KNOB AND TUBE WIRING SYSTEM

I.  KNOB AND TUBE WIRING SYSTEM:
Knob and tube wiring systems get the name from the porcelain knobs (Fig. I1, Item A) and tubes (Fig. I1, Item B) that were used to attach wires onto and through the building’s framing members.  The system used two, individual, insulated conductors to make a very limited number of circuits through a house.  Since there were only two conductors, obviously there is no separate ground conductor.  Many individual code inspectors consider this type of system the best ever installed because of the clearance between conductors, etc.  Some will allow it to remain, while others will insist that any remodeling work be accompanied by a modern electrical system.

Fig. I1

Wiring systems that are considered functionally obsolete (fused neutrals, overly extended, inadequate or unprotected connections) or unable to adequately support the electrical usage of a building will generally be deemed a major problem.  Old systems, which have been incompletely, upgraded (some knob & tube materials remaining) will generally not be deemed major problems.  These systems are invariably old and have frequently been extended to provide increased service.

To extend the service, the insulation on the conductors was scraped away, and a new run of wiring was twisted around the bare spot and taped in position (Fig. I1, Item C).  These splices and the extension of the circuitry are very suspect.  Today’s standards usually dismiss this entire system due to the inadequacy of distribution, the aged insulation on the conductors, the high likelihood of splicing, and the lack of grounding.

Knob and Tube wiring has recently become a major issue in obtaining homeowners insurance. It is now common for the insurance company to require all “knob and tube” wiring to be abandoned/removed prior to issuing insurance. When insurance is still available, it is at a higher premium.

WHAT TO:  HOW TO:
If you have no intention of upgrading an electrical system or increasing the usage, it is a good idea to have a professional electrician completely check out the system anyway.

Generally speaking, houses with knob and tube electrical service are excellent candidates for complete rewiring.

REFERENCE: E1, E2, E9