# Knob and tube inspection in San Francisco

> Knob and tube inspection in San Francisco: every circuit traced and tested, live and abandoned runs mapped, splices and boxes checked, and a clear scope.

URL: https://fogcitysfknobandtube.com/knob-and-tube-inspection/

**Service**

Every circuit traced and tested, live runs separated from dead ones, and findings that say plainly what was tested and what was only seen.

Last Updated: October 30, 2026

**Where**

San Francisco and the East Bay: Berkeley, Oakland, Alameda and Piedmont.

**Local rules**

- An electrical permit from DBI before new wiring goes in, then a pre-cover and a final inspection.San Francisco Electrical Code 89.127
- A separate permit for each dwelling unit, except in single-family (R-3) buildings.DBI fee schedule
- Only owner-occupants of a single-family home may pull their own electrical permit.SF Department of Building Inspection
- No insulation around concealed knob and tube without a licensed electrical contractor's certification.California Electrical Code 394.12
- Up to $2,000 toward active knob and tube for income-qualified PG&E customers in an EASE Home project.BayREN

Before anyone prices a rewire, it helps to know which wiring is still live. [Fog City SF Knob and Tube](https://fogcitysfknobandtube.com/) arranges knob and tube inspections in San Francisco and the East Bay: every circuit in the house is traced and tested so you know which wiring is still live on knob and tube, which is dead, and which has been joined to later cable. Licensed and insured electrical contractors (California C-10) carry out the testing and write the findings.

An inspection comes before any decision about scope. A San Francisco house with knob and tube may have had work done since it was wired: a kitchen remodel, a new panel, an outlet added for a bathroom heater. Each change may have retired some original runs and left others in service. The point of the visit is to replace guesses about age with a map of what each circuit actually runs on. It is an electrical inspection by an electrical contractor, not a general home inspection and not an engineering report, and it does not decide whether an insurer will write a policy. When a carrier is the one asking, the [electrical inspection for insurance](https://fogcitysfknobandtube.com/electrical-inspection-for-insurance/) turns the same testing into the written account an underwriter needs.

## What happens on the first visit

The visit starts at the panel or fuse box. Each circuit is identified, its fuse or breaker size noted, and the circuit followed out to the outlets, switches, lights and appliances it serves. Some circuits are switched off one at a time to see exactly what goes dead; runs seen in the attic or basement are tested for voltage to show whether they still carry power. The result is a map of the house by circuit rather than by room, because one original circuit often served parts of three rooms on two floors.

Then the reachable spaces are walked: the attic, the basement or crawlspace, and in many west-side houses the garage ceiling under the living floor. In DBI permit descriptions that name knob and tube, filed from January 2016 to October 2026, about one in eleven mention a basement, crawlspace or garage, which is where much of this wiring can still be seen. Nothing is pulled, unwrapped or moved to look at it. Old conductor insulation can crack when handled, and a run that looks abandoned may still be live.

## Circuit tracing: telling live runs from dead ones

Circuit tracing is the core of the inspection, and it is why the inspection comes before a quote. A house can show porcelain knobs in the attic and still have no live knob and tube at all, because a past rewire disconnected the old runs and left them in place. The opposite happens just as easily: a remodeled kitchen with modern outlets can sit on a cable that was spliced into an original circuit inside a ceiling, so the knob and tube upstream is still carrying the kitchen’s load.

Tracing sorts every run into one of four groups: active knob and tube; abandoned knob and tube that is disconnected; later wiring on its own circuit; and mixed circuits where the two are joined. The [active vs abandoned guide](https://fogcitysfknobandtube.com/guide/active-vs-abandoned-knob-and-tube/) explains why that distinction carries more weight than whether knob and tube is present at all. Mixed circuits get the most attention, because the joint between eras is often where a problem sits.

## Reading the condition of the wiring

Once a run is known to be live, its condition is recorded where it can be seen. The inspection looks for conductor insulation that has gone brittle or cracked, most often near ceiling light boxes where decades of heat have baked it, and for damage from rodents, later framing work or a nail. It checks that wires still sit on their knobs and pass through their tubes, rather than sagging onto wood or resting against each other.

Two conditions matter more than appearance. The first is protection: a fuse or breaker larger than the old conductors can carry means the wire can overheat before the device opens, and that is recorded circuit by circuit. The second is insulation contact. Knob and tube was designed to shed heat into open air, and California Electrical Code 394.12 bars concealed knob and tube in wall, ceiling and attic spaces where loose, rolled or foamed insulation envelops the conductors, unless a licensed electrical contractor surveys it and certifies it under the code’s California exception. Insulation found over live runs goes in the findings, and the [attic insulation service](https://fogcitysfknobandtube.com/knob-and-tube-attic-insulation/) covers what to do about it.

## Splices and junction boxes

Original knob and tube branches were spliced in the open framing and wrapped, which was the method of the time. The splices that worry an inspector are the later ones: a length of modern cable twisted onto an old run inside a ceiling with no box, a connection buried under attic insulation, or a junction box whose cover is missing or whose cable enters with no clamp. Each one is a point where two wiring systems with different insulation and different protection meet.

Junction boxes in the reachable spaces are checked for what they join: two later cables, a later cable and knob and tube, or a mix that ties a modern circuit to an original neutral. Boxes buried under insulation, or connections that should be in a box and are not, are listed with their location. They are the main reason a modern-looking circuit can still depend on knob and tube, and the main reason a [partial replacement](https://fogcitysfknobandtube.com/partial-knob-and-tube-replacement/) has to define its boundary carefully.

## What the findings can and cannot tell you

An inspection can tell you which circuits are live on knob and tube, what they feed, how they are protected and what condition their visible parts are in. It cannot see inside every closed wall. A plaster cavity between an attic entry point and an outlet box is inferred from testing, not observed. The written findings should say which conclusions come from testing, which from looking, and which spaces could not be reached, so nobody later mistakes a careful inference for a guarantee.

It also cannot answer questions only other parties decide. Whether existing knob and tube is acceptable is partly a code question and partly a carrier’s or lender’s decision. The code side is narrower than many owners expect: San Francisco’s Electrical Code says existing wiring that was legal when installed and has been maintained need not be altered, and the [California legality guide](https://fogcitysfknobandtube.com/guide/is-knob-and-tube-legal-in-california/) covers the details. A house is not declared safe or unsafe by an inspection; the findings describe conditions and leave the scope decision to you.

## From inspection to scope

The findings point to one of a few scopes. Where only a few circuits remain live and they are sound, unburied and correctly protected, leaving them in service and monitoring them can be a reasonable choice. A single damaged run or bad splice may call for a targeted repair. Where a remodel already replaced most of the house, [partial replacement](https://fogcitysfknobandtube.com/partial-knob-and-tube-replacement/) of the remaining circuits is usually the better value; where most circuits are original or several eras are spliced together, [whole-house rewiring](https://fogcitysfknobandtube.com/whole-house-rewiring/) replaces them all. Runs found dead are disconnected at both ends and removed where reachable, which is the work of [knob and tube removal](https://fogcitysfknobandtube.com/knob-and-tube-removal/).

If replacement follows, the inspection’s circuit map becomes the basis of the written scope and price, and the panel’s condition and spare capacity show whether an [electrical panel upgrade](https://fogcitysfknobandtube.com/electrical-panel-upgrade/) belongs in the same plan. The [is knob and tube dangerous guide](https://fogcitysfknobandtube.com/guide/is-knob-and-tube-wiring-dangerous/) explains which findings make work more urgent and which do not.

## Buying, selling and insulating

Three events tend to put an inspection on the calendar. On a sale, California Civil Code 1102.6i requires sellers or their agents, for transfers on or after January 1, 2026, to give buyers a statement that it may be advisable to have a qualified professional inspect the electrical system, and that substandard or faulty wiring may make insurance difficult to obtain. Buyers use the findings to price a rewire into an offer, which the [buying guide](https://fogcitysfknobandtube.com/guide/buying-a-house-with-knob-and-tube/) covers; sellers use them to disclose accurately, covered in the [selling guide](https://fogcitysfknobandtube.com/guide/selling-a-house-with-knob-and-tube/).

The third event is insulation. An attic cannot be insulated over concealed live knob and tube without the electrician’s survey and certification the code describes, or removal first. BayREN announced $300 toward a licensed electrician’s certification that knob and tube is inactive, for income-qualified PG&E customers in homes of four or fewer units that are doing an EASE Home weatherization project; its program page shows only the $2,000 remediation offer, so confirm the $300 with BayREN. The contractors supply the certification and Fog City SF Knob and Tube helps with the rebate paperwork; BayREN decides eligibility. In Berkeley, the city’s Form #146 asks the electrician to certify one of three conditions before attic insulation is inspected, which the [Berkeley page](https://fogcitysfknobandtube.com/service-areas/berkeley/) explains.

If you want a first read before booking, the [knob and tube risk assessment](https://fogcitysfknobandtube.com/tools/knob-and-tube-assessment/) sorts what you already know about the house, and the [identification guide](https://fogcitysfknobandtube.com/guide/how-to-identify-knob-and-tube/) shows what to look for without touching anything.

**Scope**

## What's included

- A test of every circuit, recording which run on knob and tube, which on later cable and which are joined between the two
- A circuit-by-circuit map from the panel or fuse box to the outlets, lights and appliances each circuit feeds
- A look through the reachable attic, basement, crawlspace and garage framing without disturbing the wiring
- A record of fuse and breaker sizes against the conductors they protect
- Notes on splices, junction boxes, insulation contact and damaged conductor insulation where they can be seen
- A check of panel condition and spare capacity, in case replacement follows
- A written summary that separates tested conclusions from visual ones and lists what could not be reached
- A recommended scope, from leaving sound circuits alone to partial or whole-house replacement

**Choosing the scope**

## Knob and tube inspection, an insurance inspection or a rewire walk-through

| Compare | Knob and tube inspection | Electrical inspection for insurance | Walk-through for a rewire quote |
| --- | --- | --- | --- |
| The question it answers | What is live, what is dead and what condition it is in | What an underwriter asked about this house | What replacing the old wiring would take |
| Circuits tested | Every circuit | Every circuit | Every circuit |
| Main deliverable | A circuit map and written findings | A written account shaped to the carrier's questions | A written scope and price listing each opening |
| Who usually reads it | The owner, a buyer or a seller | The insurer or broker | The owner comparing quotes |
| Route planning for new cable | Noted, not designed | Not part of it | Designed in detail |
| What usually comes next | A choice of scope | Replacement, disconnection or nothing | Permit, work and inspections |

**Read next**

## Related resources

- [LearnIs knob and tube wiring dangerous?The danger sits in what has happened to the wiring since it went in. Here is how each failure works, and which signs mean stop using a circuit today.](https://fogcitysfknobandtube.com/guide/is-knob-and-tube-wiring-dangerous/)
- [LearnHow to identify knob and tube wiringWhat the porcelain, the wires and the splices look like, where to look in a San Francisco house without touching anything, and which clues mislead.](https://fogcitysfknobandtube.com/guide/how-to-identify-knob-and-tube/)
- [Decision GuideShould you buy a house with knob and tube wiring?Plenty of San Francisco buyers do. The ones who come out ahead settle insurance, test the wiring and price the work before their contingencies run out.](https://fogcitysfknobandtube.com/guide/buying-a-house-with-knob-and-tube/)
- [Decision GuideCan you sell a house with knob and tube wiring?The law asks sellers for an advisory, not a rewire. The buyer's insurer is what sets the terms, and the records you bring decide how that conversation goes.](https://fogcitysfknobandtube.com/guide/selling-a-house-with-knob-and-tube/)
- [LearnHow can you tell active knob and tube wiring from abandoned runs?Live and dead knob and tube look identical. Insurers, the insulation rule, the BayREN rebate and the real hazard all turn on which one you have.](https://fogcitysfknobandtube.com/guide/active-vs-abandoned-knob-and-tube/)

**How a job runs**

## From the first call to the Green Tag.

### First call

Tell us what started this: an insurer's letter, an inspection report, a remodel, an attic you want to insulate.

The age of the house, the number of units and any deadline shape the next step.

### Walk-through

Circuits are tested to separate live knob and tube from abandoned runs, and attic, basement and garage access is checked.

Plaster, trim and finishes are noted, and you get a written scope and price.

### The work

The permit is filed with DBI. New cable is fished along the planned routes and openings are kept small.

The pre-cover inspection happens before anything is closed.

### Patching, inspection and the Green Tag

Once the pre-cover inspection passes, the openings are patched. Plaster patching is included and shown as its own line in the quote.

After the final inspection and the Green Tag, old runs are removed or terminated and the panel directory is updated.

**On the job**

## What this work looks like

A plug-in receptacle tester with glowing indicator lights in a wall outlet of an older home

A handheld circuit tracer receiver held against a breaker in an open panel

A borescope cable fed through a small hole in a plaster wall, its handheld unit on a drop cloth

A digital insulation resistance tester with red and black test leads on a basement workbench

**Questions**

## Knob and tube inspection: questions

### Will existing knob and tube fail a San Francisco electrical inspection?

Not by being there. DBI inspects permitted new work, and the San Francisco Electrical Code says existing wiring that was legal when installed and has been maintained need not be altered. The risk is different: any wiring added to it, or knob and tube buried in insulation, has to meet current rules, and insurers and buyers apply their own standards.

### What drives the cost of a knob and tube inspection?

The number of circuits, the size and layout of the house, how easily the attic, basement and garage can be reached, and whether a written letter for an insurer or buyer is needed. There is no reliable published local price for this inspection. BayREN announced $300 toward a licensed electrician's certification that knob and tube is inactive for eligible EASE Home households; confirm with BayREN that it is still offered.

### Can an inspection see knob and tube inside closed plaster walls?

Not directly. Testing shows which circuits are live and what each one feeds, and the attic, basement and box openings show how runs enter the walls. What sits between those points is inferred, not seen. A good report says which conclusions rest on testing and which on the visible ends of a run.

### Is a knob and tube inspection the same as a home inspection?

No. A home inspection surveys the whole house, usually by looking. A knob and tube inspection is electrical testing by a licensed electrical contractor: circuits are traced, live and abandoned runs separated, and protection checked against the wire. It is not an engineering report and does not replace a buyer's general inspection.

### What should I have ready before a knob and tube inspection?

Anything that explains the house's history: remodel dates, earlier electrical permits, a previous inspection report, a panel or service change, and any letter from an insurer or lender. Clear a path to the panel, the attic hatch and the basement or garage ceiling, and note outlets or lights that already flicker, buzz or run warm.

**Related services**

## Other work on the same wiring

### Electrical inspection for insurance

Tested, written findings on knob and tube, the panel and circuit protection, shaped to the questions an insurer or broker has asked.

### Partial knob and tube replacement

The circuits still running on knob and tube replaced from the panel out, with a written record of exactly what remains.

### Knob and tube removal

Dead and abandoned runs verified, disconnected at both ends, taken out where reachable and recorded, so nobody mistakes them for live wiring later.