| Knob and tube wiring was a system of open | | | | These insulators can carry very high voltage |
| electrical conductors supported on ceramic knobs. | | | | surges without the risk of shorting to ground. |
| It also used ceramic knobs where wires cross | | | | Many electricians understand that by running the |
| each other or pass through framing. This type of | | | | hot and neutral conductors farther apart from |
| wiring was in common use from the 1880's to | | | | each other than today's electrical cabling does |
| 1930's, in North America. | | | | make shorts between those conductors less |
| During the era when knob and tube was in | | | | likely, at the same time realizing there is not a |
| common use, a typical home would have 30 or | | | | grounding conductor. |
| 60 amp electrical service, sufficient for general | | | | Wiring installation standards and methods were |
| lighting and maybe a radio. These were the days | | | | less stringent in the era of knob and tube versus |
| of "iceboxes", wood or gas stoves, coal heat, and | | | | modern day electrical codes and standards. The |
| no air conditioning. Wiring homes was a new trade, | | | | main shortcomings of knob and tube include the |
| with very few rules. | | | | following: lack of a safety grounding conductor, |
| The first home wiring rules appeared around the | | | | inability to confine switching to the hot conductor, |
| turn of the century, when Terrell Croft wrote the | | | | the permitted use of in-line splices in walls without |
| American Electricians Handbook. Crofts books | | | | a junction box. Compared to modern |
| defined early wiring practices. One of Croft's rules | | | | thermoplastic wiring insulation, typically known as |
| was that knob and tube wiring should always be | | | | "romex", the knob and tube wiring is less resistant |
| run through, never over the house framing. The | | | | to damage. |
| problem with running wiring over framing is that it | | | | Knob and tube wiring is very vulnerable to |
| can get bumped, snagged, and damaged. Most | | | | mechanical damage, such as from stored |
| electrical experts would agree that wiring run over | | | | materials in the attic or basement. Bending the |
| the framing is a sign of amateur, cobbled-together | | | | wire can cause older insulation to crumble and fall |
| work, and it never conformed to any electrical | | | | off the wires. |
| code. | | | | Knob and tube is insufficient for current usage |
| Good Points | | | | patterns. Contemporary households use a wide |
| The theoretical advantage of knob and tube | | | | range and intensity of electrical equipment that |
| wiring is that it dissipates heat into free air, and | | | | was unforeseen at the time of knob and tube |
| therefore has a higher ampacity than cable | | | | inception. Household power use increased following |
| systems with equivalent wire size. When originally | | | | World War II, first generation wiring systems |
| installed in the 1900's knob and tube wiring was | | | | became susceptible to abuse by homeowners, |
| less expensive than other wiring methods. Due to | | | | who would avoid repeated blown fuses by |
| the installation cost, owners and electricians would | | | | overfusing the circuits, thus subjecting the wiring |
| opt for knob and tube versus conduit wiring and | | | | to heat damage due to higher levels of current. |
| metal junction boxes. The conduit methods were | | | | In many instances deterioration and abuse have |
| known to be of better quality, but their cost was | | | | rendered the wiring unsafe. During building |
| significantly higher than knob and tube wiring. | | | | renovations and remodeling wiring can become |
| Modern wiring methods assume two or more load | | | | damaged. The rubber insulation will become dried |
| carrying conductors will lie against each other, for | | | | out, brittle when handled, damage by rodent, or |
| instance the standard non-metallic - 2 cable. Since | | | | simple carelessness - for example hanging objects |
| the load carrying wires are in close proximity, | | | | off wiring in accessible places like attics and |
| when they heat up, the heating is shared across | | | | basements. |
| the wires, limiting the overall current load they can | | | | Covering knob and tube wiring with thermal |
| support. Since the load carrying wires in knob and | | | | insulation is unsafe and prohibited. This is a known |
| tube wiring are widely spaced, the wires are | | | | fire hazard and it is specifically condemned in the |
| capable of carrying higher loads without risk of | | | | National Electrical Code, Section 324-4. No form of |
| fire. | | | | insulation - loose, blown-in, or expanding foam |
| Bad Points | | | | insulation can be placed over the wiring. Knob and |
| Knob and tube wiring was commonly insulated | | | | tube wiring was designed to let heat dissipate to |
| with cotton cloth and soft rubber, in addition to | | | | the surrounding air. As a result, energy efficiency |
| the porcelain insulators. Although the actual wire | | | | upgrades that involve insulating previously |
| covering may have degraded over the decades, | | | | uninsulated walls usually also require replacement |
| the porcelain insulators have a very long lifespan | | | | of the wiring in affected homes. |
| and will keep any bare wires safely insulated. | | | | Homeowners insurance underwriters may refuse |
| These same porcelain insulators are commonly | | | | and deny coverage for homes and buildings that |
| used with bare wire electric fencing for livestock. | | | | have this type of wiring installed. |