SCT-013-000 100A not accurate enough?


I only seem to be able to get the 100A CTs here in Australia at the moment. I’m concerned that they won’t be accurate enough for my branch circuits (10A, 16A, 32A)…? Also, my 3 phase mains are 63A. This is all 230V.

Any advice muchly appreciated :slight_smile:

Edit: is there anywhere I can buy different CTs (like the 50A and 100A ones recommended in the Docs)? Reading these forums, it seems like the SCT-013-000’s aren’t recommended for anything but the main power inputs? I’ve tried several messages to the smartguys here in Australia but I never get a response.

I am surprised you have not received a response so will give it a go. However I am a beginner with energy monitoring so take that into account.

I have 4 of the SCT-013-000 devices and am quite satisfied with accuracy. What accuracy are you looking for?

Are you aware of the prolific number of clones on the market, labelled YHDQ instead of YHDC?
YHDQ SCT-013 clones

Thanks so much for your reply!

I was just wanting to confirm, before buying 10 of the SCT’s, that they were accurate enough for my lower Amp circuits (10A, 16A, 32A). I’m fairly certain they are fine for my main inputs (63A). The iotawatt docs say the SCT’s are ok as CTs but I’m not sure how accurate they are for the lower amp branch circuits. I can’t put any sort of accuracy number on this (I’m a complete beginner) but the fact that you’re using 4 of these (I assume some are on lower amp circuits?) is encouraging.

Yes, I read about the dodgy ‘Q’ clones. The vendor here in fact is selling that type also, but has warned people against them. The ones I want to buy from him are the correct ‘C’ type, so thanks for checking that.

I had considered getting some of the 50A CTs from other vendors recommended here, but I can’t seem to find a vendor for those who ships to Australia.

Posting the comment inspired me to do a test of low amperage. The attached image shows the results of progressively turning on 24 LED lights, most rated at 9W, some 8W and some slightly more. At the start there is a standby current of 7-8 watts. I think accuracy at that level would be difficult, but it seems reasonable with several smart lights and dimmers on standby. Viewed in isolation with no lights on it is quite choppy, but with peaks and troughs within a 2 watt range. That seems negligible to me.

In the attached chart, the first light is switched on just before 11:40am, then in groups of 4 … so just under 40w per jump. The last two jumps each included 2 x 12W lights. Sometimes there is a little delay in the chart, accounted for by me adjusting the brightness. So based on light specs, I estimate 228W altogether and that seems to match the chart pretty closely. I have also been testing with my cooktop for higher usage up to 4,800 watts and am equally satisfied with the result.

Lighting test

You’re right - the numbers do appear to show that the accuracy is indeed quite good! I think I’ll go ahead and just buy the 10 x SCT’s for my initial setup (even though they are physically a bit bigger than other CTs).

Just out of curiosity because I’m interested in your setup and I’m thinking about how I’ll do my setup, what CTs and on what circuits do you have everything connected?

I originally started with an Emporia but was unable to get it working on my Australian 3 phase power. The documentation says it should work but after numerous support chats, Emporia finally agreed they could not get meaningful readings. Reported voltages were crazy, so the usage reported was similarly wrong.

That kit came with 3 x 200A CTs and 8 x 50A CTs so I decided to use them with the IoTaWatt I then bought. They are voltage based rather than current based and if you look on here, you will see my post about using them. I am also quite satisfied with them (mainly the 50A ones) but still experimenting.

You asked about my setup. I have 3 phase power (no solar) to an external meter box. My HWS runs on Phase A from there. The 3 phase power runs from the meter box to an internal sub panel in my garage. Everything except the HWS runs from that sub panel. The 3 phase air conditioner is the only 3 phase item.

I have 3 x SCT013 attached to the mains at the meter box and 1 Emporia 50A on the HWS. I made a 27 metre extension cable using 22AWG Cat6 cable (just for the conductors, not data), and feed these four signals via conduit to my IoTaWatt which is mounted next to my sub panel in the garage.

The sub panel has 8 breakers … 3 for the air conditioner, 2 power, lights, oven and cooktop. So I monitor all those, making a total of 12 out of the available 14 inputs in use.

Thanks for the detailed explanation, that’s really useful (and I’ll definitely have a look at your other post). Your home electricity setup is 99% identical to mine (I’ll post a couple of images).

This is a new house in Brisbane. I have 3 phase coming into the external main box on the outside of the house. It has 2 x 63A breakers (I always wondered why there were 2 of them - do you know if that’s normal? I do have that question sitting with the sparkies who did the power install, so I should eventually get an answer about that). All of that power then routes to the internal breaker box in my garage. The only difference vs your setup is that I don’t (yet) have an electric HWS. I have AC, stove, lights, power points and no solar. I don’t yet know which phase each sub-circuit is on yet, but I’ll audit that during my iotawatt install.

It looks like you are using the 50A CTs to monitor your internal circuits like power and lights. I was looking at using the 100A SCT’s for that so I was wondering about the accuracy. I still might try to buy some 50A CTs. I think I can buy some eChun’s from Aliexpress, but still need to figure out the best model (I was going to use the amp type, not the voltage type).

Regarding your extension lead, that’s a smart solution and saves having to buy 2 x iotawatts and also saves having to mess around with an external, waterproof box. As I don’t have a HWS, I was wondering if it was possible to monitor the main incoming power at the internal breaker box (so no need for the extension cable) where it comes in before it splits out to the branch circuits. Do you think that is possible?

No I don’t know why there are 2 x 63A breakers at the external box. I would ask the electrician to make a drawing.

Wlth everything at the sub panel I would monitor the 3 phase inputs from there. I can’t see any benefit in doing it at the external box, and you don’t introduce any issues due to a long extension cable.

The lighting test I did was with the 100A SCT013. But that is temporary and I will be using 50A Emporias for the 9 usage circuits. Still working out correct settings.

With 3 phase, one thing you may not have considered is the voltage reference transformer. If you only have one as I do, obviously it only senses voltage for one phase. The “Derived Reference Voltage” setting manages this, so IoTaWatt then assumes all phases are the same voltage. I find when I run several high amperage devices on that phase, the voltage drops maybe 5v. Anything on the other phases is then reported with slightly different usage. And the opposite applies when devices on one of the other phases are consuming high amps. I will live with that because I don’t have power points on either of the other two phases.

Oh right, thanks for clarifying (and sorry for my misunderstanding!). In that case, the simplest option for me will be to order the SCT’s and deploy them for everything in my internal breaker box.

I’ll be using the “derived” method for my 3 phase main input so that’s a good tip re: the small differences between phases - I’ll keep an eye on that. Then 1 x SCT per each of the sub-circuits. That will leave a few spare ports on the iotawatt (I’m hoping to get an electric HWS after Christmas which will add a circuit, and I’ll ask the sparky to install that into the internal breaker box).

This has been incredibly helpful, I can’t thank you enough. I was kinda stuck not having the confidence to proceed with using the SCT’s for the sub-circuits.

I have contacted the original installer and asked them if they can give me a layout diagram of how everything connects, what phases each device/circuit is on, etc. Hopefully they can supply that. I’ll also be doing a complete on/off test of each circuit during my moving in at the house next week so should give me the start of a “map” of all the connections.

Cheers :slight_smile:

What got lost in this discussion is that the Emporia CTs are 333mV (rms) output which works out to about 25% of scale with the IoTaWatt ADCs measuring 0-1.16V (rms). When you use the SCT013-000 with the 20 Ohm burden, you use the whole range. So while the IoTaWatt has 12 bit ADCs, the effective resolution is about 10 bits with the Emporia CTs. When you use an SCT013-000 in place of a full scale 50A CT, you have an effective 11 bits. In either case, the IoTaWatt produces good results, but neither is optimal.

If I understand what you’re saying (probably not LOL) … the SCT’s will use less bits = lower resolution measurement figures captured, so the measurements are rounded a bit more, kinda like comparing 9.5 and 9.48 … less precision measurement. But otherwise good results (I don’t need super precise measurements). Thanks for confirming that. I have everything ordered and being shipped now. Looking forward to my Christmas project :wink:

I think Overeasy was saying the opposite. I hope I am not overstepping by commenting here but it would be good to get my understanding confirmed.

From Overeasy;s comment. the way I understand it, the 50 amp Emporia CTs would have less precision than the SCT. With 12 bits the IoTaWatt has 4096 possible values when the CT is matched to use that range, as the SCT is. The Emporia 50 amp CT specs show 333mv at 50 amps, so only (approx) one quarter of the IoTaWatt’s range is being used … 10 bits, or 1024 possible values.

So 50 amps divided by 1024 gives a resolution of ~0.05 amps. At 240 volts that woulld be about 12 watts.

For the 100 amp SCT, the resolution would be 100/4096 = 0.024 amps, or 6 watts. A 50 amp SCT would have a resolution of 3 watts. Hence, the benefit in sizing the CT for the anticipated maximum amps.

In practice, as you can see from the graph of the SCT I posted earlier, the IoTaWatt smooths these readings.

By the way, it was never my intention to suggest using the Emporia CTs.

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Thanks for that. I ended up ordering a bunch of the SCTs, and the more I thought about it, the more I realised I’m happy with “good enough” stats which I can cross-reference to what my main meter says. Iotawatt will help me track down any energy hogs (including the deadliest one of all time … teenage kids taking looooooong showers! LOL). Yours and @overeasy’s comment have helped me get to the “let’s do this!” stage :slight_smile:

For the things many people want to do, “good enough” is usually quite easy to do. Many people assume they need super precise measurements (or they want it, because). But is it 200W or 205W rarely matters. Knowing if it is 20W or 200W frequently does.