can improve things with one-wire busses.

indeed, or in fact i would expect to see no stubs as the 2 cores would be cut and wired into each device. I noticed the reference to “in countries where that’s allowed” in the guide, but have not experienced any such restrictions myself, my “installation” of such devices has only occurred at home or in the workshop, onsite installation of the few devices I have out in the field were installed by the electricians at build time.

On a more general note, why is the sdm120 so common or in demand? I have many different devices and see no obvious reason for that device to stand out. Am I missing something?

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Paul, thanks for taking the time to share your experiences and to set some things straight.

As for popularity of the SDM device, we shouldn’t confuse mentions here with popularity :-D. If like me you’re new to this, then you tend to pick up on the one that folks have had success with. Google certainly turns up several other makes - some a lot cheaper than the SDM 120.

For those of us new to this, it would certainly be easier to work from an existing solution, which probably means using the device others have had some success with. Maybe that’s why only the SDM120 has been mentioned so much.

Simon

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I’ve only just found my way onto this forum so after browsing through some of the threads saw your question. Have you looked at Schneider Power Tag, I’ve not used them myself but have heard good reports and are an off the shelf item made by Schneider so comply fully with regulation 536.4.203. If I’ve got the wrong end of the stick please ignore my comment.

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They look very interesting.

From the spec and pictures it looks as though they are a CT with a RF wireless device built into the housing. Seems like the power cable goes through the large hole and there’s a neutral connection that has to be added.

Certainly look neat. Wonder how accurate they are? Are they meter grade?

Simon