Electrical Outlet Grounding In Old Residences

A real-estate agent asked me about outlet grounding in response to her customer's request for having the standard, 120-volt outlets converted to the modern, grounding type.

First of all, in my opinion: purchasers of a house asking for the grounding of all the outlets, is a bit much as houses dating before circa 1958 weren't built with the ground-providing outlets.

However, if the house was built using armored (BX) cable as often was in the 1930's & '40's, grounding all the outlets is a fairly simple process as the grounded armored cable provides a ground connection to the metal outlet boxes in the wall. So, the outlets would become grounded incidentally with receptacle replacement of the modern, grounding type. I say "incidentally" because the new receptacle's grounding provision becomes electrically connected to the grounded metal box via the attaching screws as the new receptacle is installed.

If the house was built before ca. 1958 using knob & tube wiring or non-armored (early Romex) cable, there is no grounding capability. So, to ground such a house or apartment, without the huge job of cable replacement, a grounding wire has to be installed--a grounding conductor connecting each outlet to a proper ground source.

Only the outlets on a particular branch circuit may be served by its own additional / remedial grounding circuit--not in a cross-circuit fashion. So, it's not permitted to simply run one grounding conductor to connect the outlets of one branch circuit with the outlets of another branch circuit.

After ca. 1958, Romex cable includes a grounding conductor with the supply conductors to facilitate the use of the modern grounding receptacles.

If you're are remodeling, this remedial grounding work is moot as new cable should replace the old.

In some areas, as per local electrical codes dictate, the government doesn't permit the soft-sheathing Romex cable. In this case, BX cable must be installed. If local codes require even better construction (mainly larger buildings), the best wiring method, steel conduit must be installed. Of course, you may use the better, BX cable where Romex is permitted.

Richard Stuart Otto, 25 Nov. 2007