HIgh Current (20amp) Smart Switch?

Are there any switches which can be controlled by Alexa which can handle at least 20amp at 240v?

I am trying to trying to make a 3.6kw electric storage heater smart, if there’s not a switch I am considering using a contactor which is controlled by an arduino or Sonoff.

Thanks in advance.


There is this but its expensive !

I had seen that but as you say it’s expensive and even more so by the time I add a z-wave hub and temperature sensor, if I didn’t need to buy the hub I would probably just have bought it as it should be a safer route to what I want to do.

You can connect a contactor to the switch. The switch will then simply trigger the contactor, and the contactor will handle the 20amp load (assuming it’s rated for it, of course).
I’m using a similar setup to control a water heater.

I had considered a Sonoff TH10 or 16 and use it to switch a 25amp contactor, putting it in an enclosure but wasn’t sure if I would need a heat sink.

Is that similar to what you have?

In my case it’s a Secure SIR321 switch controlling a 25amp NO contactor (BTW in my case the contactor also disconnects the neutral lead, which might be required in your case as well, depending on regulation).

Yes, I was planning a double pole contactor as it would need to disconnect both live and neutral, what contactor did you use?

My electrician got one for me, so I’m not sure, I can check when I got home, but it was based on my specifications of:

  • 25 amps
  • double pole
  • NO

Also, based on the fact you’re awake to respond now I’m assuming you need 220-240V one, but not 100% sure :slight_smile:

Yes, I’m in the UK so 220-240V, something like this rather than a panel mount https://www.tlc-direct.co.uk/Products/WYMESB20.html

I’m not an electrician, so take this with a grain of salt, but I believe 20 amps should be enough with your 3.6kw load (which is about 16 amps), but I opted to get a contactor that is 25 amps, just to be on the safe side.

It’s on a 20amp spur so 20 should be plenty but I may well use a higher current contactor as the price difference isn’t much!

Just built one using a shelly1 and a 40A contactor to control my water heater.
A replacement for a 30A load switch from insteon that croaked.
Contactor and case from Amazon, shelly1 from Allterco

CONTACTOR 2 POLES 40A 240V (2 Pole 40 Amp 240 Volts)

[LeMotech Waterproof Dustproof IP65 ABS Plastic Junction Box Universal Electric Project Enclosure Pale Gray and Fixed Ear 6.3"x6.3"x3.54"(160mmx160mmx90mm)]

I added a push button and indicator light as well.

Thanks All

I’ve gone the route of a Sonoff TH16 + 25amp 2 pole contactor and enclosure as that covers my need for a temperature sensor, Alexa control and ability to handle 3.6kw storage heater, total cost £55

I would be a bit wary of the Sonoff TH16. I had one connected to my immersion heater whilst we were having building work done recently, and the builders failed to switch back on the water leaving the tank dry. The Sonoff burned out, I think because the wire terminals are not very robust (flimsy push-type rather than screw terminals), and very nearly started a fire.

Same thing happened to this guy:


Probably worth avoiding for high load applications, despite the power rating.

I’m just using the Sonoff to switch the contactor so there will be very little load, I had read about the Sonoff and that’s the main reason I am not attempting to use it to supply the actual load current, technically I could have supplied 3.6kw but I wasn’t going to risk it!

You wouldn’t happen to have a write up of any kind on this would you?

Not really, no formal writeup. Somewhere I have a half-a$$ schematic that I can post.

The thing about the contactor I bought is that it is bristling with push-on connector blades that makes the connections to the push button, indicator lamp and shelly1 super-simple.

A schematic would be awesome! Keeping the HW heater off during peak times would cut down on the electricity bill a lot I think.

I don’t know your background, so I will caution you to be very careful with this. I have been using my shelly-controlled contactor for several months now and it has been working fine

FWIW, when I first moved into my house, the electric water heater had its own meter, so I could tell how much I was spending on hot water; about $60/month in the early 80s in New England.

I put a “Little Grey Box” mechanical timer on it and my bill was cut in half. So your bill should see a similar reduction, if you choose to run the heater about 8 hours/day.

Mine runs that over 2 4-hour blocks.