Is My Old Electrical House Wiring Safe? - The Spruce Identifying Old Wiring. The oldest type of wiring system found in homes is called knob-and-tube, named for the insulating knobs and tubes are used to run the wiring along and through the house framing. Knob-and-tube wiring was run as individual wires—one black hot wire and one white neutral wire—throughout the home. Common Types of Electrical Wiring Used in Homes THHN wire. THHN and THWN are codes for the two most common types of insulated wire used inside conduit. Conduit is often used in unfinished areas, such as basements and garages, and for short exposed runs inside the home, such as wiring connections for garbage disposers and hot water heaters. Continue to 5 of 6 below.