Up until which year was knob-and-tube wiring commonly used in homes in the United States?

Knob-and-tube wiring was commonly used in homes in the United States until the 1930s. This type of electrical system was introduced in the late 19th century and became prominent in residential construction due to its simplicity and effectiveness for the time. However, advancements in electrical codes, safety standards, and materials led to its decline in use by the 1930s.

The wiring system consists of individual conductors that are run through walls, ceilings, and floors, supported by ceramic knobs and protected by tubes where necessary. While it served its purpose when it was first developed, it does not meet modern safety standards due to issues such as lack of grounding and insufficient insulation.

By the 1940s and beyond, newer wiring methods, like non-metallic sheathed cable (Romex), became prevalent, offering improved safety features and more efficient installation techniques. Therefore, the correct timeline for common use of knob-and-tube wiring concludes in the early 1930s.