2.3 Conductors (0103)

Conductor Types and Insulation Letter Designations

Conductors are identified by a letter designation that encodes insulation material, temperature rating, and location suitability. Table 310.4(A) is the master list of conductor insulations rated 600 volts, giving each type's trade name, maximum operating temperature, and permitted application. Common types:

The suffix "-2" means the conductor is rated 90°C in wet locations (not just dry). When the inspector sees THWN-2 or XHHW-2, the 90°C column may be used for derating calculations.

Copper vs. Aluminum

• Copper (Cu) — lower resistance; smaller size for a given ampacity. Read the copper columns of Table 310.16.
• Aluminum (Al) and copper-clad aluminum (CCA) — lighter and cheaper for large feeders; read the aluminum/copper-clad aluminum columns of Table 310.16, which run roughly two AWG sizes larger than copper for the same ampacity. Aluminum and copper-clad aluminum 1/0 AWG and larger may be paralleled under 310.10(G), the same threshold as copper.

Ampacity — Table 310.16

Table 310.16 gives allowable ampacities for insulated conductors rated 0–2000 V, not more than 3 current-carrying conductors in a raceway/cable/earth, and an ambient temperature of 30°C. Excerpt (copper):

The 240.4(D) small-conductor rule caps the overcurrent protection for 14 AWG Cu at 15 A, 12 AWG Cu at 20 A, and 10 AWG Cu at 30 A, regardless of the higher numbers in the 75°C/90°C columns (with exceptions listed in 240.4(G)).

Termination Temperature Limits — 110.14(C)

The conductor's insulation may be rated 90°C, but terminations are limited by 110.14(C):

• Equipment rated 100 A or less, or marked for conductors 14 AWG through 1 AWG: use the 60°C column unless the equipment is marked 60/75°C, in which case the 75°C column is permitted.
• Equipment rated over 100 A, or marked for conductors larger than 1 AWG: use the 75°C column unless marked for 90°C.
• A conductor with higher-rated insulation (e.g., 90°C THHN) may be used, but the ampacity at the termination is taken from the termination-rated column. The 90°C column is the starting point only for derating (correction/adjustment).

The Practical Rule

• Size the conductor at the termination using 110.14(C) (usually the 75°C column).
• Size the conductor in the raceway for derating using the 90°C column (if the insulation is 90°C).
• The smaller of the two is the final ampacity.

Derating (Correction and Adjustment Factors) — 310.15

Table 310.16 is built on two stated conditions: an ambient of 30°C (86°F) and not more than three current-carrying conductors in the raceway, cable, or earth. When the installation departs from either condition, apply the factors printed with Article 310:

• Ambient temperature correction — Table 310.15(B)(1): multiply the ampacity from the column matching the conductor insulation by the correction factor for the actual ambient. Representative factors (90°C column):
  • 31–35°C: 0.96
  • 36–40°C: 0.91
  • 41–45°C: 0.87
  • 46–50°C: 0.82
• Adjustment for more than three current-carrying conductors — 310.15(C)(1):
  • 4–6 CCC: 80%
  • 7–9 CCC: 70%
  • 10–20 CCC: 50%
  • 21–30 CCC: 45%

Worked Example

A feeder is run with 6 current-carrying conductors of #2 AWG THHN copper in a single conduit, in a mechanical room where the ambient is 40°C.

1. Base ampacity from Table 310.16, 90°C column (THHN is 90°C): 130 A.
2. Ambient correction at 40°C, 90°C column (Table 310.15(B)(1)): 0.91. 130 × 0.91 = 118.3 A.
3. Adjustment for 6 CCC (310.15(C)(1)): 0.80. 118.3 × 0.80 = 94.6 A.
4. Termination limit — #2 AWG Cu at 75°C (equipment >100 A, 110.14(C)) = 115 A.
5. The derated value (94.6 A) is less than the termination limit (115 A), so the derated ampacity governs: ~94 A.

If this conductor served a continuous load, the OCPD would be sized at 125% of the continuous load per 215.3 and must not exceed 94 A.

Conductor Connections, Splices, and Terminal Identification

• 110.14(B) — Splices: conductor splices and joints must be made with listed splicing devices or by brazing, welding, or soldering with a fusible metal or alloy — and a soldered splice must first be mechanically and electrically secure without the solder, then soldered. Splices must be made in a box or conduit body.
• 300.15 — Boxes and conduit bodies required: A box or conduit body must be installed at each conductor splice, tap, termination, or junction point, and must remain accessible (314.29).
• 110.14(D) — Terminal connection torque: where a tightening torque is indicated as a numeric value on equipment or in the installation instructions, it must be applied with a calibrated torque tool (torque wrench, screwdriver, or driver).
• Terminal identification: Terminals must be marked to indicate the conductor material (Cu/Al/Cu-Al) they are listed for. Aluminum-rated terminals used with aluminum conductors must be listed and installed with antioxidant compound if required.

Paralleling — 310.10(G)

Paralleling means using two or more conductors per phase instead of one large conductor. Under 310.10(G):

• Allowed for 1/0 AWG and larger copper, aluminum, and copper-clad aluminum.
• Every conductor of a given parallel set (all the phase A conductors, all the phase B conductors, and so on) must be the same length, same conductor material, same circular-mil area, and same insulation type, and must be terminated in the same manner.
• Where the parallel sets are run in separate raceways or cables, each raceway or cable must have the same number of conductors and the same electrical characteristics, so that current divides evenly. All conductors of the same circuit — every phase, the neutral, and the equipment grounding conductor — must be in each raceway (300.3(B)); putting all the phase A conductors in one steel conduit is an inductive-heating violation, and it is a classic exam trap.
• The equipment grounding conductor in each parallel raceway is full-size per Table 250.122 for the circuit overcurrent device, not divided among the raceways.

Paralleling is common on large commercial feeders and services; the inspector verifies that each conductor in a set is identical and that sets are distributed across phases to avoid inductive heating.