Knob and tube is replaced by running new grounded cable from the panel to every outlet, switch and light the old circuits served, then disconnecting the old wiring so it can never carry power again. The order is map, permit, rewire, rough-in inspection, disconnect, test, then close the walls. Decker Boston Knob and Tube arranges that work across Boston and nearby cities, carried out by licensed and insured electricians.
Each step depends on the one before it. Skipping the map leads to missed circuits; closing a wall before the inspection means reopening it.
Stage 1: the circuit map becomes the plan
Everything else rests on the map. A home inspection that notes “knob and tube observed” says the method exists, not how much is live or where it goes.
What the map records
The first visit starts at the panel, traces every circuit to the rooms it feeds, and checks the attic and basement for the routes old runs take and new cable could take. For each circuit it records the wiring type, the outlets and fixtures on it, and any place one type is spliced into another. The inspection service describes that tracing.
How the map turns into a layout
Mapping is where the old system’s limits become a design question. Original circuits were laid out for a few lights and outlets per room; the new layout is planned around what the house runs now. That’s usually the cheapest moment to add outlets, split a crowded circuit or give a kitchen its own circuits, because the walls are being reached anyway.
The written scope
The plan ends up as a written scope: each new circuit, each old run to be disconnected, the openings the routes need, whether the panel stays, and a separate line for patching. The quote comparison guide explains how to read one.
Stage 2: the permit and the panel
The permit is filed before work begins. In Boston, Inspectional Services requires a licensed contractor to apply; homeowners can’t pull an electrical permit there. The Boston permit guide covers fees and the other cities’ wire offices.
The panel decision is made here too. Massachusetts requires arc-fault protection on all 10, 15 and 20 amp, 120 volt branch circuits supplying outlets or devices in a home, so each new circuit takes a breaker space. A fuse box or a full panel often can’t take them, and the panel guide explains why that’s a separate scope.
Stage 3: running the new cable
Most of a rewire is fished through the house’s own cavities rather than cut into the walls. That’s what keeps plaster damage small.
What the new cable is
The usual replacement is nonmetallic sheathed cable, NM-B: two insulated circuit conductors and a bare equipment grounding conductor in one plastic jacket. That ground wire is what knob and tube never had. Armored cable or conduit is used where the setting calls for it, and in buildings over three stories the Massachusetts text of the code limits cable circuits to the floor or dwelling unit where they start, which affects taller rowhouses.
How it gets through plaster
The state code allows cable to be fished between access points for concealed work in finished buildings where strapping it to the framing isn’t practical. Runs drop from a walkable attic into top-floor walls and rise from an open basement into first-floor walls. Small openings go where a box will sit, at top plates and at fire blocking that stops the fish tape.
Strapped ceilings
Where a ceiling was plastered over wood strapping, cable run diagonally behind strapping of about three-quarters of an inch is treated as supported if it isn’t pulled taut. That lets ceiling lights be reached across a room without opening the whole ceiling. The plaster wall rewiring page covers how openings are cut.
Stage 4: boxes, outlets and switches
New cable ends in new connections. This is where the rewire becomes visible in the rooms.
| Item | What changes |
|---|---|
| Boxes | Old shallow boxes are often replaced with ones large enough for modern devices and the ground connection |
| Receptacles | Grounded three-prong outlets on the new circuits, tamper-resistant where the code requires |
| Switches and fixtures | Reconnected to the new cable, with a ground at each box |
| Kitchens, baths and laundry | Ground-fault protection for receptacles within 6 feet of a sink and inside a sink cabinet, under the Massachusetts amendments |
| Panel end | Each new circuit lands on its own breaker, with arc-fault protection |
The AFCI and GFCI guide explains what each protective device does.
Stage 5: rough-in inspection
The inspector of wires looks at the new wiring before anything covers it. Massachusetts Rule 10 says installations can’t be concealed until inspected, within 72 hours of proper notice, Saturdays, Sundays and holidays excluded.
This is why patching never starts while a room is being wired. Openings stay open until the rough-in passes, and a schedule should show where each inspection falls.
Stage 6: disconnecting and verifying the old wiring
Once a new circuit is live, the knob and tube it replaces is disconnected at both ends. Disconnection matters more than removal, because a dead wire in a wall carries no current.
Removal where it’s reachable
Old runs in a reachable attic or basement are taken out. That clears the space for insulation and removes any doubt about what the wire is. Concealed runs inside finished walls are usually left in place, disconnected, because pulling them would mean opening the walls the plan avoided.
Proving nothing is left in service
Testing then confirms that every outlet and light is fed by the new circuits and that no old run is still connected through a hidden splice. That matters most in houses where earlier owners spliced newer cable onto knob and tube; the guide to mixed wiring covers those junctions. The removal service explains how dead runs are documented.
Stage 7: closing the walls and the final record
After the inspection and testing, the openings are closed to the finish agreed in writing. In a house built before 1978, disturbing more than six square feet of painted surface in a room brings in the state’s lead-safe renovation rules, which is why opening size is planned from the start.
What you keep
- A labeled panel schedule showing what each breaker feeds.
- A written summary stating that no knob and tube remains in service, and which disconnected runs were left in walls.
- The closed permit with its inspections.
Those records answer an insurer, a buyer or a Mass Save certification. The insulation barrier service covers the Mass Save paperwork.
How long it takes
There’s no single figure. Duration follows the number of live circuits, the number of floors, how open the attic and basement are, and how many inspections the staging needs. A partial replacement of a few attic-fed circuits is a different job from a three-story house still on its original wiring. Ask for a written schedule that shows the working days per area and where each inspection falls, and see the living at home guide for the household side.
A whole-house replacement falls in a typical Boston-area budgeting range of $8,000 to $25,000 or more, and the cost guide shows what moves it. To find out how your own house would be sequenced, call Decker Boston Knob and Tube at (617) 405-3599 or ask for an estimate.