An insulated conductor and the flame retardant insulation composition used in plenum cables. The insulation composition includes as a polyolefin blend from about 50 to 85 parts by weight of polypropylene, 10 to about 40 parts by weight of a very low density polyethylene and from about 5 to about 20 parts by weight of a high and/or low density polyethylene. To one hundred parts by weight of the polyolefin blend there is about 6 to about 20 parts by weight of a chlorinated flame retardant, from about 4 to about 12 parts by weight of a flame retardant intumescent, from about 20 to 60 parts by weight of a non-halogen flame retardant, from about 0.2 to about 1.0 parts by weight of hindered phenolic stabilizer; and from about 0.1 to about 0.4 parts by weight of additional stabilizers. One of the non-halogen flame retardants in the polyolefin composition, may be zinc borate which triggers polymer and flame retardant decomposition when the polyolefin composition is exposed to an igniting flame. Alternatives to zinc borate include zinc stearate, magnesium stearate, ammonium octamolybdate (AOM), or zinc stannate.