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In Israel the houses usually have a hot water cylinder on the roof of the building which is attached to a solar water heating unit, but which also is connected to an electric heating element which is switched on from the inside of the house if there is no sunshine.

My goal is to rewire the indoor switch to be connected to a relay and then control the heater from remote. I plan to do this using either Arduino or Rasperry Pi connected to the internet.

My question is, assuming (I will double check this) that the heater uses a maximum power of 4000 Watts (4 kW), which divided by 220 volts is around 18.2 Amps, will a relay that is capable of handling 20 amps be able to handle such a load? I'm thinking of using the SparkFun Beefcake Relay Control Kit

I'm under the impression that with heating elements there is generally no surge in electricity and that I don't have to worry about it spiking, unless something goes horribly wrong, G-d forbid.

Also, SparkFun writes that, "There are some pretty beefy traces connecting the relay to the load pins, but the 2-pin terminals are only rated for 8A max! If you plan on connecting a larger load you’ll need to solder directly to the board." Will soldering directly to the board allow me to pass such a high current?

Thank you.

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  • \$\begingroup\$ From just eye balling the photo's it shows the traces are doubled up (top and bottom) and they are wide I would say probably. I can't give a yes or no as it would need the files to be downloaded and the trace width measured then use an online trace calculator to get a good idea if it's ok. Then I would test it by measuring how hot it get in real life... If the tracks are getting above 65 degC then it's time to look at the PCB and Relay temperature ratings... \$\endgroup\$ – Spoon Feb 3 '15 at 12:53
  • \$\begingroup\$ I'm not much of a power guy, but that pcb looks wimpy to me. I'd have fatter traces, better connectors, and I'd use a 40A relay. (I assume there is already a fuse in the power line... 20 or 30 A ?) \$\endgroup\$ – George Herold Feb 3 '15 at 14:52
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Probably it will, but it may not last for long, because it is close to maximum relay current.

Here are some suggestions:

  • Use higher current relay. It will prolong the life of the relay. (30A, 40A or even 50A)
  • Make your own PCB with wider tracks for AC.
  • Use optocoupler instead of a single transistor. It will isolate your controller from the AC.
  • Add snubber to your PCB.
  • Read this question, asked by me, provides a lot of information about switching AC loads.
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  • \$\begingroup\$ can you suggest any 30A relay which is directly connectable with Arduino ?? \$\endgroup\$ – N.K Dec 27 '16 at 9:53
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I would absolutely not do this. I think it would be a major fire hazard. Find a high voltage relay rated for 25 Amps or whatever, but with a coil rated for some low voltage. E.g., 12V or 24V or 5V. This won't be all that expensive. Install it according to local electric code. If it is a 5V relay, you may be able to control it directly with the arduino (double-check the current requirements), but in the worst case, you could use a 24V transformer to energize the coil, and switch the 24V with a smaller relay. As much as possible, avoid putting low and high voltage stuff together in one box.

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    \$\begingroup\$ Would some like this be okay to use if I had the 5v control from the Arduino and the 220v wiring on the other side? \$\endgroup\$ – Big Joe Feb 3 '15 at 19:45
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I would recommend against using a board mounted relay for this current. Instead, go with a higher din rail rated contactor.

Such as Finder 22 series. (eg: 22.32.0.012.4340)

You can still use the arduino board relay to control the contactor coil.

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you can use Magnetic Contactor as a final switch, and use the relay as a on/off switch for the MC. and you can choose what capacity of MC you prefer to use.

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