Regency Wire 11588101 500' Roll #12 White THHN Solid Wire. Despite the final gauge being the same, a solid wire will have less surface area than a stranded or bundled one, giving it a higher load capacity and less impedance. Solid wire also has a lower voltage drop over long runs but heats up faster than stranded cable. The Thermoplastic High Heat-resistant Nylon-coated wire (THHN) has PVC insulation and a nylon jacket.