I've no experience with electrical work so everything I've learned about this circuit has been cobbled together from youtube videos and subreddits.

I have a water pump in my back garden to pump water from a rain barrel to the garden. It runs off a battery that's solar-powered via a charge controller. Here are some details about parts:

Battery: Mighty Max ML18-12 | 12 V | 18 Ah

Pump: Shurflo 2088-554-144 | 12 V | 5.2 A | 62.4 W

I need to install a float switch into the rain barrel so that the pump won't run when the water is below a threshold. I've learned that I need a relay because the amperage will destroy the float switch. I've also been advised to include a diode to protect the float switch when the relay discharges. I've no experience with diodes or relays, but I've put together the following diagram to try to make sense of it.

float switch relay diagram

There is 16 AWG wire used throughout except for the horizontal float switch which comes with 22 AWG wire. The relay I'm looking at online is the 12V Viair 93940.

Is this correct? If not, why not?

  • \$\begingroup\$ What current does the motor take under full-load and stall conditions? You can get 10 amp float switches BTW because I have bought them for my koi pond. \$\endgroup\$
    – Andy aka
    Oct 1, 2022 at 19:13
  • \$\begingroup\$ @Andyaka According to this pdf online, the highest amperage I can expect is 5.2 A, although it doesn't specifically say anything about stalling. pumpagents.com/pdf/ShurfloPumps/2088-554-144.pdf \$\endgroup\$ Oct 1, 2022 at 19:23
  • \$\begingroup\$ @Andyaka I'm assuming your 10A float switch is the kind on the end of a long cable. Is that correct? I've considered those, but I've decided to use a horizontal float switch instead because then I have access to water more immediately. They can only handle very small amperage however. \$\endgroup\$ Oct 1, 2022 at 19:24

1 Answer 1


Yes, that is the correct way to wire a relay and diode.

When selecting the relay, allow a good safety margin on the current rating. Motors, especially in pumps, can take quite a large current surge when they switch on. And if you use a 7A fuse, make sure it's a slow blow one.

  • \$\begingroup\$ The fuse I have is one of these 7.5A blade fuses: tinyurl.com/bladefuse. Will this be adequate? \$\endgroup\$ Oct 1, 2022 at 21:31
  • \$\begingroup\$ @KorganRivera probably. But 16AWG wire can handle larger currents. So if it blows, try a higher rating. \$\endgroup\$
    – Simon B
    Oct 1, 2022 at 22:08
  • \$\begingroup\$ It might be wise to add a commutating diode across the pump motor terminals, and perhaps an RC snubber across the relay contacts. 100 ohms and 220 nF are typical values. These will reduce arcing and provide longer life for the relay contacts. \$\endgroup\$
    – PStechPaul
    Oct 1, 2022 at 22:46

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