7.1 Conductor Ampacity (Table 310.16)

How to Read NEC 2020 Table 310.16

Table 310.16 lists the allowable ampacity of insulated conductors rated up to 2000 V, in raceway or cable or earth, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors. The table is organized by three variables you must combine correctly:

1. Conductor size in AWG or kcmil (rows, from 18 AWG through 2000 kcmil).
2. Conductor material: copper (first block), aluminum or copper-clad aluminum (second block). The aluminum ampacities are roughly one size smaller than copper for the same AWG.
3. Insulation temperature rating column: 60°C, 75°C, or 90°C. Each column lists the insulation types that qualify (e.g., 60°C includes TW and UF; 75°C includes RHW, THW, THWN, XHHW; 90°C includes THHN, THWN-2, XHHW-2, RHH, RHW-2).

The 90°C column value is larger than the 75°C value, which is larger than the 60°C value for the same size — but you cannot simply pick the highest number. Termination limits (110.14(C)) decide which column actually governs the final circuit ampacity.

Copper Ampacity Excerpt (Table 310.16)

• Asterisked sizes are capped by the 240.4(D) small-conductor rule — see Section 7.3.

Termination Limits — 110.14(C)

The termination temperature rule is the most-tested ampacity concept on the Virginia journeyman exam. 110.14(C)(1)(a) requires that conductors be sized based on the temperature rating of the equipment terminations, not on the insulation rating printed on the wire:

• For equipment rated 100 A or less or for conductors #1 AWG and smaller, use the 60°C column unless the equipment is marked 75°C.
• For equipment rated above 100 A or for conductors larger than #1 AWG, use the 75°C column unless marked otherwise.

Because most residential circuit-breaker panels and device terminations are rated 60°C (or 75°C only when explicitly marked), a #6 THHN copper conductor on a 60 A breaker whose terminations are not marked 75°C must be sized using the 60°C column (#6 Cu = 55 A), even though THHN itself is a 90°C-rated insulation. You may use the 90°C ampacity as the starting point only for applying ambient and bundling derating factors (Section 7.2).

Worked Example: Sizing a 50 A Branch-Circuit Conductor

A 50 A continuous-duty circuit (say, a 9 kW electric range) uses standard circuit-breaker terminations rated for 60°C only.

Step 1 — Termination column. Equipment ≤100 A, not marked 75°C → use the 60°C column.

Step 2 — Find a size whose 60°C copper ampacity ≥50 A. From the excerpt: #6 Cu = 55 A (OK), #8 Cu = 40 A (too small).

Step 3 — Verify the overcurrent limit. A 50 A breaker protects #6 Cu (55 A ≥50 A), and 240.4(D) does not restrict #6, so the breaker can be 50 A. Answer: #6 AWG copper THWN (or THHN run at the 60°C column value).

If the breaker and panel were marked 75°C, you could use the 75°C column: #8 Cu = 50 A exactly, which would be acceptable on a 50 A device (assuming the calculated load is exactly 50 A and the next-size-up rule of 240.4(B) is not needed).

Why Termination Limits Cap the Column

The rule exists because a conductor's insulation may survive 90°C, but the lug or breaker terminal it lands on is usually listed only for 60°C or 75°C. Heat migrates from the copper into the lug, so 110.14(C) forbids loading a conductor above the ampacity in the column matching the lowest-rated termination, regardless of insulation rating. Equipment marked 75°C lets you use the 75°C column; otherwise the 60°C column governs for ≤100 A equipment.

Worked Example 2: 100 A Feeder — Copper vs Aluminum at 75°C

A 100 A feeder lands on a subpanel marked 75°C, so 110.14(C) allows sizing from the 75°C column.

#3 AWG copper THHN = 100 A at 75°C — matches the breaker exactly; 240.4(D) does not restrict #3. Acceptable.

#1 AWG aluminum = 100 A at 75°C — also matches (use lugs marked AL/AL-CU). Lighter and cheaper for long runs.

If the terminations were 60°C only, both drop to 85 A — too small — and you would step up to #1 Cu (110 A at 60°C) or 1/0 Al (100 A at 60°C).

Common Exam Traps

• Picking the 90°C number directly. A #12 THHN Cu conductor is not a 30 A conductor on a 20 A breaker; 240.4(D) caps #12 Cu at 20 A and 110.14(C) usually forces the 60°C column (20 A) anyway.
• Forgetting the 60°C column for ≤100 A equipment. The exam will offer THHN/THWN-2 in the answer choices to bait you into the 90°C ampacity.
• Using aluminum ampacities for copper sizes. Always read the correct material block of the table.