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If your spa is delivered during the winter months, I would have the electrician turn on the spa for a few seconds (then shut it back off) just to make sure his work is correct. Then start filling the spa.
I would absolutely, positively NOT do this. See my post above. Repeat: DO NOT do this. Fill the thing before you power it up.
One thing that hasn't been mentioned:Also: there is a tip which I think has finally made it into the owners manual about turning on the breakers. You need to turn on the 30 amp first, and wait for everything to settle. Run the jets for a few minutes, shut them off. Look at the surface of the water: when it is calm again, you should be able to see the water moving a bit directly above the floor fitting. This indicates that the small circ pump has caught its prime and is ready for you to power up the heater - that is - turn on the 20 amp breaker.....