How to secure supply wires to panel
The wall isn't enough, you need a conduit there
What you're missing is that you can't have loose wires outside of a recognized Chapter 3 wiring method (i.e. cable, conduit, or some other sort of raceway), and &quot;wires loose in a wall&quot; isn't a wiring method that the NEC provides for. So, you need to run a short length of conduit between the LB and the panel; most often, though, you can do this with a PVC box adapter fitting instead of a short length of conduit, though, as the LB will be set up to be directly opposite a knockout on the back of the panel if you've laid out your run properly.