Most homes contain one of a small number of wiring systems, and which one you have matters far more than age alone. Modern non-metallic cable — commonly called Romex, technically named Non-metallic (NM) cable — includes a grounding conductor and is the modern standard for household wiring. Knob-and-tube wiring, phased out in the 1940s, has no ground wire at all and cannot service three-pronged appliances. Armored and metal-clad cables sit between the two, using a metal sheath for physical protection. This guide explains how to tell them apart visually, why the presence or absence of a ground wire is the single most important safety distinction, and when a professional inspection is warranted.
Identifying Common Residential Wiring Types
Homeowners rarely encounter bare single wires; they see cable assemblies — grouped conductors wrapped in a common covering. Romex is encased in a plastic jacket, typically yellow for 12-gauge wire and white for 14-gauge. Armored cable, often called BX, is wrapped in a flexible metal sheath. Knob-and-tube is the outlier: instead of a single cable, it uses single insulated copper conductors, with a black cable and a white cable running separately to each electrical point.
Location also offers clues. According to electrical training material from CBT Technology Institute, location influences the suitable cable type: UF cable is required for underground use, while BX cable may be appropriate for exposed locations in a basement. All installations, the same source notes, must comply with the National Electrical Code (NEC) or local electrical codes, which specify the types of wiring permitted in different applications.
Knob & Tube Wiring: An Obsolete System
Knob-and-tube is the oldest wiring a homeowner is likely to find, and it was phased out in the 1940s after better alternatives came along. The system consists of insulated copper conductors passing through lumber framing drill-holes via protective porcelain insulating tubes, supported along their length by nailed-down porcelain knobs. Its coating — rubber or asphalt-saturated cotton cloth — is a reliable visual marker.
The system does not meet modern electrical safety codes, as the Center for Energy and Environment explains: it was designed to carry a light electrical load and is overloaded easily in a modern home with multiple appliances often running at once. Over time, heat from the wires causes the rubber coating to become brittle and break, which can lead to exposed live wires. Because it is often hidden in attics or wall cavities, its condition is difficult to assess.
Its legal status is more nuanced than many assume. InterNACHI, the home inspector association, reports that while knob-and-tube is considered obsolete, there is no code requiring its complete removal — treatment varies by jurisdiction — but it is not permitted in any new construction.
