I seem to remember something in the code that talked about the minimum size feeder conductor. I know the feeder conductor must be capable of carrying the connected load and that's not what I referencing. I seem to remember that the minimum size feeder being size #10 AWG. Please tell me I'm right or wrong.
You have to scratch your head to come up with a scenario where a feeder would be less than 30a but I suppose it can happen. A possibility would be a feeder to a shed that terminates in a 2 pole <20a breaker> disconnect in the shed, serving as the disconnect and also supplemental overcurrent protection. Technically any conductor on the load side of the service disconnect and on the line side of an overcurrent device is a feeder.