I am not an electrician by trade but have done a lot of wiring. The breaker is designed to protect the wiring. (The wiring between the breaker and the outlet). There is an exception when it comes to the cord on the appliance it can be smaller. So I would rate the breaker needed as at least 10% higher than the full current draw on the appliance. Then the wiring from the breaker to the outlet rated at or better than that capacity.
An example is when you need to run a power tool at 12 amps 100 feet away. You need a 12 gauge extension cord that is ran off a 20 amp outlet but the power tool has only a 16 gauge cord on it. Clear as mud right???
BTW if you do run another service all the way back and plan to use larger power tools. Run a 2-2-2 Alum wire. The cost is probably going to be close to the number 6 copper anyway and it will help with less of a voltage drop. You sure don't want to be welding and have a compressor kick on and all the lights go out.
Brian
Shade tree MIG welder.
Now a Shade tree TIG welder.