Older homes: aluminum branch wiring, knob-and-tube, ungrounded receptacles and the GFCI replacement rules, and cloth wiring
In one paragraph
Four things you will find in the walls of a California house built before 1980, and what the code and the federal safety agency say to do about each: aluminum branch wiring, which the Consumer Product Safety Commission says has only three acceptable permanent repairs, none of which is the pigtail-with-a-wirenut most electricians were taught; knob-and-tube, which California allows under insulation only after a licensed electrical contractor surveys it and files a certification; ungrounded two-prong receptacles, which may be replaced with a GFCI or a GFCI-protected grounding receptacle carrying the exact markings 406.4(D)(2) prescribes; and cloth-insulated wiring, which is a condition to inspect, not a code violation by itself. Each becomes a rung on the options ladder with an honest cost band.
The rule
The Consumer Product Safety Commission recognizes three permanent repairs for aluminum branch-circuit wiring, complete replacement with copper, the COPALUM crimp method and the AlumiConn connector, and does not consider twist-on pigtailing or CO/ALR devices acceptable permanent repairs (CPSC Publication 516). Terminals and connectors must be identified for aluminum where aluminum is used (2025 CEC 110.14). Concealed knob-and-tube may not lie in insulated hollow spaces except under the California exception requiring a licensed electrical contractor's survey and certification filed with the enforcing agency and bilingual caution notices (394.12). A nongrounding receptacle may be replaced with a GFCI receptacle marked No Equipment Ground, or a grounding receptacle protected by a GFCI and marked GFCI Protected and No Equipment Ground, or another nongrounding receptacle (406.4(D)(2)).
Why it matters
Older-home work is where a new contractor either sells the right repair or inherits a fire. The aluminum finding in particular is where good intentions go wrong: the industry habit is a twist-on pigtail, the federal safety agency says that is not a permanent repair, and the customer will be told one of those things by whoever comes after you. Knowing the three approved methods, the California knob-and-tube certification, and the three replacement options for a two-prong receptacle is the difference between a professional and a handyman with a license.