I’ve wanted to add power monitoring for our dishwasher, washing machine and tumble dryer for a while. In the scullery, where these are stationed, there is a dual socket under the counter, which means a multiplug needs to be used to power all three appliances. The implication is that you cannot install e.g. Shelly’s PM units as (1) there are only two sockets and (2) it’s unlikely to fit into the wallbox.
Another option is to install power monitors inside the appliances, but there are two potential issues: (1) the metal casing which will affect the wireless signal and (2) voiding any warranties.
A third option is to just cut the power cord and install them inline, using a 3D printed housing. I don’t own a 3D printer, although I could easily enough have asked someone to print these for me. But again, I wasn’t sure how strict the warranty T&Cs for our new dryer would be, so I opted for not making any changes to any of the appliances. Another factor influencing this decision was that I wanted to try something more generic which I could repeat in other places around the house, e.g. the unit where all the multimedia appliances are in order to clean up the cable and adapter mess while adding some power monitoring.
For this reason, I though I’d make my own multiplug using three Shelly PM minis (I didn’t have a need for relays in this case).
The bill of materials:
A piece of power rail, like what’s used in offices around the skirtings for power.
Endplates for the power rail.
Suitable sockets that fits in the rail.
A hacksaw to cut things up.
Cabtyre for wiring (check your power ratings to ensure you use a safe wire gauge).
A plug. (I drilled a whole for the extension cord and used a grommet to make it neat.)
Connector blocks, but if you fancy you can go for WAGO connectors (again, check the power ratings).
We made things out of paper, boxes, plastic pieces and wood! Of course, not nearly with the complexity that you can now, and you had to get a bit more creative.
Looking at your power graph, if all 3 appliances were on you could be pulling nearly 5000W, at 230V that’s 21.7A, can that single plug handle that, I thought SA plugs were 15A max ?
Yes, you’re right: there is a max rating (16A). It is typically to protect the wiring. To answer: It can handle it, but within limits. It’s a bit complex to explain quickly, and I’m not a qualified electrician (while I am en electronic engineer by education), but it’s similar to why breakers don’t trip immediately. There are tolerances for certain periods.
We also don’t run everything at the same time, typically, and I’d say I’m in a better position with this monitoring than before, as I can take appropriate action should there be an overload risk. Appliances also don’t all run at their max all the time, so that helps too (the inverter tumble dryer has a pretty steady load, which is different to the others).
As for my DIY solution, I wanted to make sure that at least what I’m installing won’t contribute to a disaster, by using at least the same or higher ratings. My house’s wiring is also adhering to regulations, so I feel pretty comfortable otherwise, since I use everything as installed/designed.
The first (and most common) misconception is that a breaker trips when its nameplate rating is exceeded. One fire text has stated (incorrectly) that a circuit breaker will trip in several minutes with a small increase in current over its rating. Actually, a 20 amp breaker must trip at a sustained current of 27 amperes (135 percent) at less than one hour…
Fair enough, I’d still be uncomfortable with the possibility of running nearly 22A through a plug and flex that is rated at 16A, but if you are happy with it, it is a good solution.
Just read what I sent (the quote and the PDF): It’s not about me being happy. It’s about understanding what these ratings actually mean. There is a myth section in that PDF, and it explains how to read the ratings: It’s about continuous loads and nothing will happen at exactly the 16A point when that threshold is crossed.
I’m not so sure about this statement, although the document is dated. Here is a time current curve for a standard QO 20A-1P breaker quite regularly used in the US. It at appears that this breaker will trip at 25A (125% of the rating) between 45 and 400 seconds. Mind you, most breakers are 80% rated, so the continuous load calcs shouldn’t exceed 16A on a 20A breaker… in the US at least.
Edit: In either case, tripping quicker is safer… so I wouldn’t be too worried about it - if your home wiring is to code. The breaker will protect the house wiring. The only wiring not necessarily protected would be the cord, but that is a short length.
I think we’re illustrating the same principal. The main point being that a breaker doesn’t trip instantly at its rated value — that is the misconception. I don’t know anything about other countries, but would imagine that there will be some variation.
100%, but only when there is a risk, and understanding a bit of physics here is important.
In my case I used the same gauge wire that’s used in the conduits. The connector blocks have a very high rating too, and I made very, very sure everything is secure to avoid any kind of arcing, with no wiring exposed.
That’s a good excuse for adding a microcontroller (like an ESP32 with ESPHome) to monitor the current on each socket
I have started thinking about doing this for my laundry downstairs to let me monitor the chest freezer and let me know when washer or dryer has finished. Got to be a better solution than 3 of those smart power plugs each with its own power monitoring, microcontroller and wi-fi. I particularly like Pieter’s use of the power rail as a housing.
The current is monitored for each socket – with 3 Shelly PM Minis. (EDIT: I think I misunderstood you at first, and thought you didn’t realise it’s individual monitoring.) The overcurrent detection can be done in HA, but would be possible with Shelly’s own scripting too.
I agree, I would’ve like to avoid that, or at least gone for BT over WiFi (I don’t have a Z-Wave or Zigbee network). Shelly has the 2-channel PMs, but I haven’t really seen devices with more, but I also haven’t search too hard, not wanting to bring in another brand.
