Originally Posted by Audiotekm
What kind of issues would I run into if I run the cable inside each piece of conduit?
And why I can't run NM cable inside a conduit? It's a 15A circuit to run some LED Xmas light in my backyard garden. The lights probably will draw 3~5A
Here's the cable i got... for free Carol 12/3 Type SJ FT2
The NEC requires that conduit be fully assembled before the conductors are pulled into it. The basis for this is to avoid damage to the conductors during assembly, and to ensure that the conductors can be removed from the conduit assembly later if needed. With that said, people do sometimes assemble conduit with the conductors in place. It's not code compliant and it usually ends up being harder to do than pulling them through later, even though it seems like it will be easier.
NM cable is not permitted in wet locations, and buried conduit is a wet location. Not just according the the NEC, but for real. Buried conduit ALWAYS becomes very wet due to condensation. NM cable falls apart eventually in contact with water.
The cable you have is not usable for this application. It is not code compliant for use in conduit or in any concealed application at all. It is also not water resistant. You need THWN. You should be able to pull 4 THWN #12 conductors through 1/2" with no problem if you use plenty of lube and don't have more than 360 degrees of total bends in the conduit (the code limit, and the practical limit as well). Your conduit needs to be 18" deep, or 12" if you use GFCI protection.