NEC conductor ampacity: Table 310.16, correction and adjustment

Where to turn

• Table 310.16 holds allowable ampacities for insulated conductors in raceway, cable or earth, at an ambient of 30°C. Pick the material (copper or aluminum), then the temperature column that matches the insulation, then the size.
• The ambient correction table and the adjustment table sit in 310.15 (2023 numbering: Table 310.15(B)(1) and Table 310.15(C)(1)). Confirm the exact numbers in your 2026 book.
• 110.14(C) limits the temperature column you may use at terminals and splices.
• 240.4 ties the overcurrent device to the conductor, and 240.6(A) lists the standard ratings.

Put tabs on all five. You will cross between them several times in one problem.

The method

1. Find the load current. For a continuous load, 125% of it is the minimum for the conductor and the device (210.19(A)(1) and 215.2(A)(1)).
2. Find the ampacity from the 90°C column if the insulation allows it. That is the starting value for correction and adjustment.
3. Multiply by the ambient correction factor, then by the adjustment factor for the number of current-carrying conductors in the raceway.
4. Confirm the result is at least the load.
5. Separately, confirm the load fits the terminal rating in 110.14(C). The 90°C figure can be used for derating, but the circuit may not exceed the ampacity in the column that matches the terminal rating. Use whichever result is lower.

Worked example

Six current-carrying copper THHN conductors in one conduit serve a 60-ampere noncontinuous load. Ambient is 38°C and the terminals are marked 75°C. Test 6 AWG first, then 4 AWG.

• 6 AWG: the 90°C column gives 75 A. The correction factor for 38°C at the 90°C column is 0.91. Six conductors take 80%. So 75 × 0.91 × 0.80 = 54.6 A. That is below 60 A, so 6 AWG fails.
• 4 AWG: 95 A × 0.91 × 0.80 = 69.2 A. That is above 60 A.
• Terminal check: 4 AWG at 75°C is 85 A, which is above 60 A, so the terminals do not limit it.
• Answer: 4 AWG copper.

Notice that 240.4(B) lets you round a device up to the next standard rating, but that never rescues the 6 AWG here. The conductor itself must carry the load.

Common traps

• Using the 90°C column as the final answer. It is the starting point for derating, and the terminal column can still be lower.
• Counting the wrong conductors. Only current-carrying conductors count for adjustment. Check the rules for neutrals and equipment grounding conductors in 310.15 (2023 numbering) before you count.
• Correcting at the wrong column. The correction factor changes by temperature column, so read the factor from the column you used.
• Forgetting that small circuits limit the column. For many circuits at 100 A or less, 110.14(C) points to the 60°C column unless equipment is marked otherwise.
• Mixing the 125% rule and the factors. The continuous-load 125% raises the required ampacity. The factors lower the available ampacity. Work them on opposite sides of the comparison.
• Missing the edition. The exam is referenced to the 2026 NEC, so mark any rule you learned from an older book.

How to practice

Invent six or seven loads with different ambient temperatures and conductor counts. Work each from the load to the size, then check yourself by working backward from the size to a maximum load. Time yourself. A good target is two to three minutes a problem, since the Calculations portion gives you 110 minutes.

After you size for ampacity, test the run with the voltage drop calculator.

Related guides: conduit fill and voltage drop.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.