I live in SoCal and just purchased a Sundance Maxxus Spa. I had a contractor lay conduit for me for my Spa. The distance from the main panel to the Spa Shut off switch is about 90 ft and then about 8ft from there to the spa. The contractor walked off the job before completing the work (long story) but now the new electrician I am talking to says the previous guy laid the wrong size conduit. It's 3/4 inch and the electrician says he has to pull two #6 Gauge Black (Hot) a #6 Gauge Neutral and a #6 Gauge Ground. He says the 3/4 inch conduit is not big enough to cope with these wires!! Is he correct or can I run the hots in #6 and the ground and neutral in #10, will the conduit I have be big enough for this? I really don't want to dig up the existing conduit but the other thing is that he only laid it about 6 inches below the surface and I thought it was supposed to be 18" down if it was PVC?

There are a few variables, however, first and foremost is the conduit depth. NEC is a minimum of 18" and many municipalities require it even deeper. Check with the local codes to be sure.

Second, depending on the type of wire, you can run up to 3 #6 wires in a 3/4" ridigid non-metallic tubing. That would be if it is THHN, THWN, or THWN-2. Notice there isn't "room" for a ground?

What I would do, and this is true in many spa cases, is tear out the 3/4", replace it with 1 1/4" (at proper depth), run 2 #6's for your hots, run a #8 for your neutral and a #8 for your ground. Most spas can get by with a smaller neutral since almost nothing runs at only 120V. Your pumps, heater, and blower are usually the 240V. The ground conductor can be one size smaller than your largest load lines, hence the #8.

That "electrician" that "walked off the job" could not have been a properly licensed electrician. I would be double checking everything that guy touched. Anyone in the industry knows you bury the rigid non metallic conduit at least 18" regardless what it is made of or it's size.