NEC 240.4(D): The Small-Conductor Rule on the Electrician Exam
You can do a derating problem perfectly and still miss the question. The reason is usually 240.4(D), the small-conductor rule. It caps the overcurrent protection on small conductors no matter what the ampacity table says. It shows up on journeyman and master exams constantly, because it is easy to forget at the end of a long calculation.
Study aid only, not legal or design advice. Confirm every value in the edition your state has adopted and in your own code book.
What 240.4(D) says, in plain terms
For the small sizes, the overcurrent protective device cannot exceed a set value, even if Table 310.16 shows a higher ampacity. The values most people need for the exam:
| Conductor | Max overcurrent protection |
|---|---|
| 18 AWG copper | 7 A (with conditions) |
| 16 AWG copper | 10 A (with conditions) |
| 14 AWG copper | 15 A |
| 12 AWG aluminum / copper-clad aluminum | 15 A |
| 12 AWG copper | 20 A |
| 10 AWG aluminum / copper-clad aluminum | 25 A |
| 10 AWG copper | 30 A |
The 18 and 16 AWG allowances come with conditions (continuous load limits, specific listed conductor or cable types, and specific overcurrent devices). Read the list items in your code book. Newer editions also expanded where 16 AWG can be used, so check your edition.
Why the 90°C column fools people
Table 310.16 lists 14 AWG THHN copper at 25 A in the 90°C column. 12 AWG is 30 A and 10 AWG is 40 A. Those numbers are real, and you are allowed to use the 90°C column as the starting point for derating. But 240.4(D) still limits the breaker: 15, 20 and 30 A. A typical exam question gives you a 12 AWG THHN with a derating factor that leaves it at, say, 24 A, then asks for the maximum breaker. The answer is 20 A, not 25 A.
The order of operations
- Start with the ampacity from Table 310.16 (90°C column if the insulation allows it and you are derating).
- Apply corrections: ambient temperature correction and the adjustment for more than three current-carrying conductors.
- Check terminations under 110.14(C). Most equipment is rated 60°C or 75°C, so the final usable ampacity cannot exceed that column.
- Apply 240.4(D) last for the small sizes. This is the cap.
- Pick the overcurrent device at or below the result, using 240.4(B) only where it applies (it does not override 240.4(D) for the small sizes).
Worked example
Six current-carrying 12 AWG THHN copper conductors in one conduit, 30°C ambient. What is the maximum overcurrent protection?
- Table 310.16, 90°C column: 30 A.
- Adjustment for 4 to 6 current-carrying conductors: 80%. 30 × 0.80 = 24 A.
- Termination check: the 60°C value for 12 AWG copper is 20 A; the 75°C is 25 A. You also can't exceed 24 A.
- 240.4(D): 12 AWG copper is capped at 20 A.
- Answer: 20 A.
Notice that three different rules land on or near 20 A. Exam writers like that, because every shortcut gives a slightly different wrong answer: 24 A, 25 A, or 30 A.
Where 240.4(D) does not apply
240.4(D) has a short list of exceptions pointed to in 240.4(E) and 240.4(G). Motor circuits (Article 430), air-conditioning and refrigeration equipment (Article 440), fire alarm, Class 1 remote-control, and some specific equipment have their own protection rules. A motor branch circuit on 14 AWG copper can be protected by a larger breaker sized under Article 430 because the motor overload protection handles the overload, and the breaker only handles short circuit and ground fault.
The exam trap here runs the other way: a question about a motor or an AC unit with a nameplate maximum overcurrent rating, where applying 240.4(D) gives you the wrong (too small) answer. Read the load type before you reach for the cap.
Exam traps to watch for
- Stopping at the derated ampacity and picking the next breaker size up.
- Using 240.4(B) "next size up" on a 14, 12 or 10 AWG conductor.
- Forgetting that aluminum has its own lower caps (12 AL at 15 A, 10 AL at 25 A).
- Applying the cap to a motor or HVAC circuit that is covered by Article 430 or 440.
How to tab it
Put a tab on 240.4 and highlight the (D) list. Write "430 / 440 exempt, see (G)" in the margin. When you finish any ampacity problem, glance at the tab before you circle an answer. That habit alone picks up points.
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