I have a 100 amp subbreaker with no circuits on it yet that sits
against a block wall adjacent to the crawlspace. I want to run some
new circuits through the crawlspace to the kitchen. My main breaker
(which is in an inconvenient wall to pull cable to) has metal
knockouts and I am familiar with the use of romex cable clamps to
secure the conduit and prevent sharp edges. But the subbreaker just
has a short 3" dia. plastic pipe that terminates at the other side of
the block wall. How do I secure the romex? My wiring book and home
inspection book do not discuss this situation--they only say to use
clamps to secure cable when exiting any box.

If I run the conduit directly out the pipe and staple it to the joists
about 1' above the pipe termination, there's nothing sharp to cut the
wire, but isn't there a danger that someone could snag the loose
conduit hanging out of the pipe and it off a breaker? What's the
correct way to do this? Is there an NEC code for this? The wire
feeding the subbreaker is completely enclosed in the same type of PVC
conduit running into the box--but I can't do this for multiple
circuits running out, can I?