Quick Question 30 amp RV hookup

[bevans6]

On a 3 wire feed to a sub-panel the old practice was to tie ground to neutral inside the sub-panel, with no other external ground in the panel.  That preserved the single ground-neutral bond in the main panel, and in the absence of a fourth grounding wire the 3rd grounded neutral was used to provide the ground fault path to make sure breakers could blow if a ground fault developed.  It's still a good fault path to ground, it just shares the load carrying neutral to provide it.

This stuff gives me a headache sometimes.  The reason they went to a fourth wire is to allow for current carrying capacity in the grounding conductor in the event of a ground fault failure.  The fourth conductor never carries any current until a failure occurs, and then it carries the current that blows the breaker.  In the three wire hookup Sean has, the third conductor normally carries the sum of the currents in the two load paths, which because they are opposite phase would normally add up to a quite small current if the panel is properly balanced.  In the event of a ground fault failure, the third conductor would carry an overload current from one of the two load conductors, plus whatever other load current was present, so a possibility of a momentary overload on the conductor.  A pretty small possibility, but still present.