Chapter 6 Conductors 85 Copyright Goodheart-Willcox Co., Inc. 6.1.1 Conductor and Cable Classifi cations Conductors are identifi by letter codes (THW, fied THHN, and XHHW, for example). These letter designations identify the type of insulation and the appropriate environments for the conductor. Some of the letter designations indicate insula- tion composition: A = Asbestos MI = Mineral insulated R = Rubber T = Thermoplastic V = Varnished cambric X = Cross-linked synthetic polymers. Letters also identify the physical characteris- tics of the conductor: H = Heat resistant to 167°F (75°C) HH = Heat resistant to 194°F (90°C) UF = Appropriate for direct burial W = Appropriate for wet conditions C = Corrosion resistant. Letter designations also indicate the type of outer jacket, sheath outer covering, or overall outer layer: AC = Armored cable (having an interlocked metal sheath) L = Lead covered MC = Metal clad N = Nylon jacketed NM= Nonmetallic sheath. Numerous other letter designations are included in Table 310.104(A), Conductor Applica- tions and Insulations Rated 600 Volts, of the Code. Letter designations are marked on the prod- uct for easy identification. For example, THHN fi is a conductor having a thermoplastic, high heat-resistant (194°F) insulation and nylon (or equivalent) jacketing. THW indicates a thermo- plastic heat-resistant insulation that can be used in wet locations. UF is suitable as an under- ground (directly buried) feeder. USE is under- ground service-entrance cable. Conductor Size Designation The size of a conductor is determined by its cross-sectional area. There are two methods of expressing conductor size: American Wire Gage and circular mils. Within the American Wire Gage (AWG) ( ( sys- tem, the smaller the number, the larger the wire. For power wiring, 14 AWG is the smallest size used in commercial wiring. Control wiring may be smaller sizes, such as 16 AWG or 18 AWG. The AWG system designates wires as large as 4/0 AWG (four-aught). Branch-circuit conductors Branch-circuit conductors Motors Main feeders A B C Subfeeders Motor feeders Motor control center General lighting panel General power panel Service disconnect Transformer Utility supply Service-entrance conductors Main Panel A Main Panel B Goodheart-Willcox Publisher Figure 6-2. This diagram shows different conductor classifications.