ECN Forum Forums Code Related Discussion NEC & other Code issues Another stupid ampacity question

I routinely fuse #8 copper at 40 amps. Table 310-16 says #8 THHN is good for 50 amps. A customer is replacing an existing spa with a spa with a 30 amp 240 v GFI and a 20 amp 120 v GFI. The disconnect box is fed with 4#8 CU conductors from a 50 amp CB in the main panel about 90 feet away. Do I need to replace the #8 CU with #6 CU?
Bob