Knob-and-tube, fuse boxes and two-prong outlets: old wiring explained

Updated November 10, 2026 · KeystoneProGuide editorial team

How common is old wiring in Pennsylvania?

Common enough to plan for. The Census Bureau estimates that 25.0 percent of Pennsylvania's housing was first built in 1939 or earlier, and in Philadelphia the figure is 39.9 percent. A house that old was wired, at least originally, with systems that the Electrical Safety Foundation, a nonprofit safety group, says are no longer considered safe. Some will have been partly rewired since, so the only way to know what is behind your walls is to have someone look.

What is knob-and-tube wiring?

The earliest system in most old houses. The foundation dates it to the 1800s through the 1930s and describes it as an open-air system: single wires were held away from the wooden framing on ceramic knobs and passed through ceramic tubes where they went through joists, so that they would not touch the wood. It says knob-and-tube is now considered a fire hazard because it is not a grounded system and because it is more easily damaged by aging and by faulty renovation work.

You may see it in an unfinished basement or attic: separate wires held on ceramic knobs. Leave it alone and point it out to an electrician.

What came after knob-and-tube?

Grounded systems. The foundation dates grounded wiring from the 1940s to the present. It says a third, grounding wire runs to all outlets and metal boxes and connects to the earth through a grounding rod or a cold water pipe, and that it is designed to greatly reduce the chance of shock or electrocution if a short circuit occurs.

The difference shows at the outlet. The foundation says most homes since 1920 have polarized outlets, with one slot wider than the other, and that grounded outlets add a round hole for the ground. It says grounded outlets are required in all modern homes, and that a home without them is likely missing critical safety features.

Why do two-slot outlets matter?

Because people work around them. The U.S. Fire Administration says never to force a three-prong cord into a two-slot outlet, and the foundation warns against running extension cords through walls or ceilings, where they can overheat. The National Fire Protection Association says extension cords are intended for temporary use only, not as a substitute for outlets. The fix the foundation suggests is to consult an electrician about updating the system.

What about a fuse box?

It dates the panel. The foundation says service panels installed before 1965 used fuses for each circuit, that early fuse panels were commonly 30 or 60 amps, and that new homes today need 100- to 200-amp panels. The foundation says fuses and breakers both exist to protect each circuit's wires from overheating and causing a fire, which only works if each fuse is the right size: a replacement must match the circuit's amperage, and calls a larger fuse a very dangerous practice and a serious fire hazard. Capacity is the other question. The foundation says the demand from modern appliances and devices can overburden an older home's electrical system, resulting in fires or electrocutions.

What do safety bodies recommend for an older house?

Inspection first. The NFPA advises having a home inspected by a qualified private inspector, or according to local requirements, when you buy or remodel it. The foundation advises asking a qualified electrician whether the home would benefit from arc-fault protection, especially during inspections of older homes or upgrades, and lists the devices to add during any renovation: arc-fault and ground-fault protection, surge protection and tamper-resistant receptacles. It notes that the National Electrical Code is revised every three years and that many older homes may not meet today's electrical demands.

Do you have to rewire the whole house?

Not necessarily, and no page read for this guide says that existing old wiring must be removed because it is old. Ask an electrician who has seen inside the walls whether targeted work, such as replacing damaged sections, adding grounded circuits and adding protection devices, would do, or whether the house needs a full rewire.

If the work adds or moves wiring, Pennsylvania's code regulation says it is not an ordinary repair and needs a permit, so either route will normally be inspected. In a house built before 1978, the Environmental Protection Agency requires anyone paid to disturb painted surfaces to be certified in lead-safe practices, which matters when plaster walls are opened to run new cable.

What does rewiring cost?

The published ranges are wide. Angi, a home-services marketplace, gives $1,500 to $10,000 to rewire a house, and Fixr gives $7,191 to $13,384 for whole-home rewiring. Fixr also says an older home with its original outlets, fixtures and wiring may cost more to repair, and that work requiring access through drywall, attics or crawlspaces can raise the price. These are national figures. The only reliable number for your house is a written estimate from an electrician who has inspected it, and it is worth getting two.

What should you ask the electrician?

• Is there any knob-and-tube or other ungrounded wiring still in use, and where?
• Which circuits would you replace first, and why?
• Would you add arc-fault and ground-fault protection as part of the work?
• Will the walls need opening, and how will lead dust be controlled?
• Is the panel big enough for what the house now uses?

Arc-fault and ground-fault protection are explained in the GFCI and AFCI guide, the signs that a circuit is in trouble are in the warning signs guide, and prices for other jobs are in the cost guide. Electricians are listed on the electricians directory page.

Sources

1. Understanding Your Home Electrical System, Electrical Safety Foundation International (ESFI)
2. Know the Dangers in Your Older Home, Electrical Safety Foundation International (ESFI)
3. Home Safety Devices - Renovate Your Home to Code, Electrical Safety Foundation International (ESFI)
4. Home Electrical Fires, Electrical Safety Foundation International (ESFI)
5. Appliance and Electrical Fire Safety, U.S. Fire Administration
6. Electrical Home Fire Safety, National Fire Protection Association (NFPA)
7. 2020-2024 ACS 5-Year Narrative Profile: Pennsylvania, U.S. Census Bureau
8. 2020-2024 ACS 5-Year Narrative Profile: Philadelphia County, Pennsylvania, U.S. Census Bureau
9. Lead Renovation, Repair and Painting Program, U.S. Environmental Protection Agency
10. 34 Pa. Code § 403.62. Permit requirements and exemptions., Commonwealth of Pennsylvania (Pennsylvania Code)
11. How Much Does an Electrician Cost to Hire? [2026 Data], Angi
12. How Much Does it Cost to Hire an Electrician?, Fixr