Can I run 8thhn from my service disconnect 100 ft to my shed sub-panel 6/12 box? Only asking because I really don't see the difference with what I was going to do, run 8thhn from my main circuit breaker box through two 50 amp cb's... across my garage... down my wall... buried in PVC under ground to the shed... connected to a sub-panel.

NO, you cannot do this. Your service disconnect will not have the breaker space to install the sub-feed breaker. You CANNOT put the wires under the same lugs as the main feeder either.

I hope you are describing all this that the WHOLE run is going to be in conduit. You cannot run THHN exposed at all.

Thanks, yes it will all be PVC'd

An 8 gauge wire can safely handle a 40 amp load. It MUST be fused / protected as such. If it were wired to the main feeder it would basically be unprotected which is not allowed for many reasons. 1, it's a fire hazard... 2, is't expressly forbidden by any electrical code... 3, it's plain *** stupid.

Thanks, I already got pretty much the same answer already, going with original plan... route the wire from my main breaker box.

Missouri, #8 THHN and THWN is rated 50 Amps. This includes pulled through conduit or if MC cable is used.

#8 Romex or UF cable is rated 40 Amps.

There does need to be a 2 pole 50 Amp breaker at the main panel to protect this circuit using #8 THHN/THWN pulled in PVC.

If any section of the circuit is not THHN in conduit, say a section of Romex or UF, then it can only be rated 40 amps.