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I have an existing outdoor GFCI outlet. Can I wire a pump relay to the existing outlet, and put another GFCI outlet after the relay? The goal is to have the second outlet only powered when the relay is "switched on."

Background:
I've got a tank that is collecting rainwater. I'm wanting to use that water to feed some raised beds using an opensprinkler pi (OSpi). I want to use the OSpi to activate a pump start relay, which would then give power to the outlet and turn the pump on (pump is activated when hit with power, no on/off switch). I already have a pump that has a three-prong plug, and I'd like to keep that intact if possible, hence to wiring the outlet.

Setup:
-Dedicated circuit on a 20A breaker
-Existing outdoor GFCI outlet on the previously mentioned circuit
-All outlets are/will be housed in weather resistant bubble enclosures (like this)
-Pump Start relay comes with/will be housed in weatherproof enclosure
-Pump and sprinkler valves will also be in weatherproof housings/covered

Proposed Design/Wiring Diagram pump start relay outlet design image Link to the above image can be found here: https://i.imgur.com/pia2Zry.png

  • Have you bought the AFCI breaker yet? – Harper Oct 17 '18 at 1:02
  • There's no need for an additional GFCI after the relay. The GFCI feeding it will catch any ground fault. – Ecnerwal Oct 17 '18 at 1:33
  • @Harper Yes, and sorry, I botched the name. The CAFI breaker has already been purchased and is installed. For clarity, everything up to and including the first GFCI outlet is already installed. – C Wagon Oct 17 '18 at 3:52
  • @Ecnerwal, excellent thanks. Other than chasing down which outlet might be tripping, is there any downside/negative if that second outlet is also GFCI? – C Wagon Oct 17 '18 at 3:54
  • Yes. Your arrangement will have you severing power to the GFCI's inputs. Many GFCIs do not like that and will have weird trip problems. You might find one that is ok, but GFCIs don't live long outdoors, and you'll be changing it periodically and you'll have to hunt down a power-sever-tolerant GFCI everytime. Also it means that some other circuit in your house isn't protected at all because you wasted a GFCI. It is a complete and total waste to do that. – Harper Oct 17 '18 at 6:06
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This is why GFCIs have LOAD terminals

A receptacle-type GFCI has two sets of terminals: LINE and LOAD. The use of the LINE terminals is simple -- these are where power is applied to the GFCI. The LOAD terminals, though, are interesting -- they are effectively a third receptacle on the back of the GFCI, only implemented using screw terminals or backstabs instead of normal NEMA receptacle contacts. In other words, they extend GFCI protection to whatever is connected to them.

So, you can connect the wires feeding power to your pump start relay box (hot + neutral) to the LOAD terminals on the existing GFCI. This way, you don't need to put in another GFCI, just a standard receptacle, and you don't have to worry about oddities that might be caused by the second GFCI having power cut to it on a regular basis. Just remember to put one of those "GFCI PROTECTED" stickers on the regular outlet that you're wiring up to the pump start box if you do this!

  • Excellent, this is exactly what I was looking for. I'll go with your suggestion above, use a standard receptacle, and be sure to label with a "GFCI Protected" sticker. Thanks! – C Wagon Oct 23 '18 at 20:35
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It will work to use the contacts of the relay to switch the pump power, tapped from the load side terminals of the existing GFCI to run the pump receptacle.

I personally might use a single (not duplex) non-GFCI receptacle for the pump. If you use a duplex receptacle it seems like someone will eventually unintentionally plug something in there expecting constant power.

If you don't want to risk whatever gets plugged into that GFCI receptacle overloading the pump circuit - make it really dedicated - you could even use a dead front GFCI there instead of a receptacle.

  • Great idea, I think I'll go with the single non-GFCI receptacle for the pump so there's no confusion on that side. – C Wagon Oct 23 '18 at 20:33

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