unless i am mistaken the wire itself has to be THWN, not THHN, for burial. even if it is in conduit.

also i think to only go a foot down, the whole conduit must be metal. i know in town here it has to be metal unless you go 18" deep. it can be PVC after it gets above grade, but everythign underground is to be metal.

my best advice for you is to call your local inspector.

We are allowed to use high-density polyethylene conduit underground with THWN here in NorCal, but it must be buried at least 18 inches. If it is protected by a GFCI you can go as shallow as 12 inches, but we don't do that. It's too easy for a worker or child to dig that deep.

Definitely check with your local building department or electric supply store. They will know what is allowed in your area.

I have had particular customers spec rigid steel 90's and even rigid steel for anything above ground. You may have local codes that require it. Check with your electrical inspector.

Remember that if you do use any steel in the manner you are describing, you need to bond the steel to the EGC. If you use only a steel 90, then you would need to use an external EGC with the proper ground clamps and tie it into the rest of the EGC run at some point. If you use a steel riser (and pvc boxes) you would need to use bonding bushings to connect the EGC to.

I know here in the Phoenix area, PVC conduit must be 18" deep. Metal conduit is only required to be 6" deep. If you run metal conduit the entire way, you can help yourself with some of the grounding issues mentioned too by using the conduit as a ground conductor. But PLEASE get clarification on this from a sparky here.

Article 300.5 and the corresponding table in the NEC have the needed information. It is more than I wish to type out here.

The 12 inch rule for gfci protected circuits only applies to 120 volt circuits 20 amps or less. The spa circuit would have to be min 18" if PVC and 6" if RMC or IMC.

The spa/hot tub also requires an insulated grounding conductor, you cannot depend solely on the conduit for grounding. Also, in many areas, rigid conduit buried underground will rust away in 15 to 20 years.