If you find knob-and-tube wiring in your Philadelphia home, you don't have to panic, but you shouldn't ignore it either. Have a licensed electrician evaluate it, keep insulation away from it, make sure every fuse matches the wire it protects, and plan to replace the circuits that are damaged, overloaded or in wet areas first. Stop using a circuit and call right away if you see sparks, smell hot insulation or feel a hot outlet. One rule matters more in Philadelphia than in many places: the City says a licensed electrical contractor must perform all electrical work, so most fixes here are a job for a pro, not a weekend project.
This is general information, not an electrical inspection. For code questions in the city, the Department of Licenses and Inspections (L&I) has the final word; in Bucks and Montgomery counties, ask your township or borough.
What knob-and-tube wiring is and how to recognize it
Knob-and-tube is the earliest common way American houses were wired. Instead of a cable with the wires bundled inside one jacket, it uses two separate single wires, a hot and a neutral, run apart from each other. They're held off the wood by white porcelain knobs where they run along joists, and they pass through porcelain tubes where they go through framing. The wire insulation is usually cloth or rubber.
Many Philadelphia rowhomes and twins are old enough to have had it, and pieces of it often survive behind walls even after a kitchen or a panel was updated. Places to look:
- The basement ceiling and the attic or crawl space above the top floor. Look for pairs of single wires on porcelain knobs, a few inches apart.
- Where wires pass through joists. Porcelain tubes in the holes are the giveaway.
- Your outlets. Two-slot outlets with no round ground hole are a hint that the circuit has no ground wire. They don't prove the wiring is knob-and-tube, and a three-slot outlet doesn't prove it isn't, because three-prong outlets are sometimes installed on circuits with no ground.
- The panel. A fuse box with screw-in fuses doesn't mean knob-and-tube is still in use, but it tells you the electrical system has a lot of history.
Seeing knobs in the basement doesn't mean the wiring is live. Old runs are often abandoned and left in place. Only an electrician testing the circuits can tell you what is still connected.
Why old wiring is a concern
Knob-and-tube that's intact and lightly loaded can keep working. The problems are age, changes and load:
- No ground wire. These circuits have no equipment ground, so three-prong appliances and surge protectors don't get the protection they're designed for.
- Insulation that's brittle or damaged. Cloth and rubber insulation dries out over decades, and wires get nicked during later renovations.
- Amateur splices and add-ons. Decades of "just one more outlet" can leave loose connections hidden in walls.
- Insulation piled on top. Knob-and-tube was designed to shed heat into open air. The National Electrical Code's article on knob-and-tube (Article 394, published by NFPA) doesn't allow concealed knob-and-tube in hollow walls, ceilings and attics where loose, rolled or foamed-in-place insulation surrounds the wires. That's why you should have the wiring checked before anyone blows insulation into the attic or walls.
- Oversized fuses. This one is common and dangerous, so it gets its own section.
The fuse box: the cheapest check you can do
A fuse protects the wire, not the appliance. If a 15-amp circuit keeps blowing its fuse, the tempting fix is a 20- or 30-amp fuse. The fuse stops blowing, and the wire inside your wall can overheat instead.
The Consumer Product Safety Commission's Guide to Home Wiring Hazards is blunt about it. It says that in a typical four-fuse (60-amp) panel most fuses should be rated at 15 or at most 20 amps, and that a fuse above 20 amps may mean someone was trying to stop outages by substituting a bigger fuse. It recommends going back to 15-amp fuses unless you're absolutely certain the circuit is wired for 20 amps. Because the old screw-in (Edison-base) fuses of different ratings all fit the same socket, CPSC recommends fitting the sockets with "S type" inserts that accept only a fuse of the correct rating. And it says never to put anything other than a fuse in a fuse holder.
What you can do today:
- Open the panel cover and read the number on each fuse. Write it down. Don't touch anything inside beyond what you'd normally do to change a fuse.
- If you see 25- or 30-amp plug fuses on ordinary lighting and outlet circuits, or a penny or foil behind a fuse, treat it as urgent and call an electrician.
- If a fuse keeps blowing, note which circuit it is and what was running at the time. That tells the electrician whether you have an overload or a fault.
Warning signs that mean "call now"
CPSC's guide lists warning signs of electrical hazards. These are the ones to act on immediately, by turning off the circuit at the panel if you can do it safely and calling an electrician:
- Sparks or bright flashes anywhere in the system.
- Sizzling or buzzing from outlets, switches or the panel.
- A smell of hot or burning insulation.
- Switch plates, outlet covers, cords or plugs that are hot (painful to touch) or discolored. CPSC notes they may be warm but should never be hot.
- Any shock, even a mild tingle, from a switch, outlet, appliance or faucet.
Less urgent but worth a visit: fuses that blow often, lights that dim when something turns on, plugs that fall out of worn outlets, and extension cords that have become permanent wiring. CPSC calls extension cords short-term only and says to have permanent wiring installed instead.
If anything is burning or smoking, leave the house and call 911.
Space heaters on old circuits
Heating season is when old wiring gets tested hardest. CPSC's portable heater safety sheet says never to power a heater with an extension cord or power strip, never to run the cord under a rug, and, if any part of the outlet gets hot, to contact an electrician. On a knob-and-tube or fused circuit, also avoid running a heater on the same circuit as a microwave, a hair dryer or another heater. If you need a heater because a room never warms up, the real fix may be on the heating side: see our guide to a furnace and boiler tune-up before winter.
What about the missing ground?
The usual partial fix for two-slot outlets is GFCI protection. CPSC's GFCI fact sheet notes that a GFCI works even on two-slot receptacles. It protects you from a serious shock, but it doesn't create a ground. The electrical code generally allows a GFCI receptacle, or a GFCI breaker feeding three-prong outlets, on a circuit with no ground, as long as the outlets are marked "No Equipment Ground," and some outlets would also need "GFCI Protected."
Two cautions. First, don't swap two-slot outlets for ordinary three-prong ones without GFCI protection: it looks grounded and isn't. Second, a GFCI on top of damaged knob-and-tube doesn't fix the damaged wire. It's a reasonable step on a circuit that's in good shape, not a substitute for replacing one that isn't.
Philadelphia rules: who can do the work
The City's electrical permit page sets out the basics:
- A licensed electrical contractor must perform all electrical work. The page doesn't describe an exception for homeowners. A Pennsylvania Home Improvement Contractor registration doesn't count for electrical work.
- An Electrical Permit is required for installing, altering, replacing or repairing electrical systems. Routine maintenance, such as replacing lamps, fuses and receptacles, may not need a permit.
- Inspections are done by a licensed electrical inspection agency that the owner or the owner's agent hires. Your electrician usually arranges this, but confirm who's responsible in writing.
- Fees are based on the estimated cost of the electrical work, with a minimum fee. The current schedule is on the same page.
Before you hire, check that the contractor holds a City electrical contractor license with L&I's find a licensed contractor tool, and after the job, you can look up the permit on Atlas by your address. If you're unsure whether something you want to do yourself is allowed, ask L&I before you start.
In Bucks and Montgomery counties, permit and inspection rules are set locally under the state's construction code, so call your township or borough.
Checklist: what to do if your home has knob-and-tube
- Note where you can see knobs, tubes, two-slot outlets and the panel type. Take photos.
- Read every fuse rating. Anything above 20 amps on a general circuit, or anything that isn't a fuse in a fuse holder, needs an electrician soon.
- Don't add insulation in the attic or walls until an electrician has checked the wiring there.
- Keep space heaters on wall outlets, one per circuit, never on cords or power strips.
- Ask a licensed electrician to test which knob-and-tube circuits are live and assess their condition.
- Ask for a written, prioritized plan: what's urgent, what can wait, and what can be replaced room by room.
- If you're buying or refinancing, ask your insurance agent early whether older wiring affects your coverage.
- Make sure any work has a permit and an inspection, and keep the paperwork for when you sell.
When to call a pro
In Philadelphia, almost always: the City requires a licensed electrical contractor for electrical work. Call right away for any of the warning signs above. Call soon if your fuses are oversized, if you're planning insulation, a kitchen or bathroom remodel, central air or an EV charger, or if you want three-prong outlets on circuits that have no ground. Rewiring a rowhome is often done in stages, and walls can sometimes be opened selectively, so ask how the electrician plans to run new wire and who patches the plaster afterward.
When comparing estimates, ask each electrician which circuits are still live, what they'd replace first and why, whether the panel or service needs an upgrade, and who pulls the permit and books the inspection. Our guide on how to check a contractor in Philadelphia covers the written contract and what to verify.
On HomeKept you can describe the job once, with photos of the panel and any wiring you can see, and compare up to three estimates from local electricians in Philadelphia. It's free for homeowners.
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