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I had a hot tub that I removed. I would like to repurpose the wiring (currently capped off) to add a generator inlet box. It's supplied by a 60 amp double pole breaker in the main panel, which feeds a sub panel outside that has a GFCI breaker.
Are there any issues with using the old hot tub wiring and adding the generator inlet box to a post where the hot tub used to be?
The wire should be fine to reuse assuming it is in good condition and has the appropriate number of conductors.
More of a challenge will be how you actually use it for a generator. You will need to properly interlock the feed at both the subpanel and at the main panel. I'm not certain that will be possible for loads off the main panel since any interlock that disables the feed to the subpanel so you can enable the generator will also block you from sending power back to the main panel. It's probably going to be easier to install a dedicated inlet at the main panel and leave this wire be.
That is, unless you're referring to a single breaker 'spa panel'. In that case it's probably best to add the interlock at your main panel and then put the generator inlet box where the spa panel is currently. Alternately you could replace the spa panel with a junction box to get the power all the way to the spa location and allow you to install a main panel interlock.
The key thing is interlock. There is a secondary issue that GFCI does not play well with reverse power. If you use the subpanel then there are a number of possible problems - I can explain in detail if needed.
I think the easiest solution is to either put the inlet at the location of the subpanel or, if the former hot tub location where you want to put the inlet is a significant distance away, use the subpanel box simply as a place to splice the wires.
You definitely need an interlock in the main panel between the main breaker and the generator inlet breaker, which will likely mean rearranging breakers as interlocks generally only work in one specific position in a panel. An interlock is usually just two specially sized/shaped pieces of metal, designed so that it surrounds access to either the main breaker and a branch double breaker below it or a pair of branch double breakers (for a panel using a backfed breaker instead of a "main breaker") so that one piece slides to block either one or the other breaker - i.e., so that you can't have both ON at the same time.
You notably didn't mention adding an interlock to prevent both the main breaker and the generator breaker from being on at the same time (both can be off), which is the minimum required (The inlet breaker also needs to be held down, but a listed interlock kit for your panel will generally do both.)
