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...

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