A hermetic lead wire for use in refrigerant systems which is insulated by (A) an inner layer which comprises a blend of (1) a thermoplastic aromatic polyester, preferably polybutylene terephthalate, and (2) a polyester block copolymer having a recrystallization temperature greater than 150 DEG C, preferably a thermoplastic elastomer containing aromatic polyester blocks and aliphatic polyester blocks, the ratio of the block copolymer to the aromatic polyester being 0.05:1 to 0.35:1; and (B) an outer layer of a fluoropolymer which prevents cracking of the polyester layer when contacted by the varnishes conventionally used to provide hermetic seals.