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We have heard of faulty or older GFI's doing as you descibe. You're right in getting an electrician to check it out, but my guess would be a faulty GFI.
It controls 3 outside outlets (just installed) on the deck and a ceiling fan (rated for wet locations), it's 20 Amps and nothing besides the ceiling fan is attached to it yet ... and it's too D*#n cold to use it now - LOL. All the connections are in outdoor recepticles. I have turned on the ceiling fan and it's light 4 or 5 times before and it worked OK. Actually the GFI is working fine now, but it tripped last night.
The tub has a regular 50 Amp circuit breaker in the box and a GFI disconnect at the tub. Reading around and hearing some eletrician's precautions - it was a long run, almost 100' so I did it that way to avoid nusiance trips ... can this be the problem with this GFI (same general distance)?
Vin, I think its the brand of spa you bought. Nothing but trouble !!
Not sure if this answers your question, but some ozonators can trip the breaker. Some ozonators have thier own fuse/GFI built in to the unit. If you tell us the make/model of the ozonator someone could probably help you.
Make sure that moisture didn't get into that outdoor outlet box somehow. That'd trip a GFI. If you spa isn't on the same circuit as the outlets, it shouldn't be impacting the other GFI. I'd focus on the wiring in the circuit where that the GFI is tripping, and on the GFI itself. It's possible for the breaker to be defective- unlikely but possible.