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First you need to find out what the spa feed is in amps.Your best bet is to run a 60 amp feed.6 AWG wire is good for 60 amps. Since you will not be loading above 80% of the 60 amp breaker you will be good for a run of 125 feet based on a 50 amp load with a 3% peak voltage drop at 240 VAC. A 40 amp load on the same wire will be good for a 153 foot run. Most spas will actually run at less than 40 amps even if it is rated for a 60 amp feed. Remember that this is cable length. Your dealer should have included (hopefully) a GFI Spa kit which has the 2 pole GFI breaker and a box. I recommend that you install a 60 amp 2 pole breaker in the main panel, wire thru a 3/4" box connector to the GFI panel either inside your basement or outside. The breaker will last longer if installed indoors. You will need to provide a disconnecting means not closer that 10 feet (5 feet in some places) and not farther than 30 feet (up to 50 feet in some places) and within site of the tub for service use. The cable that you use for the run is 6/3 which is 6 AWG 3 conductors insulated with a ground (fourth) conductor. Your spa might not require the neutral but run it anyways. I recommend that you pull the cable through properly installed PVC conduit. A small stone could in time poke through the underground cable and cause the GFI to trip. Glue it well and tape the ends until you do the final pull. Good luck and happy diggin'Steve