I am doing a job soon that will involve taking down old fan coil units and installing new ones where the old ones came out. Some of these were on 50 amp breakers with number 8 wire. All of the new ones are 30 amp heat coils. I am trying to find a code reference were I can still use the 50 amp breakers on the 30 amp units because the units are fused at 30 amps on the inside of the units themselves. All help is GREATLY appreciated

If you will count on the fuse for the overcurrent/short circuit protection of the motor, then you will have to ensure that the termination is rated at least 75 degrees. If not, then the #8 will not be protected at its ampacity.
On motor circuits, it is acceptable for the breaker to be sized somewhat larger than the conductor ampacity, but you will not be taking advantage of that aspect since the 30A fuse is protecting the motor.

Well it has a small motor 1.2 amps I believe and the rest is heating strips. And yes the fuses(30 amps) are protecting the motor and heat strips. Trying to save a good customer of mine a few pennies and I am thinking the 50A breaker will suffice as long as the motor is fused at 30 amps like the 10ft tap rule maybe. I figured to look here because this is a very knowledgeable bunch of guys. Would this not be like setting a 100 amp disconnect and feeding it with number 3 and then fusing it for 50 amps and having number 8 on the load side of the disconnect.

waymag,
from the hip, what do you think of 430.63?

Well that seems to be for lighting and appliance. I will look into it further to see what I come up with
Thanks a bunch Sparky