Conductor Ampacity on the Exam: Table 310.16, Terminations and Derating
Ampacity questions trip people up because there are three or four rules stacked on top of each other, and the table alone won't give you the answer. Here's the order I work them in, every time.
References follow the NEC's Article 310, 110.14(C) and 240.4(D) as they've read for the last few editions. Confirm the numbers against the edition your exam uses.
1. Start with Table 310.16
Table 310.16 covers up to three current-carrying conductors in a raceway, cable or earth, at a 30°C (86°F) ambient. Find the size and material (copper or aluminum), then pick the column that matches the insulation's temperature rating: 60°C, 75°C or 90°C.
2. Check the terminations (110.14(C))
A conductor is only as good as the lug it lands on. For circuits rated 100 amps or less, or conductors 14 through 1 AWG, you use the 60°C column unless the equipment is listed and marked for higher. Over 100 amps, or larger than 1 AWG, use 75°C unless marked otherwise. Most equipment today is marked 60/75°C, so on the job 75°C is common, but read what the question tells you about the equipment.
The 90°C column is still useful. You can start derating from the 90°C value for THHN, as long as the final answer doesn't exceed what the terminations allow.
3. Apply corrections and adjustments (310.15)
- Ambient temperature correction, from the correction tables in 310.15(B), when the ambient isn't 30°C.
- Adjustment for more than three current-carrying conductors in a raceway or cable, from 310.15(C)(1):
| Current-carrying conductors | Percent of table value |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Equipment grounding conductors don't count. A neutral that carries only the unbalanced current of a multiwire circuit usually doesn't count either, but read 310.15(E) for when it does.
4. Don't forget the small-conductor rule (240.4(D))
Whatever the table says, copper overcurrent protection is capped at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG, unless a specific exception applies.
Worked example
Eight current-carrying 12 AWG THHN copper conductors in one conduit, 30°C ambient, 75°C terminations.
- 90°C value: 30 A
- Adjust for 7–9 conductors: 30 × 0.70 = 21 A
- Termination check: the 75°C value is 25 A, so 21 A is fine
- Small-conductor rule: maximum 20 A overcurrent device
Answer: the conductors are good for 21 A, protected at 20 A.
Common traps
- Using the 90°C column for the final answer.
- Counting grounds as current-carrying.
- Forgetting 240.4(D) after doing all the math right.
- Reading the aluminum side of the table.
Related: box fill calculations and sizing grounding and bonding conductors.
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More guides: How to navigate the NEC · 2026 NEC changes for exam takers · Massachusetts exam: 527 CMR and the NEC · How to tab and mark your NEC · 30-day journeyman study plan · Box fill calculations (314.16) · Conductor ampacity and derating · Grounding conductor sizing (250.66 vs 250.122) · Voltage drop calculations