Knob and Tube Safety Wiring Requirements

Knob and tube wiring is an early standardized method of electrical wiring used in buildings until the 1940's. It consisted of single-insulated copper conductors run within wall or ceiling cavities, passing through joist and studs via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators.

New installation of this wiring system is no longer allowed  by the California Electrical Code (CEC), except by special permission form the Building official, and CEC Article 394.12(5) does not allow concealed knob and tube wiring to be used in hollow spaces of walls, ceilings and attics that are insulated loose, rolled or foamed-in-place insulating material that envelopes the conductors. If knob and tube wiring is discovered during the course of construction, it is recommended to have it removed entirely or in part, to the maximum extent practical. 

Installation of Insulation Where Knob and Tube Wiring is Present

There is an exception to allow the installation of insulation in residential structures that have knob and tube wiring which requires:

• The wiring shall be surveyed and certified by an electrical contractor licensed by the State of California certifying that the existing wiring is in good condition with no evidence of deterioration with proper overcurrent protection and no improper connections or splices. Any repairs, alterations or extensions of the existing system will require permits and inspections. 
• The certification form shall be submitted to the building department and a copy given to the property owner. 
• Printed notices in both English and Spanish stating that caution is required shall be posted at all access points to the area where insulation has been installed around knob-and-tube wiring. 
• The insulation shall be noncombustible as defined in the CA Building Code (2025). 
• The insulation shall not have any electrical conductive material as part of or supporting the insulation material. 
• The Building Division may still require permits and inspections for the installation of the insulation. 

Where knob and tube wiring is to remain and insulation installed, whether new or replacement of existing insulation, a Knob and Tube Certification Form(PDF, 136KB)  is required to be submitted to the Building Division for review and approval prior to the installation of the insulation.