# What Wiring Replaces Knob-and-Tube in Philadelphia Homes?

> What replaces knob and tube wiring: grounded NM-B or metal-clad cable, 15 and 20 amp circuits, real boxes, AFCI and GFCI protection and a new layout. See how.

URL: https://phillyknobandtube.com/guide/what-replaces-knob-and-tube-wiring/

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A knob-and-tube replacement is new branch wiring from the panel to every outlet, switch and light, with a ground in every box and circuits planned around modern rooms. This is what that system is made of and how it gets into an old house.

Last Updated: October 26, 2026

Knob-and-tube is replaced with modern cable, usually type NM-B, that bundles a hot, a neutral and a bare equipment ground in one jacket: 14-gauge copper on 15-amp circuits and 12-gauge on 20-amp circuits. Metal-clad cable is used where protection or the building requires it. Every splice and device sits in a box, and the circuits are laid out anew.

That is the system [Philly Knob & Tube](https://phillyknobandtube.com/) delivers when it manages a rewire in Philadelphia; licensed electrical contractors run the cable, set the boxes and connect the panel, and a licensed inspection agency checks the work. The important idea is that replacement is the branch wiring itself, not a swap of devices on the old conductors. This guide describes the parts, the protection, how cable gets through plaster walls and what inspectors look for.

Key takeaways

- Replacement means new conductors from the panel to every outlet, switch and fixture
- NM-B cable carries an equipment ground to every box; metal-clad cable fits where conditions call for it
- 15-amp circuits use 14-gauge copper, 20-amp circuits use 12-gauge
- AFCI, GFCI and tamper-resistant receptacles are designed in from the start
- Rough wiring is inspected before walls are closed, then again at final

## The New Circuit and Cable System

The workhorse of residential rewiring is nonmetallic sheathed cable, designated NM-B. Inside a plastic jacket are an insulated black conductor (hot), an insulated white conductor (neutral) and a bare copper equipment grounding conductor. The “B” denotes conductor insulation rated for 90°C, although sizing rules for this cable use its lower 60°C ampacity, which builds in margin. Unlike knob-and-tube, where each conductor ran separately and relied on open air, the conductors travel together and are rated for being enclosed in walls.

| Circuit | Typical cable | Typical use |
| --- | --- | --- |
| 15 A, 120 V | 14/2 with ground | General lighting, bedroom and living room receptacles |
| 20 A, 120 V | 12/2 with ground | Kitchen countertop, bathroom, laundry, dedicated appliance circuits |
| Three-way switching | 14/3 or 12/3 with ground | Stair and hall lights controlled from two places |
| 240 V appliance | Heavier cable sized to the load | Electric range, dryer or other large equipment, when part of the scope |

The numbers before the slash are the copper gauge; the number after is the count of insulated conductors, with the ground added. Manufacturers color the jackets by gauge, white for 14 and yellow for 12, which helps inspectors check that cable matches its breaker. Exact sizes come from the electrician’s design and the loads on each circuit; the [appliance loads guide](https://phillyknobandtube.com/guide/knob-and-tube-appliance-loads/) explains why some rooms now need more than one.

Boxes are the other big change. Many original outlets and switches were mounted directly to lath, and many splices were soldered and taped in open air. New work puts every device, fixture and splice in an accessible box sized for the number of conductors it holds. In plaster walls, electricians use old-work boxes that clamp to the back of the wall surface through a neat cutout. Ceiling boxes are rated for the fixture or fan they carry.

## Where Metal-Clad Cable Fits

NM-B is not the only answer. Metal-clad cable (type MC) wraps the conductors, including an insulated green ground, in an interlocking aluminum or steel armor. Electricians choose it where cable runs exposed and could be struck, such as along an unfinished basement ceiling below joist level, where a building’s construction type or use limits nonmetallic cable, or simply where the route benefits from a tougher jacket. Some two-to-four-unit buildings fall into categories where those limits come into play, which the contractor confirms during design. The 2020 National Electrical Code permits nonmetallic cable in one- and two-family dwellings, so in most houses the choice between cable types is made run by run. Older armored cable (BX) is a different, earlier product with its own grounding questions, compared in the [knob-and-tube vs BX guide](https://phillyknobandtube.com/guide/knob-and-tube-vs-bx-wiring/).

## Protection and Grounding Decisions

Each new circuit lands on a breaker sized to its wire, so the overfusing common in old fuse boxes cannot carry over. The ground in every cable bonds each metal box and every receptacle’s ground terminal back to the panel, so three-prong outlets are grounded for real instead of labeled as ungrounded, the distinction drawn in the [three-prong outlets guide](https://phillyknobandtube.com/guide/three-prong-outlets-and-ungrounded-wiring/).

Under the 2020 National Electrical Code, which Philadelphia enforces, new circuits feeding bedrooms, living areas, kitchens, hallways, closets, laundry areas and similar rooms get arc-fault protection, and receptacles in bathrooms, kitchen countertop areas, basements, laundry areas, outdoors and near sinks get ground-fault protection. Often a combination arc-fault breaker or a dual-function breaker in the panel does both jobs. New receptacles in a dwelling must be tamper-resistant, with shutters that block objects pushed into one slot. The [AFCI vs GFCI guide](https://phillyknobandtube.com/guide/afci-vs-gfci-for-old-wiring/) explains what each device detects.

The panel has to accept all of this. Old fuse boxes cannot take AFCI or GFCI breakers, and many older panels have no spare spaces, so panel work frequently travels with a rewire; the [panel upgrade comparison](https://phillyknobandtube.com/guide/panel-upgrade-vs-knob-and-tube-replacement/) explains how the two scopes relate.

## A New Circuit Layout, Not a Copy of the Old One

Old circuits wandered across rooms and floors. New ones are planned by use. Lighting may run on its own circuits so a tripped receptacle circuit does not leave a room dark. Kitchens get at least two 20-amp small-appliance circuits for countertop receptacles. Bathrooms, laundry and appliances such as dishwashers and built-in microwaves get their own circuits. Where walls are opened, receptacle locations are planned with current spacing rules in mind, which typically means more outlets than an old room had and fewer extension cords.

When the work is done, each breaker is labeled in the panel directory by what it serves. That directory, plus a record of which original circuits were disconnected, is what a future buyer, insurer or electrician relies on. The [guide to what to expect after replacement](https://phillyknobandtube.com/guide/what-to-expect-after-knob-and-tube-replacement/) covers those records.

## Switches, Fixtures and the Details You Will Notice

Some changes show up in daily use. Switch boxes on new circuits generally carry a neutral conductor, which current code requires at most switch locations so that occupancy sensors, timers and smart switches can be added later without new wiring. Knob-and-tube switch loops never had one. Ceiling fixtures are reconnected to new boxes, and an old fixture whose internal leads have gone brittle from decades of heat is a candidate for rewiring or replacement before it goes back up; that is often a separate line in the scope.

The number and placement of outlets is usually the most visible difference. Rooms that had one receptacle may get three or four, placed where furniture and appliances actually sit. Exterior and basement receptacles come with ground-fault protection, and in a rowhouse with the laundry in the basement, that corner typically gets its own circuit.

Owners of older houses sometimes want the original look kept. Reproduction push-button switches and period cover plates are made to fit standard modern boxes, so new wiring does not have to mean new-looking hardware. The [historic home service](https://phillyknobandtube.com/historic-home-knob-and-tube-replacement/) covers projects where finishes and appearance carry extra weight.

## Routing Through Finished Walls

Getting new cable into a finished house without tearing it apart is the craft of a rewire. Electricians look for paths that already exist: up from an open basement ceiling into interior partition walls, down from an accessible attic into top-floor walls and ceilings, and through closets, chases and stacked walls that connect floors. Long flexible fish tapes and drill bits pull cable through cavities from one opening to the next. Where a path is blocked by fire blocking, a beam or a header, a small access opening in the plaster gets the cable past, and the opening is restored later.

Philadelphia rowhouses add their own conditions. Where party walls and exterior walls are solid masonry with plaster applied directly to the brick, there is no cavity to fish through, so outlets on those walls may be fed from the floor below, along baseboards or in surface raceway. Interior framed partitions are usually where cable travels vertically. The [plaster-wall rewiring service](https://phillyknobandtube.com/plaster-wall-knob-and-tube-rewiring/) and the [guide to replacing knob-and-tube without gutting walls](https://phillyknobandtube.com/guide/replacing-knob-and-tube-without-gutting-walls/) cover access strategy and patching expectations in more depth.

The old conductors are disconnected at the panel and at their last device so they can never be re-energized. Accessible runs in basements and attics are commonly removed; runs sealed in walls are usually left in place dead. The [removal service](https://phillyknobandtube.com/knob-and-tube-removal/) explains that choice.

## What Permitting and Inspection Check

Replacing branch wiring is not a minor repair. In Philadelphia, a licensed electrical contractor obtains an electrical permit from the Department of Licenses and Inspections through eCLIPSE, and the owner or the owner’s agent hires a licensed third-party electrical inspection agency to perform the inspections. The [permits guide](https://phillyknobandtube.com/guide/philadelphia-electrical-permits-for-rewiring/) covers fees and paperwork.

The rough-in inspection happens before any wall or ceiling is closed. The inspector looks at the cable type and gauge for each circuit, how cable is supported and protected where it passes through framing, box fill, grounding connections in every box and how runs enter the panel. Steel plates over cable near the face of studs, and bored holes set back from framing edges, are details checked here. The final inspection looks at installed devices, working arc-fault and ground-fault protection, the panel’s labeling and the finished terminations. The [final inspection guide](https://phillyknobandtube.com/guide/final-inspection-after-knob-and-tube-rewiring-philadelphia/) describes that visit and the paperwork it produces.

## Getting a Plan for Your House

The materials are standard. What differs from house to house is the route, the number of circuits, the panel and how much plaster has to be opened. To see how a project is sequenced, read the [rewiring process guide](https://phillyknobandtube.com/guide/knob-and-tube-rewiring-process/) or the [whole-home rewiring service](https://phillyknobandtube.com/whole-home-knob-and-tube-rewiring/), then [request an estimate](https://phillyknobandtube.com/contact/) for a circuit-by-circuit plan.

## Questions about this guide

### What gauge wire replaces knob-and-tube?

New 15-amp circuits typically use 14-gauge copper and 20-amp circuits use 12-gauge copper, each as cable with an insulated hot, an insulated neutral and a bare or green equipment ground. Kitchens, bathrooms and laundry areas get 20-amp circuits. Larger appliances get heavier cable sized to their nameplate and the breaker that protects them.

### Will new wiring use the old knob-and-tube holes in the joists?

Sometimes the same paths make sense, especially vertical routes from the basement or down from the attic, but the porcelain tubes themselves are not reused. New cable needs bored holes set back from the edge of framing, or steel protection plates where it is close to a face that could be nailed. The new layout also differs from the old, so many routes are new.

### Do all the old switches and outlets get replaced in a rewire?

On circuits being replaced, yes. Old devices are rated for the old two-wire system, often sit without a box, and cannot accept the ground. Replaced receptacles are grounded, tamper-resistant and protected as current code requires for the room. Decorative push-button switch styles are still made if a historic look matters to you.

## Related service

### Whole-home rewiring

Replace every active knob-and-tube branch circuit in the house, with the permit, inspections and a written record of what changed.

## More guides

- Learn [Can Knob-and-Tube Wiring Handle Modern Appliances?](https://phillyknobandtube.com/guide/knob-and-tube-appliance-loads/)Knob-and-tube circuits were laid out for lights and a few outlets. A single space heater can use most of one. Here is the arithmetic, what it does to old conductors, and how new dedicated circuits fit into a replacement plan.
- Learn [AFCI vs GFCI for Old House Wiring](https://phillyknobandtube.com/guide/afci-vs-gfci-for-old-wiring/)Arc-fault devices look for the electrical signature of a fire starting. Ground-fault devices look for current leaking through a person. Neither one restores a century-old circuit, and both can trip constantly on wiring that shares neutrals.
- Learn [Does a Three-Prong Outlet Mean Old Wiring Is Grounded?](https://phillyknobandtube.com/guide/three-prong-outlets-and-ungrounded-wiring/)Three holes in a receptacle describe its shape, not what is behind it. Here is what knob-and-tube circuits lack, the replacement options the electrical code allows, and the shortcuts that create new hazards.
- Decision Guide [Does a Panel Upgrade Replace Knob-and-Tube Wiring?](https://phillyknobandtube.com/guide/panel-upgrade-vs-knob-and-tube-replacement/)A new breaker panel and a knob-and-tube replacement are separate jobs that often get confused on invoices and insurance forms. Here is what each one changes, why old circuits survive a panel swap, and when a project needs both.
- Learn [How Does Knob-and-Tube Rewiring Work?](https://phillyknobandtube.com/guide/knob-and-tube-rewiring-process/)The technical sequence behind a rewire, from tracing which old circuits are live to the moment the last original run is cut loose at the panel and the inspection agency signs off.