The typical maintenance department has several spools of THHN wire on a rack or pallet, ready for projects or repairs. THHN is a good, general-purpose insulation. However, suppose you need to rewire a machine tool. Would THHN be the most appropriate kind of wire? Would it be appropriate at all?

To ensure you use the correct insulation type for the application, turn to Chapter 3 of the NEC. Article 310 contains Table 310.13(A), Conductor Applications and Insulations.

Most application descriptions are very general. For example, you can use moisture-retardant thermoset (RHW) in "dry and wet locations." A few application descriptions are specific. For example, you can use moisture, heat, and oil-resistant thermoplastic (MTW) in machine tool wiring. Before using wire from those maintenance spools, check the table for applicability. If that insulation isn't allowed for your application, then look for that application in the table to find the allowed insulation types.

For most applications, this table will give you several insulation types from which to choose. Your ultimate selection will depend first on the temperature range and ampacity (Tables 310.17 through 310.20) and then on other factors (e.g., cost and availability).