I have an issue where I recently needed to change a meter on the outside of the house to allow for a pool installation. The reason for this was because the main panel in the house is on the second story with no attic access to allow for new circuits. It is placed on an interior wall that is directly above the ceiling in a dining room, so to run any wires would involve renovation (sheetrock cuts, painting, etc.)-a proposition that the homeowner refused. My solution was to change the meter for a meter/sub panel combination. The catch is that the meter can on the outside was a meter only with no primary disconnect or main breaker. The inspector initially approved the change, but then later (after the work was done) said that the feed wires to the main panel upstairs (which is now effectively a sub-panel) have to be upgraded to a 4-wire bundle instead of the existing 3-wire bundle. I didn't change the amperage coming into the house, and I didn't change anything existing other than the meter to allow for additional outside circuits. If anything, I feel that the service is safer because it provides protection for the 30+ feet of main feed wire that runs through the walls and attics of the house. Be that as it may, I know that the code requires 4-wire conductors for new installations. But is there not a "grandfather" rule for existing installations?