CON-2 — Ampacity Tables and Terminal Temperature Limits

Read the table conditions before the number

Table 310.15(B)(16) gives allowable ampacities for insulated conductors rated through 2000 V. Its heading assumes not more than three current-carrying conductors in a raceway, cable, or earth when directly buried, and an ambient temperature of 30°C (86°F). If the installation differs, the table value is a starting point, not the answer.

Choose the row by AWG or kcmil, then the material and temperature column. Selected copper values in the 60°C/75°C/90°C columns are:

• 12 AWG: 20/25/30 A;
• 10 AWG: 30/35/40 A;
• 8 AWG: 40/50/55 A;
• 6 AWG: 55/65/75 A;
• 3 AWG: 85/100/115 A; and
• 1/0 AWG: 125/150/170 A.

Selected aluminum or copper-clad aluminum values are lower for the same size: 12 AWG is 15/20/25 A, 8 AWG is 35/40/45 A, 6 AWG is 40/50/55 A, 4 AWG is 55/65/75 A, and 1/0 AWG is 100/120/135 A. Use the actual table rather than extrapolating a ratio between materials or sizes.

The temperature column must be permitted by the conductor's insulation in the actual location. THHN/THWN-2 copper can use a 90°C insulation basis in a wet or dry location. THHN/THWN without the “-2” is limited to the 75°C THWN rating in a wet location. That insulation rating still does not establish the terminal limit.

Apply the termination provisions

Section 110.14(C)(1)(a) governs equipment for circuits rated 100 A or less or marked for 14 AWG through 1 AWG conductors. Unless equipment is listed and identified otherwise, use the 60°C termination basis. Higher-temperature conductors are permitted, but final ampacity remains based on the 60°C value unless the equipment is listed and identified for the higher-temperature conductor or a specific motor provision applies.

For equipment over 100 A or marked for conductors larger than 1 AWG, 110.14(C)(1)(b) generally uses the 75°C basis. A higher-temperature conductor is allowed, but the final ampacity cannot exceed the 75°C value unless the equipment is listed and identified otherwise. Both ends matter: a 75°C breaker does not erase a 60°C disconnect termination at the other end. Connector, equipment, and conductor markings control, not an assumed rating based on equipment size. A separately installed splice connector also has a temperature and material rating; passing a 90°C conductor through a lower-rated connector does not bypass that limit. Where a listed equipment marking differs from the general assumption, apply the marking within its identified scope.

Example 1: 8 AWG copper THWN-2 has Table 310.15(B)(16) values of 40 A at 60°C, 50 A at 75°C, and 55 A at 90°C. With no correction or adjustment and equipment identified for 75°C conductors at both ends, usable table ampacity is 50 A—not 55 A. The 90°C insulation marking does not make the lugs 90°C.

Example 2: 4 AWG aluminum XHHW-2 has values of 55/65/75 A. With 75°C terminals, no other correction or adjustment, and compatible aluminum lugs, the termination-limited ampacity is 65 A. The copper row cannot be used, and the equipment must accept 4 AWG aluminum.

Use 90°C correctly for adjustment and correction

The second sentence of 110.14(C) permits conductors with temperature ratings higher than the termination rating to be used for ampacity adjustment, correction, or both. Start from the insulation column that applies to the conductor and environment, multiply the required factors, and then compare that result with the termination-column limit. The final allowable ampacity is the lower value.

Example: 8 AWG copper THWN-2 is installed with 75°C terminals in conditions requiring an 80 percent adjustment factor. Start with the 90°C value: 55 A × 0.80 = 44 A. Compare 44 A with the 75°C terminal cap of 50 A. The allowable ampacity is 44 A. Multiplying the 75°C value to get 40 A misses the permission to use the 90°C insulation rating for adjustment; simply using 55 A ignores the terminals.

If the same conductor were marked THHN/THWN and installed in a wet location, the wet insulation basis would be 75°C, so a 90°C starting point would not be available. The calculation must honor the lowest location-specific insulation rating before the termination comparison.

Check other limits after the table

Section 240.4(D) generally limits 14, 12, and 10 AWG copper to 15, 20, and 30 A overcurrent protection, respectively, unless a specifically permitted application applies. Thus, 10 AWG copper THWN-2 does not become an ordinary 40 A branch-circuit conductor merely because the 90°C table column shows 40 A. Temperature ratings can still provide calculation headroom for adjustment and correction.

Type NM cable is another boundary. Under 334.80, its ampacity generally uses the 60°C conductor column, although the 90°C insulation rating can be used for adjustment and correction if the final result does not exceed the 60°C ampacity. Equipment articles for motors, air-conditioning, appliances, transformers, and other loads can impose different sizing or overcurrent rules.

Finally, table ampacity is not load ampacity. Apply continuous-load treatment, largest-motor additions, demand factors, neutral treatment, and other Article 220 or equipment-article calculations separately. Check voltage drop as a design issue, overcurrent protection under Article 240, and terminal and installation instructions. The safe workflow is: calculate the load, identify conductor material and insulation, read the correct table, apply adjustment and correction, enforce the termination and wiring-method caps, then select protection.