Knob-and-Tube Replace vs MC Cable vs Conduit Retrofit Decision Matrix

Why this matters

Knob-and-tube (K&T) wiring was installed roughly 1900 to 1945 and is still energized in millions of homes. It has no equipment grounding conductor, relies on open-air heat dissipation around individual conductors, and degrades where rubber insulation has gone brittle or where insulation has been blown over it (a code violation under NEC 394.12). Replacing it is rarely optional once you are opening walls, insurers deny coverage, or a circuit is overfused. The retrofit method you choose, full home-run rewire, MC cable through accessible cavities, or EMT/conduit on surface or in chases, determines labor hours, drywall damage, inspection scope, and whether you can salvage existing device boxes. Picking wrong means either tearing out finished walls you did not need to, or fishing flexible cable through a structure that has no path and burning a day per circuit.

The options

Three remediation paths cover almost every residential K&T job. Each assumes the old conductors are abandoned (de-energized, capped, and ideally removed where accessible) rather than spliced into, because K&T cannot be extended with modern methods under most AHJ interpretations of NEC 394.

Full NM-cable rewire (home runs to panel): New nonmetallic-sheathed cable (Type NM-B) pulled from each device back to the panel or a junction, replacing the circuit end to end. This is the gold standard for finished single-family homes where attic and basement access exists. It restores grounding, lets you correct circuit loading, and passes inspection cleanly. The cost is wall and ceiling access: every drop point typically needs a fish or a small opening.

MC cable (metal-clad) through accessible cavities and along framing: Type MC armored cable is the workhorse where NM is not permitted (some jurisdictions require MC in multifamily or where cable runs through plenum-adjacent spaces) or where the armor is wanted for physical protection in exposed runs like unfinished basements and garages. The metallic armor of listed MC also provides the equipment grounding path when the cable includes a dedicated green EGC (older interlocked MC armor alone is not an approved ground; verify the assembly).

EMT/conduit retrofit (raceway, surface or chased): Electrical metallic tubing or, for damp/wet locations, rigid or PVC. This wins in masonry buildings, unfinished basements and shops, exposed garage walls, and anywhere conductors must be physically protected or future-pulled. You install raceway and pull THHN/THWN conductors. It is the most labor per linear foot but the most serviceable result.

When NM rewire wins

Choose full NM home-run rewire when the structure is wood-framed, you have working access from an attic or basement, and the goal is a clean code-compliant result with minimal visible raceway. NM-B is the lowest material cost and fastest pull in open framing. It is the default for occupied single-family homes during a panel upgrade or kitchen/bath remodel. Constraints: NM is not permitted exposed where subject to physical damage (NEC 334.15), not allowed in conduit as a wet-location wiring method, and many jurisdictions bar it from commercial assembly occupancies. If the home has plaster-and-lath walls with no stud cavities you can fish (common in true K&T-era homes), NM rewire labor balloons because every run becomes a wall-opening job, and one of the other two methods may net out cheaper.

When MC cable wins

Choose MC when you need a grounded, flexible cable that is rated for exposed installation or required by the AHJ. MC shines for: surface runs in unfinished basements and garages where NM would need protection; multifamily and mixed-use where local amendments mandate metallic wiring methods; and runs through fire-rated assemblies where the listing supports it. MC fishes nearly as well as NM in open cavities but is stiffer and heavier, so long fishes through tight plaster walls are harder. Verify the specific MC type carries an insulated EGC; do not rely on the armor alone as the ground unless it is a listed MCAP/MC-PCS assembly designed for that.

When conduit retrofit wins

Choose EMT or PVC conduit when conductors are exposed and need physical protection, when the building is masonry or concrete with no fishable cavities, in shops and garages, in damp or wet locations (PVC or RMC with proper fittings), and anywhere the owner values future pull-ability. Conduit is the most labor-intensive (bending, mounting, pulling) but gives the most durable and serviceable circuit. It is frequently the right call for the panel-feeder and large appliance circuits even when branch circuits go NM. In retrofit, surface raceway (e.g., metal wiremold) is a legitimate conduit-family answer for finished masonry where you cannot chase.

Field decision flow

1. Map the K&T extent. Open the panel, identify which breakers feed legacy conductors, and walk attic/basement to see actual wiring method and access.
2. Check insulation contact and overfuse. If K&T is buried in insulation or fed by an oversized fuse/breaker, that circuit is a priority replacement regardless of method.
3. Assess cavity access. Fishable wood-framed cavities favor NM. No-cavity plaster or masonry favors conduit/surface raceway.
4. Check AHJ and occupancy. Multifamily or local metallic-method amendments push you to MC or conduit. Pull the local code amendments before quoting.
5. Match method to exposure. Concealed and protected, NM. Exposed but indoors, MC. Exposed-and-damp or maximum-protection, conduit.
6. Verify grounding path on every run, and ground existing metal boxes you reuse per NEC 250.148.

References

• NFPA 70 National Electrical Code, Article 394 (Concealed Knob-and-Tube Wiring), including 394.10 uses permitted and 394.12 uses not permitted.
• NFPA 70 Article 334 (Nonmetallic-Sheathed Cable, Types NM, NMC, NMS), 334.10/334.12/334.15.
• NFPA 70 Article 330 (Metal-Clad Cable, Type MC) and Article 250.118 for equipment grounding via cable armor.
• NFPA 70 Article 358 (Electrical Metallic Tubing) and Article 250.148 (grounding continuity at boxes).
• NECA/NEIS installation standards for cable and raceway support and securing.