What you're calling outdoor Romex is actually called type UF (underground feeder) cable, it resembles Romex, but it's not. UF cable is THHN conductors plus a ground encased completely in PVC. Romex is a brand name for non-metallic paper bonded cable (NM-B, as printed on the jacket) with a PVC jacket.hat you're calling outdoor Romex is actually called type UF (underground feeder) cable, it resembles Romex, but it's not. UF cable is THHN conductors plus a ground encased completely in PVC. Romex is a brand name for non-metallic paper bonded cable (NM-B, as printed on the jacket) with a PVC jacket.