NEC 240.21 establishes tap conductor rules. A 10-foot tap rule allows smaller conductors if they:

• Are protected by a fuse at the tap point equal to conductor ampacity
• Are installed in conduit and terminate in a disconnect within 10 feet
• Are the same size as the feeder conductors they tap from
• Have ampacity at least 1/10 of the feeder OCP rating and be enclosed in raceway