I need to know if #2-4 alum wire needs to be put in conduit if it travel under a house in a crawl space some of which is exposed , the house sits on piers.

Don't know if code requires it but I think it's a good idea, Certain animals appear attracted to electrical fields and would chew on the wires.

I wired my shed using armoured cable just because of that.

Thank you:) smoothy I'am confused I wouldn't think anyone from northern Virginia would have shermans picture up

Can you please tell me what #2-4 cable is?

Could you possibly mean that you are planning on using four(4) #2 conductors?

#2 AL conductors can only handle 100 Amps, if it is used as a main service feeder (see T.310.15(B)(6)). T.310(16) gives you 75Amps (if used any where else) in the 60c column.

4 wires of # 2 alum. To feed 100 amp sub panel with 100 breakers on both sides, this all comes from a 200 amp main panel in main house , I am putting on an addition that does not have an enclosed crawl space yet but will next spring

Must be installed in conduit.