# Is a relay with a 20 amp rating enough to handle the current from a residential hot water heater?

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.

• 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... – Spoon Feb 3 '15 at 12:53
• 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 ?) – George Herold Feb 3 '15 at 14:52

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.