Discussion and questions related to the course Professional Motorsport Wiring Harness Construction
Using the table and MILSTD 339 it is possible based on wire/ cable sizes that you can have a branch out point with wires from different layers?
Yes and this is almost essential in many harnesses. In particular you're going to want to use your shielded cables for engine speed/position as a core to your harness and it's likely these will need to be broken out at one or more branch points. in this respect you can disassemble your twisted layers at a branch point and reconstruct them as required.
Is it common to then use filler wire in the core to maintain cross-section or is a step down in CSA generally expected/implied at that point? I'm sensing a "situation dependent" type of answer but I'll send it anyway!