THHN stands for Thermoplastic High Heat-resistant Nylon-coated. THHN wire is a highly versatile building wire primarily used for general-purpose electrical wiring within various structures. Its common applications include branch and feeder circuits in homes, commercial buildings, and industrial facilities, typically running power to outlets, switches, and lighting. Thanks to its durable nylon jacket, it's well-suited for installation in conduit and cable trays, and also finds use in machine tools, appliances, and control systems. It's rated for 600 volts and 90°C in dry locations, with many versions also suitable for wet conditions.

XHHW-2 offers superior performance for demanding environments, excelling in resistance to heat, moisture, and chemicals, despite its higher cost, making it ideal for industrial and harsh conditions. THHN serves as a cost-effective choice for general-purpose wiring in dry or damp locations, though it offers less resistance to extreme conditions. Lastly, USE-2 is specifically designed for direct burial and underground service entrance applications, providing good moisture resistance for wet outdoor use. In essence, XHHW-2 is for harsh environments, THHN for general budget-friendly wiring, and USE-2 for reliable underground power.

Choosing between NM-B (Romex) and UF-B cable depends entirely on the installation environment. NM-B is designed for indoor, dry locations like typical household wiring, and is not suitable for outdoor or wet conditions. In contrast, UF-B is specifically engineered for outdoor and underground applications, including direct burial and wet locations, thanks to its robust, moisture- and sunlight-resistant construction. Essentially, use NM-B indoors, and UF-B outdoors or underground.