I'm looking for ideas and suggestions to build a 220v ~ 40 amp wifi switch for controlling my pool pump so that it can replace my mechanical timer.

Very few devices are available on the market. There are both relatively expensive and according to the reviews don't live up to expectations in terms of reliability and build:

The main issue seems to come from the terminal connections not being big or sturdy enough for 10 gage wire.

Looking at the Migro Outdoor Smart Wi-Fi Outlet Box it seems they have combined a contactor with a sonoff switch which seems like the best option. This video explains how a contactor works. Migro are charging around $140 whereas the contactor on it's own only costs $14 here and the sonoff switch $8 here.

Does building one seem the best / cheapest option?

  • I would also suggest to build something based on Sonoff. I like how Migro added a range extender. – Marcel Stör Jan 3 '20 at 6:57
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    What's best questions often lead to no answers because people do not know on what criteria they are supposed to measure best. Can you add what's important to you? – Helmar Jan 4 '20 at 9:36

Looks like you've answered your own question :)

Yes - a wifi controllable interface (likr Sonoff) hooked to act as the trigger for the contactor relay is probably your best/cheapest option. Saves you the issue of coding your own I/O control and easily manages overall power requirements.

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