Is my consumer unit too small to fit an IoTaWatt Installation? (UK)

Hello IoTaWatt Community!

I have a newly installed single-phase system with 14 circuits. All are rated to 40A or less. I’d like to monitor all circuits (using software to compute the total consumption) so I believe I need the IoTaWatt device plus 14x 50A clamps.

My concern is that the consumer unit is relatively small. I plan to install a separate electrical box to the side to house the IotaWatt, but I’m worried there won’t be enough space for all the clamps to operate without interference. I spoke to an electrician and he told me he would want to replace the whole consumer unit.

Replacing the consumer unit is something I’d rather avoid if possible! So please if anyone could take a look at the photo below and let me know if you think I can do the install without upgrading the unit? Is the electrician just looking for more work or is there a legitimate cause for concern?

Looks like it would work to me. Though might be tight in a couple of places.

It is about £8 or 9 to buy a SCT-013-000 on eBay (make sure it is the 000 variant). Price is variable on quantity or this place has sct-006 as well. Current - So you could pick up a couple in the UK and see how they fit before taking the plunge.

You will need the 13 for your incoming mains and ring main (b32) circuits, the 006 (smaller) can do the b6 (lighting). Either of them should do your radial power ( B20).

If you go for economy shipping from the states is is very very variable how long it takes to arrive and whether you get the customs bill, before delivery, after delivery or not at all. I have had units arrive in under two weeks but the last sat in Switzerland for 3 weeks before making the final jump to the UK.

Those breakers say they are 17.5mm. That puts them in-between the USA 1" full-size and 1/2" half-size breakers. I have both sizes in my different panels. The smaller breakers are challenging with some of the 50A CTs. The AcuCT one is 24x26x40mm. That means you need to have enough space to stagger them. It appears you have plenty of space to do that without even bending any wires.

So, you should be able to get them installed without needing to replace the panel, but you might need to find a different electrician.

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Not sure why you intend to monitor just the loads and not the mains, or at least the two RCDs. I’d recommend doing at least the two RCDs. You can still get all 14 loads by monitoring 5 and 7 circuits on the two RCDs and subtracting those loads from their respective RCD total.

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Thank you for all your responses. This is very helpful!

If I can get the AccuCT clamps in there it would be my preferred due to the high efficiency (0.5%). I’ll likely buy a couple and see how they fit. Would it create any issues if I mixed them with the YHDCs (sct-006) for the lights, for example?

I’m such a novice that the thought of measuring RCDs hadn’t really crossed my mind. I’m happy to learn and be pointed in a better direction if needs be.

For some additional context, I’m running Home Assistant and I have a UK SMETS2 energy meter installed which, together with a Hilderbrand Glow device publishes total load consumption at a 10-second frequency over MQTT. The reason for IoTaWatt is to provide additional granularity into circuit-level consumption. I had (probably naively) assumed that the sum of the circuits should add up to what I see on the smart meter.

Do you think it would still be worthwhile measuring the mains and RCDs as well in this case? Should I expect the sum across all 14 circuits to equal the total mains consumption reported by my smart meter? And could you explain if/why I would want to measure RCD consumption if I’m measuring the individual circuits?

Only you can answer your “Why Measure” question but circuit level measurement helps find power hogs and reduce consumption.

Yes, monitoring the individual circuits SHOULD match the ‘smart’ meter but there will be variation. Smart meters have maximum permissible errors so an error in one direction and an error in the other (iotawatt) could occur giving a significant variance. Provided what you want is to see the immediate impact of changes in behaviour the variance doesn’t matter. Less is less.

What is the accuracy of Smart Meters?

Smart Meters have to be approved under UK law to a certain accuracy. The accuracy is defined as the MPE (Maximum Permissible Error) class.

  • Electricity Meters: MPE Class A
    • ± 2.5% at 1 amp
    • ± 2.0% at 20 amps.
  • Gas meters: MPE Class 1.5
    • ± 3%

Many electricity meters fitted achieve the higher standard of MPE Class B (± 1.5% at 1 amp and ± 1% at 20 amps).
All meters will have the accuracy class printed on their front face.

Read more at: Technical information on SMETS 2 Smart Metering - The full story ©


Thank you so much everyone. Apologies it has taken me a few weeks to reply. I wouldn’t have been able to move this little project forward without all your help.

I’ve gone back and forth between the options you shared and ended up purchasing the IoTaWatt kit, 12x 50A AccuCT (5 and 7 circuits), and 2x 100A AccuCT (top and bottom RCDs). I know this will be overkill on some circuits, and a combination of SCT-013-000 and SCT-006 (as recommended by @philrob - thank you) would be more compact, but it just feels simpler in my little head minimising variation and going for maximum accuracy, even if it’s a bit more expensive or bulky.

Thank you @frogmore for explaining the sizing. And thank you @overasy for your advice about measuring top and bottom RCDs. I’ve had more time to think about what you said and can see this makes a lot more sense and it’s what I hope to do.

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