Conductors and Insulation
Copper
Copper has low resistivity, good mechanical properties, and a long history of reliable electrical terminations. Bare copper is used for grounding and bonding in appropriate applications, while insulated copper is common throughout electronics, controls, motors, appliances, and premises wiring.
Aluminum
Aluminum has higher resistivity than copper, so an aluminum conductor generally needs a larger cross-sectional area to carry comparable current with similar voltage drop. Its lower density makes large conductors much lighter. Modern electrical-grade aluminum is widely used in services, feeders, utility systems, and other large-conductor applications.
Copper-clad aluminum
Copper-clad aluminum has an aluminum core metallurgically bonded to an outer copper layer. It has its own code and product requirements and should not be treated simply as either solid copper or ordinary aluminum. Use the markings, listing, and applicable NEC rules for the actual conductor.
Solid versus stranded
Stranding changes flexibility more than basic DC conductivity. Two conductors with the same nominal size can have very different numbers of strands. Building wire may use relatively coarse stranding, while portable cords, battery leads, and welding cable use much finer stranding.
Strand count affects terminal compatibility. Lugs, set-screw terminals, spring clamps, crimp terminals, and wire connectors are tested for particular conductor classes and ranges.
Common building-wire insulation markings
| Marking | What it generally tells you | Important caution |
|---|---|---|
| THHN | Thermoplastic, high-heat-resistant, nylon-coated building wire. | Many products are dual-marked THHN/THWN-2; the complete marking determines wet-location and temperature suitability. |
| THWN / THWN-2 | Thermoplastic heat- and water-resistant nylon-coated conductor. | Use the exact temperature and wet-location ratings printed on the product. |
| XHHW / XHHW-2 | Cross-linked polyethylene insulation used in many feeder, service, and raceway applications. | The “-2” suffix and product listing affect wet/dry temperature use. |
| MTW | Machine-tool wire intended for equipment wiring under its listing. | Do not assume machine wire is automatically a premises-wiring conductor. |
| TFFN / fixture wire types | Small conductors used in listed fixture/equipment applications. | Smaller fixture conductors have their own ampacity and use rules. |
A wire may carry several markings
A single manufactured conductor may be listed for more than one use and carry several type designations. That is useful because one wire can satisfy different parts of an installation. Conversely, if the required marking is absent, the installer cannot simply assume the conductor has that rating because its insulation “looks the same.”
Temperature ratings
A conductor may have insulation rated 75°C or 90°C, yet that does not mean the circuit can automatically be sized from the highest-temperature ampacity column. Equipment terminals, conductor size, cable type, and NEC rules may limit the usable ampacity to a lower temperature column.
Wet and dry locations
Raceways installed outdoors or underground can contain moisture. A conductor inside a raceway does not automatically become a dry-location conductor. Insulation must be listed for the environment in which it is installed.
Voltage rating
Insulation voltage rating tells how much electrical stress the insulation system is designed to withstand under the conditions of its listing. It does not replace spacing, overcurrent protection, raceway, equipment, or installation requirements.
Jackets, shields, and armor
A cable can add layers around its insulated conductors for mechanical protection, moisture resistance, electromagnetic shielding, sunlight resistance, oil resistance, flame performance, or grounding. The outer jacket does not necessarily mean the conductors inside may be removed and used individually; the cable and its internal conductors must be used according to their markings and listing.