OpenEnergyMonitor Community

New build house: improving on clip on CTs

Hi, I’m new to OEM and not exactly an electrician or an electrical engineer. I have a new build house going up soon and am lucky enough to be starting from a blank slate with the electrical installation.

I can see that the clip on current sensor CTs sold here are awesome for not intruding on any existing wiring. But what if I can put whatever I want in with the consumer unit? Would you still recommend clip CTs then? They seem to take up a bit of space and I’m wondering whether something like these DIN rail mounted things might be more robust:

That one has a pulse output and I have no idea how that would integrate with OEM, but perhaps there are others that have an analogue output more suitable for the emonPi?

If clip CTs are still the way to go, it would be great to see some photos of a recommended way to lay them out next to the consumer unit.

Thanks in advance for any advice.

Welcome, Eliot, to the OEM forum.

What you have there is a complete electricity meter, you could feed ONE of those into an emonPi using the pulse input, but your emonPi won’t accept a direct voltage or 4-20 mA input, it is designed solely for a current transformer.

I haven’t yet checked the full data sheet of that, however if you are looking to have something like that, I suggest you look at those that have their output on a data bus, that you can send more or less directly to emonCMS.

You mention CTs in the plural, I think we need a little more information on your proposed setup to give you the best advice. Will yours be single phase or three-phase installation, and how many points do you want to measure independently?

I think - but heavily dependent on your answers - that if you do want a complete meter and you want to measure several separate circuits, you probably need to be looking at something like this: PZEM-016 single phase modbus energy meter
If you search here for “MODBUS meter”, there are several discussions about different models.

However, coming back to the original question - can you use a ring-core c.t, then yes you can. But you do need to take some precautions. A c.t. should never be left open-circuit, it should be short-circuited or it must have a proper load, called a ‘burden’. Some c.t’s have internal protection to prevent danger should they become disconnected, but you shouldn’t rely on that.

I had found a range of three small ring-core ones from RS Components, but there’s a problem - the web page and data sheet have conflicting information - the RS website is wrong (confirmed in a phone call). They are suitable to directly replace our standard c.t. and connect directly to an emonPi, there are 3 ratings: 25 A, 40 A & 50 A: is the 25 A, is the 40 A. & is the 50 A. (You’ll need to fit a plug, and those are not protected, they must not be open-circuited.) Those are significantly smaller than our 100 A split-core c.t.

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If I was starting with a blank page I would without any further thought definitely use din rail modbus meters. I am currently slowly installing them in my own home as I change the wiring to suit, eg different ring mains for different appliance types and/or zones etc.

Searching “modbus meter” on ebay will give you many options, you can use CT models for the large incomers and any heavier loads where you don’t want to interrupt the circuit and be led by the cable sizes and loads when choosing “direct” meters for any/all the other circuits you want to monitor.

This is less of an off the shelf solution that will involve some learning, but the result will be well worth it.

The data you can collect is (usually) MID certificated (ie the same level accuracy as your billing meter) and you will have a incredibly robust system that will record all energy usage without fail, that is the main difference between meters and monitors. Monitors will monitor usage whilst they are able (powered up, connected and fully functional etc) where as a meter will clock everything that passes through it, always. Only the realtime data (current power) is volatile to gaps if your system fails to collect the current data for some reason (power outage/network etc), the actual amount of energy passed will always be 100% correct, even if you leave it dormant for weeks between reads, it is by very definition “revenue grade” and IMO unbeatably neat, robust and informative at a far more granular and accurate level for similar spend when starting a fresh, retrofitting is a different thing altogether.

If you are considering “din rail meters” definitely get modbus enabled devices rather than pulse, pulse counting is like counting mile markers as you go along a motorway rather than reading your odometer, very accurate if you don’t miss any, but hard to ensure you definitely always count them all. And that slim possibility that you could have missed some also brings a lack of confidence into the mix. pulse counting is ideal for retro installed monitoring (even when counting a meters pulses) where changing/installing meters is impractical and/or too costly.


Even if not starting with a “blank page,” despite the learning curve, the results are definitely worth it.

Hi Robert can you please confirm what these CT’s are exactly? From what I’ve just read on both the NZ links given and the UK site for the same CT’s, these are apparently voltage output and have 33R burdens, or is that not so? The web pages say “voltage output” and the datasheets also state 23R burdens (as a spec rather than an additional suggestion/requirement)

The erroneous RS web page, both on the NZ address and the UK one, does not state it is voltage output, the error is in the ratio, which says 25 A / 40 A / 50 A primary current and 1 A secondary current. The data sheet that I am reading, which is the Talema one - and confirmed in a phone call to RS - says that the output is 50 mA as inferred by the ratios, and confirmed by the graphs.

Where does it say that? I can’t see where it states the burden is internal, I can’t see where it gives the transfer ratio in units of V/A, I can see what I’d expect of a true current transformer which is the nominal turns ratio, the secondary winding resistance, and I can see a graph that gives the output voltage with a rated burden and a value for that burden. I haven’t seen either the graph, winding resistance or the burden value on a data sheet for one with an integral burden and a voltage output.

I wasn’t challenging what you say, I was just interested in CT’s for smaller loads and you said they were direct replacements for the OEM one (bar the connection of course). But when I looked the webpage descriptions said “Very high output voltage & Linearity in small size”, assuming that could be the error you found I checked the datasheet which also states “Very high output voltage & Linearity in small size” and refers to a 33R burden without saying “optional” or “example” or “mandatory” etc, so I then believed the CT was “Very high output voltage & Linearity in small size” due to the assumed to be inbuilt 33R burden and decided it was best to ask rather than make further (incorrect) assumptions. I even checked the UK RS webpages in case the error was specific to a RS region.

So, from what you’ve said would I be right to assume then that these are current output CT’s, implied by the current to current ratio’s and that the 33R burden is just an example or perhaps even a recommendation and that they will work just fine with the in-built 22R burdens in the emonTx?

Again not challenging, just trying to understand, how will the different burden effect the range and performance? I know normally a lower burden will bring greater linearity so that is maybe less of a concern, but what would the range of (say) the “25A” CT become when used with a emonTx, given the smaller burden and limited input range?

It also appears in the graphs there is some headroom given the CT’s seem to offer some linearity beyond their rated values, perhaps suggesting they are perhaps pretty good quality. Would using the “rated” 33R burdens be better suited in your opinion?

Now looking closer, I guess the “DC Resistance in Ω Nominal” for the 40A and 50A models (35R and 72R) would confirm there is not a 33R in parallel with the coil and perhaps the word “Very” in “Very high output voltage & Linearity in small size” should of rang bells, I took that to mean high sensitivity, not so much an open-circuit warning!!!

You need to turn your brain around completely from the familiarity of voltage sources (like batteries or the mains, or even the a.c. adapter!), when dealing with current sources, which is what a pure current transformer behaves as.

The c.t. sources a current. That secondary current is proportional to the primary current. The c.t. also, like every other transformer, has a VA rating. Unfortunately, few manufacturers and distributors specify it directly.

Most ADCs are looking for a voltage. The way to convert a current into a voltage is to use a resistance, specific to a c.t, it’s called the burden. (Historically, the burden would be an electro-mechanical instrument such as an ammeter or a wattmeter current coil, or a watt-hour meter current coil, or a combination of these.) Therefore, the value of the burden directly determines the voltage that appears across it.

The value of the burden also determines the power that the c.t. needs to deliver. That’s where the VA rating comes in. When the voltage becomes too great, the c.t. is unable to deliver the power, and the voltage you expect no longer appears. You can see that in the droop of the graphs on the data sheet.

So — the value of the burden has absolutely no direct effect on the rating of the c.t. But the voltage that the ADC can withstand, reflected back as determining the maximum secondary current and hence the maximum primary current, does affect the maximum that the combination of c.t. and instrument can measure. And that is precisely the case with input 4 of the emonTx, where the higher burden reduces the rating of that input to 18.33 A when using the 100 A c.t. [It doesn’t change the c.t’s rating.]

No, that wasn’t the error. The error is the headline ratio: “25:1” which defines a 25 A rating and a 1 A secondary current, and that’s in conflict with the data sheet that gives the rated current as 25 A but the nominal turns ratio as 500 : 1. ‘Very high output voltage’ defines a high VA rating, it’s not an open-circuit warning. ‘small size’ is a consequence of the ring-core construction and (probably) a distributed secondary winding.

The simple fact that the burden value is mentioned means to me that it isn’t integral. I’ve never seen it mentioned by any other manufacturer in their data for the voltage-output types. The value is the maximum recommended value - clearly good to give 1.6 V rms at the rated current, therefore good for a 5 V span ADC, and as I’ve so often written here, any lower value to suit the lower voltage that you might need - including a short circuit (when of course the c.t. is delivering zero power into its burden, but still dissipating the same power internally - I²R using the rated secondary current and the winding resistance).

And then what? Use a voltage divider to reduce the burden voltage to the 1.1 V rms that the 3.3 V ADC of the emonTx / emonPi can handle? :exploding_head:
No, I would regard 33 Ω as the maximum recommended value. That, with the 25 A c.t, would give you an input rated at 16.66 A for the emonTx/emonPi, but it would be much more accurate at low currents than the 100 A YHDC with the 120 Ω burden.

Maybe we can persuade @glyn.hudson or @TrystanLea to obtain a sample of each so that we can verify the performance and measure the phase error etc.

Magnelab publishes the burden values used in their voltage output CTs.

Burden-Resistor-Chart.pdf (46.2 KB)

Resistance of an SCT-0750-150 CT measured 17.1 Ω The chart says 16.3 Ω
An SCT-0750-000 (Unburdened) CT measures 500 Ω
(I used a Fluke 27/FM to do the measurement, but I have no idea how far off its calibration is)

But, as you mentioned, I’ve not seen any of the other manufacturers do likewise.

And I haven’t downloaded that pdf, so it’s true that I’d never seen it. :wink:

But I have sent it to you. (many moons ago) :wink: :grin:

I’ve got nothing containing “burden” amongst my saved data sheets, so maybe I never saw it. I certainly don’t remember it.

But, wait a minute, that doesn’t say it is the value of the internal burden of a voltage output type, only that it’s the resistor value to give the standard voltage with a current-output type. And the two won’t be the same unless they keep the same number of secondary turns between the current- and voltage-output types. Magnelab might, YHDC don’t, their voltage output ones have a few less turns - presumably to enable standard value resistors to be used.

Aye, that it does. That’s why I took the two measurements.
The value I measured for the SCT-0750-150 differed from the chart by only 0.8 Ω
I realize that doesn’t mean they use the value in the chart is the internal burden value,
but the values are quite close, nonetheless.

You forgot the parallel 500 Ω. You were measuring 17.7 Ω // 500 Ω = 17.1 Ω, so 1.4 Ω (8.5%) adrift.

Yep. I sure 'nuff did. But still fairly close. Given my DMM is quite old, and I have no idea regarding
its calibration, that’s still not too bad.

I have found a definitive answer to:

but of course after I’d sent an email to the distributor.


“Custom lead lengths and optional internal 33 Ω 0.25 watt burden R can be supplied on request.”

Thanks for the quick and detailed responses, everyone. I did a it of reading here and elsewhere on MODBUS and RS485.

Single phase and I’d like to measure about ten circuits (not points) independently. I had in mind to run separate circuits for all the heaviest loads, eg. one for the air source heat pump, one for the oven, perhaps one for the fridge. All the lighting on another, sockets on another etc. I’m sure the electrician will tell me I’m wasting cable but it seems like it’ll be worth it for the learning. The goal is to get a good understanding of where all my consumption is going so that on the next house, which will have solar, I can size the PV system to just meet demand.

Did we reach a conclusion on which meters to go for?

For the DIN rail mounted ones, how are these wired back to the emonPi and where are the bus wires themselves? I’m having trouble visualising how this is all wired up.

How do I set a slave ID on any of these MODBUS meters? Do they have jumpers?

Cool that OEM has such a helpful community - thanks again for the help. Happy new year.

I put together an emonhub interfacer to read from a single SDM120 modbus meter that’s documented here the software behind this would need extending to handle multiple addresses within the single interfacer, so unfortunately not an out of the box solution for what you want to do just yet.

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That sounds normal to me. My electrician was the one telling me to put in more radial circuits. Multiple lighting circuits - one upstairs and one downstairs but with the odd one cross-wired so you’re not left totally in the dark if a circuit trips. Multiple rings for general sockets to meet current limits, and radials for all ‘power’ connections etc. The fridge lives on the kitchen ring - I measure it’s power with a plug-in meter. Ovens, hobs etc are on radials.

Sure sounds like 10 of those din rail mounted meters are going to take up more space than 10 CTs.