Originally Posted by P-J
That is it exactly.
The breakers mounted within the controller or within the Spa Panel are placed for device protection.
If your going to drop the wire size from the GFCI panel (50A breaker) it needs to be UL489. As mentioned above UL489 is to meet NEC requirement. NEC requires a 30A overcurrent protector for your 10gauge wire from the modified GFCI, a UL1077 device does NOT satisfy this requirement.
As mentioned UL1077 is supplemental protection, meaning you already have some other protection. If you are dropping to 30A with the supplemental protection you have NO primary protection at 30A. This is like saying you can use UL1077 breakers off your bus bar's in your panel because you have the 100/200A main breaker....
This is a very common mistake and I see it done in industry alot as well (and fails panel certification)
If you want to do it cheap, what about a Fused disconnect they sell for Air conditioners/etc?
EDIT: Yes, NEC ends at outlet but then UL panel specifications are supposed to take over (in industry), but as most people don't usually have access or desire to follow they stick with the NEC. Certaintly you can do whatever you want we are not building industrial panels here. So if you don't follow it thats your choice, but that doesnt change the facts.
EDIT2: Why not something like this, with a small peice of 6AWG wire and a short conduit to connect the two panels. Physically bigger, but cheap. http://www.homedepot.com/p/t/202106492?catalogId=10053&langId=-1&storeId=10051&N=5yc1vZbm0h&R=202106492
Modifying the GFCI panel is also likely a bit of an issue.... You can usually also get single breaker panels/disconnects cheaply as well (http://www.homedepot.com/p/t/100177812?catalogId=10053&langId=-1&storeId=10051&N=5yc1vZbm2w&R=100177812
), and you could throw a regular 30A breaker in it (still going to be a bit more money though)