CON-1 — Conductor Materials, Insulation, Markings, and Permitted Use

Select the conductor material and its terminations

Article 310 recognizes copper, aluminum, and copper-clad aluminum conductors where permitted. The same AWG size does not have the same ampacity in every material. Table 310.15(B)(16) has separate copper and aluminum or copper-clad aluminum columns, so a size-for-size substitution can reduce ampacity. Resistance, voltage drop, bending space, lug range, and equipment instructions also change with material and size.

Section 110.14 requires terminals and connectors to be identified for the conductor material. A copper-only lug is not made suitable for aluminum by adding oxide inhibitor. Use equipment marked for aluminum, copper, or both as applicable; prepare the conductor, use inhibitor when the listing or manufacturer requires it, and tighten with the specified tool and torque. Aluminum strands must not be nicked or wire-brushed contrary to instructions. Recheck conductor range because a larger aluminum conductor selected for ampacity might not fit the original lug.

Conductors 8 AWG and larger installed in raceways are generally stranded under 310.106(C), subject to stated exceptions. Stranding improves handling but does not by itself authorize flexible-equipment use. Fine-stranded conductors can require connectors specifically identified for that strand class; an ordinary building-wire lug may not be suitable.

Read the complete insulation marking

Table 310.104(A) identifies insulation types, construction, maximum operating temperatures, and permitted locations. Do not infer wet-location permission from a 90°C number alone. Common distinctions include:

• THHN: 90°C in dry and damp locations; THHN alone is not a wet-location designation.
• THWN: 75°C in wet locations; a dual THHN/THWN marking uses the rating applicable to the actual environment.
• THWN-2: 90°C in dry and wet locations.
• XHHW: generally 90°C dry and 75°C wet under the table marking.
• XHHW-2: 90°C in dry and wet locations.
• RHW-2: 90°C in dry and wet locations.

Many modern conductors are marked with multiple types, such as THHN/THWN-2. That complete marking permits the 90°C insulation basis in a wet raceway, subject to terminal and other limits. A conductor marked only THHN/THWN is limited by its 75°C THWN wet rating in that environment; it cannot start a wet-location adjustment calculation from the 90°C column merely because THHN appears on the jacket.

Section 310.10(C) requires insulated conductors and cables in wet locations to be listed for wet use. Under 300.5(B), the interior of an underground raceway is a wet location, even when the raceway joints appear watertight. Raceway interiors in wet locations above grade are also treated as wet under 300.9. Therefore, individual THHN-only conductors in underground PVC are not compliant; use a wet-rated type such as THWN-2 or another permitted marking.

Direct burial is another permission, not a synonym for wet rated. A conductor or cable must be listed and marked for direct burial where placed directly in earth. Type USE has outside and underground uses and restrictions; it is not automatically permitted for interior building wiring unless it also carries an appropriate premises-wiring designation. Sunlight resistance, oil resistance, voltage rating, and other suffixes must likewise match the exposure.

Identify conductors by function

A grounded conductor is generally identified white or gray under 200.6. For 6 AWG and smaller, identification is generally continuous; 4 AWG and larger can use permitted reidentification at terminations and accessible points. An insulated equipment grounding conductor is generally green, green with yellow stripes, or reidentified as permitted for larger conductors under 250.119; bare conductors can also serve where allowed. White, gray, green, and green/yellow are not ordinary phase colors.

The NEC does not impose a universal black-red-blue phase-color sequence for every premises system. Where a building has more than one nominal voltage system, 210.5(C) requires ungrounded branch-circuit conductors to be identified by phase or line and system, with the identification method permanently posted or documented at each branch-circuit panelboard or similar distribution equipment. Follow the established method rather than assuming a color from habit.

Reidentification permissions are conductor-size and function specific. A small white conductor generally cannot be taped black and used as an ungrounded conductor merely for convenience, except where a particular cable-assembly rule permits permanent reidentification. Identification never substitutes for tracing, testing, or lockout/tagout.

Protect the installed conductor

Section 300.4 protects wiring against physical damage. Where a cable or raceway passes through a bored hole in a wood member, the hole generally maintains at least 1.25 in. from the nearest edge. If that setback cannot be maintained, protect the wiring with a steel plate or bushing meeting the rule before wall finish is installed. At entries, fittings and bushings protect insulation from abrasion; conductor bending radius, box fill, raceway fill, pulling tension, and support requirements still apply.

Flexible cords and cables are governed by Article 400 and their listing. They are not substitutes for fixed wiring, cannot ordinarily be run through holes in walls, structural ceilings, suspended ceilings, or floors, and cannot be concealed behind building surfaces where 400.8 prohibits the use. Where movement or vibration calls for flexibility, select a permitted wiring method, cord type, fittings, ampacity, environmental rating, and strain relief. Finish conductor selection by checking material, size, insulation letters, voltage and location rating, terminal compatibility, identification, physical protection, and the wiring-method article.