AN ELECTRICIAN SCREWED UP

Although it’s code-legal to run wiring through notches and holes in beams and girders, it has never been legal (or safe) to run the notch down the thickness of the beam. Nor has reusing wiring that no longer meets modern electrical codes.

To begin, make sure the old wire is completely disconnected from any potential power source (you may have to follow the line through gaps in the wall or ceiling to determine this). Once you’re sure it’s not hot, remove it. Since the girder is massive, this notch shouldn’t have compromised its integrity. If it’s sound, leave it in place.

As for running future wiring, try to avoid cutting into beams if at all possible. If you must, any holes or notches should go through the top of the joist or girder, not along the perimeter from top to bottom. To avoid weakening the support beam, the notch cannot fall in the center third of the joist, according to the International Residential Code book (see portions online at archive.org). The maximum notch depth should not be deeper than 1 /6 the overall depth of the joist. For an 8"-thick joist, that means the notch cannot be more than roughly 1 ¼ " deep—a tight fit for an electrical conduit.

The code is more lenient about holes drilled through joists, however. The hole can be up to 1 /3 the depth of the joist—or 2 ½ " for an 8" beam. Any hole must be at least 2" away from the edges of the joist.

Another problem with your situation is that the cable appears to be nonmetallic (NM) cable, or rag wire. In widespread use from the 1920s through the early 1960s, NM cable is sheathed in an insulated, rubberized fabric coating. It no longer meets code, and tends to deteriorate and become brittle with age. Most of these mid-20th-century wires also lack a grounding conductor, so touching a live wire can give off a shock.