NEC Table 310.16 Ampacity Explained

NEC Table 310.16 is the master ampacity table for insulated copper and aluminum conductors rated 0-2000V installed in raceway, cable, or directly buried in earth, based on an ambient of 30C (86F) with not more than three current-carrying conductors. NFPA 70 (2023) presents three columns - 60C, 75C, and 90C - corresponding to conductor insulation ratings. Selecting the correct column is governed by NEC 110.14(C)(1), the terminal-rating rule.

What Table 310.16 Says

Below is the copper portion of Table 310.16, NEC 2023:

Aluminum/copper-clad aluminum values appear in the right half of the table at roughly 78% of copper.

The 110.14(C)(1) Terminal Rule

NEC 110.14(C)(1) requires that conductor ampacity be selected based on the lowest temperature rating of any termination, conductor, or device in the circuit. For circuits rated 100A or less, terminals are assumed 60C unless listed for 75C. For circuits over 100A or conductors larger than 1 AWG, terminals are assumed 75C. This is the rule that traps many installers: a 90C THHN insulation does not authorize you to load the conductor at the 90C column unless every breaker, lug, and splice in the circuit is also 90C-rated - a rarity in field equipment.

Worked Example

Consider a 12 AWG THHN copper feeder to a 20A breaker. THHN insulation is 90C, so Table 310.16 lists 30A in the 90C column. But the typical breaker is 75C-listed at best, frequently 60C for circuits under 100A. Per NEC 110.14(C)(1), you must select from the 60C column - 20A. The 90C value is still useful: when you apply temperature correction (Table 310.15(B)(1)) or adjustment factors (310.15(C)(1)) for more than three current-carrying conductors, you start from 30A and derate from there. The final adjusted ampacity then has to be compared against the 110.14(C)(1) terminal limit.

Why the Columns Exist

The 90C column represents the conductor's intrinsic thermal limit - the temperature at which the insulation degrades. The 75C and 60C columns are de-rated to protect terminations, which are the weakest link thermally because lugs, breakers, and splice points dissipate heat through bus bars and mounting hardware. UL 489 and UL 486A test fixtures historically used 60C and 75C wire to establish breaker performance, so terminals carry those ratings forward. NEC 110.14(C)(1) ensures the field installation does not exceed those test conditions.

Common Misconceptions

• "My wire is THHN/90C, so I can use 30A on 12 AWG." False - only after derating and only if terminals are 90C.
• "Table 310.16 applies to free-air conductors." No - free-air is Table 310.17. Table 310.16 assumes raceway, cable, or direct burial.
• "Three current-carrying conductors counts neutrals." Only when the neutral carries unbalanced current of a 3-phase wye non-linear load (see NEC 310.15(E)) or where it is the sole return.

Cross-References

• NEC 110.14(C)(1) - terminal temperature rating
• NEC 240.4(D) - small conductor overcurrent cap
• NEC 310.15(B)(1) - ambient temperature correction
• NEC 310.15(C)(1) - adjustment for >3 CCCs (see our NEC 310.15 derating guide)
• NEC 310.12 - dwelling-service conductor table (separate)

How WireFillChart Uses Table 310.16

Our conduit fill calculator does not size conductors for load, but the underlying database stores 60/75/90C ampacities from Table 310.16 for every common insulation type. Use our ampacity reference to look up the controlling column before sizing your raceway. Insulation specifics are covered under THHN, THWN-2, XHHW, and RHH.

Related

• Conduit Fill Calculator
• What is Ampacity
• NEC 310.15(C)(1) Ampacity Adjustment Factors
• What is THHN
• What is THWN-2
• Wire Fill Chart