- Q. I know that knob-and-tube wiring can’t be covered with insulation, but what about BX cable and joist-mounted junction boxes? Are there any restrictions on blowing cellulose insulation into an attic and burying the cable and junction boxes?
A.Lynn Underwood, a licensed contractor and building code official in Norfolk, Va., responds: While there are no prescriptive prohibitions against covering junction boxes or electrical cables with insulation, there is always the issue of heat buildup around any electrical device or wiring. Temperature ratings (in degrees Celsius) for various types of conductors can be found in the National Electric Code (Table 310.13, 2005 NEC); unfortunately, BX cable is no longer manufactured and is not listed, but it is similar to AC cable and can generally be assumed to have at least a 60°C temperature rating.
Electrical conductors can’t be used in an environment that exceeds their temperature rating, but actually determining the operating temperature of a conductor is tricky. Factors such as ambient temperature, the amount of heat generated internally inside the conductor, the rate at which this heat dissipates (which can be affected by the presence of thermal insulation), and the presence of adjacent load-carrying conductors must all be taken into account (Article 310.10, 2005 NEC). Understanding these temperature ratings and matching the intent of the building code should be attempted only by a licensed electrician on a case-by-case basis, but the electricians I’ve consulted agree that it’s safe — under normal conditions — to bury older BX cable in cellulose insulation in an attic. Still, depending on the installation’s location (Arizona vs. Maine, for example), on whether or not the attic is vented or unvented, and on whether some of the junction boxes also contain light fixtures (which generate heat), installing insulation around existing wiring could violate one of these NEC performance standards.
Buried junction boxes can be an issue too. The NEC says that a junction box must be accessible “without removing any part of the building” (Article 314.29, 2005 NEC). In my opinion, blown-in insulation is neither part of the structure nor a finish material, and therefore wouldn’t create a violation. On the other hand, this kind of insulation would tend to obscure the location of such a box and make it difficult to find, thus making it violation of the spirit of this section.
If your inspector is concerned about concealment, a reasonable compromise would be to mark boxes and wiring with placards that are clearly visible after the insulation is installed.