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1) The National Electrical Code recommends that the voltage drops be limited to a maximum of 3%. Based on a 240 volt circuit @ 50 amps on a 130 foot run results in a 3.7% voltage drop or a loss of 8.9 volts with 231.1volts at the end of the circuit. This is based on using 6 AWG Aluminum conductors for the branch circuit. Yes, aluminum conductors are NEC code compliant and perfectly acceptable to use as long as they are sized correctly and the proper devices are used for the terminations. 6 AWG aluminum conductors are rated for a maximum of 50 amps if using XHHW insulated wire or a maximum of 60 amps if using XHHW-2 insulated wire.
2) Even though the voltage drop is (.7%.....ie.... point seven percent) a little over the NEC recommendation, you will still be OK using the 6 AWG aluminum conductors as the recommended limitation is boarder line and a very small %.
3) Make sure the contractor installs a double pole feeder breaker and a GFCI disco switch that is rated for aluminum conductors. The aluminum conductors will also require an anti-oxidant paste on all terminations. Noalox is an anti-oxidant paste that is sold at any Home Depot, Lowe's or local electrical supply stores.
3) Yes, the hot tub will require a GFCI disconnect switch.
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