What size wire should be used for bus wires ?

Hi Ken

If it's a straight DC controlled layout it should be fine as the locos speed up or slow down in direct proportion to the voltage.  If it's a DCC controlled layout running a DC loco it could also be fine, but why risk it?

It's difficult to tie it down as the problem is intermittent and hard to identify.  However it happens often enough that it's fairly well known on DCC forums as a possible explanation of erratic behaviour with DCC locos.  It's easier to eliminate the possibility at the build stage.  It's no harder to make the bus without a loop.

DC does not have the problems that DCC may have. Normally DC layouts are wired differently than DCC.