8.2 Conductor Insulations and Ampacity

Why Conductor Ampacity Matters on the TX Journeyman Exam

Conductor sizing and ampacity calculations appear in both exam parts. The NEC Knowledge part tests your recall of insulation types, temperature ratings, and 110.14(C) termination rules. The Calculations part expects you to apply Table 310.16, apply temperature correction from Table 310.15(B)(1), and apply ampacity adjustment for more than three current-carrying conductors from Table 310.15(C)(1). Getting derating right is often the difference between passing and failing the 26-question Calculations section.

Article 310 — Conductors for General Wiring

Insulation Types and Temperature Ratings

NEC Table 310.4(A) lists conductor applications and insulation ratings. The most common types on the Journeyman exam:

Key distinctions:

• The -2 suffix means the insulation is rated 90 °C in both wet and dry locations. Conductors without the -2 suffix typically drop to 75 °C in wet locations.
• USE-2 is for underground service conductors. It is not permitted inside a building because it has no flame-retardant rating unless dual-listed (for example, RHH/RHW/USE-2).
• THWN (without -2) is only 75 °C in wet locations — important when derating must start from the 75 °C column instead of 90 °C.

Table 310.16 — Allowable Ampacities (Copper, 60/75/90 °C)

This is the single most important table on the exam. Below are the copper values for common sizes:

Small-conductor rule (240.4(D)): The overcurrent protection for #14, #12, and #10 copper conductors is limited to 15 A, 20 A, and 30 A respectively, regardless of the higher ampacity in the 75 °C or 90 °C columns — unless specifically permitted otherwise (motor circuits per Article 430, for example).

110.14(C) — Electrical Connections: Temperature Limitations

This rule ties Table 310.16 to real equipment. You cannot use the 90 °C ampacity for conductor sizing unless the equipment terminations are rated for 90 °C.

The 90 °C column is used for derating only. You start with the 90 °C ampacity to apply temperature correction and bundling adjustment, then verify the result does not exceed the termination-temperature limit (60 °C or 75 °C column).

310.15(B)(1) — Temperature Correction

When the ambient temperature is not 30 °C (86 °F), correct the ampacity using Table 310.15(B)(1). Key correction factors:

310.15(C)(1) — Ampacity Adjustment for Number of Conductors

When more than three current-carrying conductors are in a raceway or cable, reduce the ampacity per Table 310.15(C)(1):

What counts as a current-carrying conductor?

• Ungrounded (hot) conductors: always count.
• Neutral conductor of a 3-wire single-phase 120/240 V circuit: does not count (balanced).
• Neutral conductor of a 4-wire 3-phase wye circuit: counts (harmonics and unbalanced current).
• Equipment grounding conductors: do not count.
• 3-way switch travelers: count (they carry current).

Worked Example — Derating Six #12 THHN at 40 °C

Problem: Six current-carrying #12 THHN copper conductors are installed in EMT in a 40 °C ambient. What is the maximum allowable ampacity?

Step 1 — Base ampacity: #12 THHN is 90 °C rated. Table 310.16, 90 °C column = 30 A.

Step 2 — Temperature correction: 40 °C ambient, 90 °C column factor = 0.91.
30 A × 0.91 = 27.3 A.

Step 3 — Adjustment for 4–6 conductors: Factor = 0.80.
27.3 A × 0.80 = 21.84 A.

Step 4 — Termination limit (110.14(C)): Equipment rated 100 A or less, not marked otherwise, uses the 60 °C column = 20 A.

Step 5 — Final ampacity: The lower of 21.84 A and 20 A = 20 A.

The derating does not reduce the conductor below the termination limit, so a standard 20 A breaker still protects this circuit. If the result had been below 20 A, you would either upsize the conductor or reduce the breaker rating.