Knob-and-tube wiring in Maryland homes.

Knob-and-tube is the wiring many of the region's oldest homes were built with, and it can still be found in service in older Baltimore City rowhomes and older houses in Baltimore County and Harford County. It isn't automatically dangerous, but it was designed for a different era of electrical use. This guide explains how to recognize it, what the code says, and how replacing it works. Our knob-and-tube and older home rewiring page covers the work we do.

What knob-and-tube is

Knob-and-tube was a standard way to wire houses in the early twentieth century. Instead of a cable with several wires bundled inside, it runs each conductor separately:

• Porcelain knobs hold the wires away from joists and studs.
• Porcelain tubes carry the wires through framing.
• Cloth or rubber-covered wire runs in open air between them, with the hot and neutral spaced apart.

The spacing and open air let the wires shed heat. That design worked for a house with a few lights and outlets. It wasn't built for a kitchen full of appliances, window air conditioners or a modern entertainment setup.

Why older Maryland homes still have it

A Stacker analysis of U.S. Census Bureau 2023 five-year estimates ranks Baltimore City first in Maryland for pre-war housing: about 41% of homes, roughly 120,800 of them, were built in 1939 or earlier. A home that age isn't guaranteed to have knob-and-tube, since many have been rewired, but it's the housing stock where we look for it first. Older farmhouses and village homes in Baltimore County and Harford County can have it too, often alongside later additions wired with modern cable.

How to tell if your home has it

• White porcelain knobs or tubes on joists in the basement or the attic
• Single wires with cloth or rubber covering, running separately rather than in a cable
• Two-prong outlets, especially on the original floors of the house
• Three-prong outlets with no ground behind them, which a plug-in tester will flag
• A home inspection report that mentions “knob-and-tube” or “ungrounded circuits”

Look, but don't touch. Old insulation can crumble when it's moved, and the wiring may still be live even if you think it's been abandoned.

The real risks

No ground

Knob-and-tube has no equipment grounding conductor. Three-prong appliances and surge protectors plugged into it don't get the ground path they're designed to have.

Brittle insulation

After decades of heat, the covering on the wire can crack or fall away, especially where it's been disturbed in an attic, at fixtures or in boxes.

Insulation packed around it

The National Electrical Code's article on concealed knob-and-tube wiring doesn't allow it in hollow spaces of walls, ceilings and attics where loose, rolled or foamed-in-place insulation envelops the conductors. Wiring that relied on open air to stay cool shouldn't be buried in insulation. If you're planning to insulate an attic or walls, deal with the wiring first.

Decades of changes

The bigger problem is often what's been done to it since: splices with no box, newer cable tied into old circuits, and fuses or breakers upsized so a circuit stops tripping. Those are the things an inspection looks for.

What the code says about ungrounded outlets

You don't have to rewire just to replace a worn two-prong outlet. When there's no ground at the box, the NEC allows the replacement to be a two-prong outlet, a GFCI outlet marked “No Equipment Ground,” or a three-prong outlet protected by a GFCI and marked “GFCI Protected” and “No Equipment Ground.” GFCI protection guards people against shock, but it doesn't create a ground, so equipment that needs one still won't have it. Adding three-prong outlets with no ground and no GFCI is a common inspection finding.

Do you have to replace it?

The electrical codes enforced in Harford County, Baltimore County and Baltimore City don't set a date by which knob-and-tube that's in service has to be removed. Practical pressures come from elsewhere: a buyer's inspector who flags it, a renovation that opens the walls, an attic that needs insulation, or a house that needs more circuits than the old wiring can safely give. Ask your insurance agent how they treat it, since that's a question only your insurer can answer.

How a replacement works

1. Find what's live. We trace circuits, open a few boxes and check the panel, so the plan is based on what's really in the walls.
2. Plan in phases. The worst or busiest circuits first, or the whole house at once. Most of the work goes through existing walls and ceilings with small, targeted openings.
3. Permit and inspection. Rewiring is permitted work in Harford County, Baltimore County and Baltimore City, built to the code edition in force there. See which NEC applies in Maryland.
4. New grounded circuits. New circuits replace the old ones, which are disconnected and retired. When branch circuits in living areas are modified, replaced or extended, the NEC generally calls for arc-fault (AFCI) protection, and GFCI protection goes where the code requires it.
5. Check the panel. A house with original wiring often has an original or undersized panel too. See rowhome service upgrades in Baltimore and panel and service upgrades.

Buying or selling a house with knob-and-tube

If it shows up on an inspection report, a written evaluation from a licensed electrician gives both sides something concrete: which circuits are still live, their condition, and what replacing them involves. Our home inspection electrical repairs page covers report-driven work on a closing timeline.

Wyatt Electric has worked in older homes across Baltimore City, Baltimore County and Harford County since 1986. We'll tell you plainly what we find, and give you a price before any work starts.