NEC Table 310.16: The Ampacity Table That Sizes Most Circuits

What Table 310.16 actually tells you

Table 310.16 lists the allowable ampacity of insulated conductors rated 0-2000 volts, installed in raceway or cable, with not more than three current-carrying conductors per raceway, at 30°C (86°F) ambient temperature. Three insulation temperature ratings, three columns:

Aluminum ampacity is roughly 80% of copper at the same size, listed in the right half of the official table. Memorize the residential rows (14 AWG through 4 AWG copper) for the most common service and branch-circuit problems.

The 110.14(C) rule that decides which column you use

110.14(C)(1) — Up to 100 amps

For circuits rated 100 amps or less:

• Default: 60°C ampacity applies (the conservative value)
• Exception: 75°C ampacity may be used if every termination is listed and identified for 75°C
• 90°C ampacity is allowed for ampacity adjustment calculations only — not as the final installed value

Practical: most modern equipment terminations are listed for 75°C use, so 75°C ampacity is what gets installed. But the device labeling has to confirm it. If unmarked, default to 60°C.

110.14(C)(1)(b) — Above 100 amps

For circuits rated above 100 amps:

• Default: 75°C ampacity applies
• 75°C ampacity may be used if equipment is listed for 75°C (most large-equipment terminations are)
• 90°C ampacity again allowed for adjustment calculations only, not final value

Ambient temperature correction (Table 310.15(B)(1)(1))

Table 310.16 is based on 30°C (86°F) ambient. Real installations are often hotter. Table 310.15(B)(1)(1) gives correction factors:

For attic installations and rooftop conduit runs, ambient correction is significant. NEC 310.15(B)(2) gives specific rooftop ambient adders.

Conductor bundling adjustment (Table 310.15(C)(1))

Table 310.16 assumes not more than three current-carrying conductors per raceway. When you bundle more than three for over 24 inches, ampacity is reduced:

• 4 to 6 current-carrying conductors: multiply by 0.80
• 7 to 9 conductors: multiply by 0.70
• 10 to 20 conductors: multiply by 0.50
• 21 to 30 conductors: multiply by 0.45
• 31 to 40 conductors: multiply by 0.40
• 41 or more conductors: multiply by 0.35

Note: the neutral counts as a current-carrying conductor only in 3-phase 4-wire wye systems carrying nonlinear loads (210.4(C)). The equipment grounding conductor never counts.

Worked example: 3/4 inch EMT in a 105°F attic

Three 12 AWG THHN current-carrying conductors plus an EGC, in 3/4 inch EMT, running through an attic with summer ambient temperature of 105°F (~41°C).

1. Start with 90°C THHN ampacity from Table 310.16: 30 A for 12 AWG copper
2. Apply ambient correction at 41-45°C from Table 310.15(B)(1)(1) for 90°C: 0.87
3. Bundling: 3 current-carrying conductors, no adjustment required (4 or more triggers 310.15(C)(1))
4. Adjusted ampacity: 30 × 0.87 = 26.1 A
5. Apply 110.14(C) termination limit: 75°C column for 12 AWG = 25 A
6. Final ampacity: 25 A (limited by 75°C termination)
7. Branch circuit OCPD: 20 A breaker is the closest standard size at or below 25 A continuous-load ampacity (210.20(A))

This is the canonical multi-step ampacity problem on every electrician exam. The trick is doing the steps in order: start at 90°C, apply ambient and bundling adjustments to that 90°C base, THEN compare to the 75°C termination limit. The lower of the two is the installed ampacity.

How Table 310.16 questions appear on the exam

1. Direct lookup: “What is the 75°C ampacity of 6 AWG copper?” → 65 A. Pure table reading.
2. 110.14(C) limitation: “A 100-amp circuit is wired with 4 AWG THHN copper terminating to 75°C-listed equipment. What is the installed ampacity?” → 85 A (75°C column, not 95 A from 90°C).
3. Ambient correction: “3 AWG THWN-2 in a 110°F ambient. What is the corrected 75°C ampacity?” → multiply 100 A × 0.82 = 82 A.
4. Bundling adjustment: “Six 10 AWG THHN current-carrying conductors in one EMT. Adjusted ampacity?” → 40 × 0.80 = 32 A. Then compare to 75°C termination limit (35 A) — use 32 A as the limiting value.
5. Combined: ambient + bundling stacked. Multiply both factors against the 90°C value.

The most common 310.16 mistakes

• Reading the 90°C column as the final answer when 110.14(C) limits to 75°C
• Forgetting to apply ambient correction in attic and rooftop installations
• Counting the EGC as a current-carrying conductor for bundling (it’s not)
• Applying ambient correction to the 60°C value when it should be applied to 90°C, then comparing
• Confusing TW (60°C only) with THW (60/75°C) with THHN (60/75/90°C)
• Skipping the 24-inch rule for bundling — short raceway sections under 24 inches don’t require adjustment