A.Peitsa Hirvonen, a licensed electrical contractor and the owner of SESCO Electrical Inc. in Berkeley, Calif., responds: Generally speaking, if an electrical circuit was properly wired and up to code when it was first installed, then it's up to code now. Most inspectors will not make you change out existing wiring if it appears to be sound and correctly wired and has not been modified or added to in an illegal way. But when a contractor completely opens the walls, floors, or ceilings and makes it easy to access the wiring, most inspectors will require those areas to be rewired to meet current code.
The problem, of course, with a two-wire circuit is that it's not grounded. The old Romex (NM) and BX (AC) cables had no separate grounding conductor, or the conductor was so small (16 or 18 AWG) that it wouldn't count by today's code. This type of cable can usually be identified by the sheathing, which looks like snake skin or tarred cloth....
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