Knob and tube and BX are different ways of running electrical wiring. K&T uses separated conductors supported by ceramic hardware. BX is a familiar name associated with armored cable, where conductors are grouped inside flexible metal armor.
An older NYC property can contain both. Finding BX in the basement does not rule out K&T elsewhere, and finding K&T does not mean every armored cable needs the same replacement treatment. The useful comparison concerns the actual route and its condition, rather than choosing one label for the whole building.
The visible difference is in the wiring assembly
With K&T, each conductor follows its own route on supports. Ceramic tubes protect it at framing penetrations. The original system does not include an equipment-grounding conductor.
Armored cable contains conductors within a flexible metal covering. HUD’s Residential Rehabilitation Inspection Guide distinguishes AC, often called BX, from K&T and from metal-clad cable. These terms describe different assemblies, even if several look like old wire when viewed at a fixture.
An electrician needs more than a glimpse of metal covering to identify the precise cable and assess it. “BX” is often used loosely in conversation. Ask what wiring type was actually found if the name affects a replacement recommendation.
Metal armor does not answer every grounding question
Original K&T lacks an equipment-grounding conductor. An armored installation needs a more specific assessment: the cable assembly and its connections must provide the grounding arrangement required for that installation.
It would be a mistake to say that all metal-covered cable is ungrounded. It would also be a mistake to assume that any visible metal covering proves an effective grounding path. The electrician needs to determine what is present and whether the complete arrangement works as required.
That is particularly relevant where a three-prong outlet has been added. Its appearance does not identify the cable supplying it, much less establish the grounding path. The outlet-grounding guide explains why the findings should describe the actual protection, not simply the number of slots.
The insulation inside still matters
The outer armor is only part of a cable assembly. It does not make the insulation around the conductors immune to aging or previous damage. If that insulation has deteriorated, keeping the metal covering does not restore it.
This can first become apparent during work at a light fixture. The accessible wire ends may be more disturbed than portions left inside the building. Moving them can expose a weakness that was not obvious while the light was operating.
UL’s aging research explains the distinction between electrical performance and the mechanical vulnerability of older rubber insulation. The finding needs to be evaluated where it occurs and in relation to the circuit. Do not assume either that one damaged end proves every cable has failed or that a good outer jacket proves everything inside is sound.
How both systems can appear in the same property
A building may have retained original lighting while later renovations introduced armored cable. Those newer circuits might be separate from the old ones. In another location, a later cable may continue from an older K&T supply.
The second situation deserves particular attention because the visible cable describes only the last part of the route. An outlet supplied through it may still depend on old wire farther back. Replacement decisions need to follow that connection.
Our guide to K&T mixed with modern wiring explores this distinction. It is also why a statement such as “the apartment has BX” should not be treated as a complete electrical history. Ask which areas were examined and whether the supply was established.
Does BX have to be replaced when K&T is found?
Not simply because the two are present in one building. The assessment should identify which wiring is deficient and which circuits are included in the intended work. Cable in a separate, suitable installation is a different finding from cable connected to an inadequate retained supply.
For an insurance requirement, the wording matters as well. A request concerning active K&T should be answered with an accurate description of what remains active. Relabeling every old wire as K&T may lead to an unnecessarily confusing proposal; calling everything BX can conceal the issue the insurer asked about.
If a contractor declined the work based on an initial description, better identification may help clarify what kind of project you need. It does not guarantee that they will accept it. Some undertake cable replacement while setting limits on work involving separated old conductors and uncertain connections.
What to ask for in the assessment and quote
The findings should locate the wiring and explain how it relates to the proposed replacement. Where different systems meet, ask how that transition affects the plan. If grounding is a concern, the answer should address the actual path rather than rely on the word “metal.”
The quote should then describe which branches will be replaced and what remains. A room-based description may need further explanation if its supply passes through older wiring outside that room. Our partial replacement guide addresses those stopping points.
NYC Knob & Tube Experts handles old-wiring replacement planning across NYC, including cloth-covered wiring. If the report uses K&T and BX together, send the relevant findings. We can discuss the replacement around what was actually identified instead of treating all old wiring as one material.