Proper identification of the type of electrical wire and wire insulation is useful for those seeking to guess at the age of a building or of its electrical system, and in some instances it is helpful in assessing the condition and safety of the building wiring system, such as taking note of the presence of branch circuit solid conductor aluminum electrical wire that has not been properly repaired - a fire hazard.
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These wires are simply put through holes in the joists or stapled to the sides of them.
They usually have Romex wrapped around the wire and it would be covered by cloth sheathing.