Short answer: Start from Table 310.16 in the column for the insulation (THHN and XHHW-2 start at 90 C). Multiply by the ambient correction factor from Table 310.15(B)(1)(1) when the ambient differs from 86 F (30 C), and by the adjustment factor from Table 310.15(C)(1) when there are more than three current-carrying conductors; if both apply, apply both. Then compare the result with the termination limit in 110.14(C)(1). The lower value governs.
Ampacity questions punish one wrong column, and the exam gives you about 4 minutes 14 seconds per Calculations item. The Practice Pack has 299 step-by-step calculation solutions that show the givens, each step and where to round, plus 10 full timed two-part mocks that score you by part and by topic. Its code-book navigation aids and ranked tab list help you reach Table 310.16 fast. US$39, one payment.
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The order of operations
Ampacity questions follow one path. Run it the same way every time and you stop guessing which column to use.
- Find the starting column. Table 310.16, in the temperature column for the insulation named in the question. THHN and XHHW-2 start at 90 C. All conductors are copper unless the question says otherwise.
- Correct for ambient. If the ambient differs from 86 F (30 C), read the factor from Table 310.15(B)(1)(1), in the same temperature column you started in.
- Adjust for conductor count. If there are more than three current-carrying conductors in the raceway or cable, read the factor from Table 310.15(C)(1).
- Apply both when both apply. Multiply the starting ampacity by both factors.
- Check the termination limit. 110.14(C)(1) sets it. Circuits rated 100 A or less, or marked for 14 through 1 AWG, use the 60 C column unless the equipment is listed and marked for 75 C. Larger circuits use 75 C. Most exam questions state the termination rating, so use the column the question names.
- Size the protection. Standard device sizes are in 240.6(A). For devices of 800 A or less, 240.4(B) allows the next size up. For small copper, 240.4(D) caps 14, 12 and 10 AWG at 15, 20 and 30 A.
A worked example with round numbers
This is an example only. The numbers below are not code table values; read the real ones from your own code book.
| Step | Example value |
|---|---|
| Table 310.16, 90 C column for the size in the question (example) | 100 A |
| Ambient factor times adjustment factor (example product) | 0.5 |
| Corrected ampacity: 100 x 0.5 | 50 A |
| Table 310.16, termination column (60 C or 75 C) for the same size | read it from your book |
| Usable ampacity | the lower of 50 A and your termination column value |
If your termination column value is above 50 A, the derating sets the answer and the usable value is 50 A. If it is below 50 A, the termination value is the answer. Light factors push the corrected value up, and then the termination limit can be the number that decides. That is why you check both every time.
The two traps that cost the most points
Our workbook flags two mistakes on this question type:
| Trap | What it looks like | The fix |
|---|---|---|
| Derating from the termination column | Applying the factors to the 75 C value instead of the 90 C value | Derate from the insulation column (90 C for THHN), then compare to the termination column |
| Counting the equipment grounding conductor | Adding the ground to the current-carrying count and picking a lower factor | Count only current-carrying conductors when you read Table 310.15(C)(1) |
Two smaller habits help too. Read the ambient factor from the same temperature column you started in, not the column next to it. And keep your finger on the row for the conductor size, so the two values you compare come from the same row.
Work out the usable ampacity
Corrected ampacity and the termination limit
The defaults are round examples to replace. Read the starting ampacity and the termination column ampacity from Table 310.16, and the two factors, from your own 2026 NEC. Enter 1 for a factor that does not apply.
- Ampacity from Table 310.16, 90 C column (your book)
- Ambient correction factor from Table 310.15(B)(1)(1), 90 C column (your book), 1 if not needed
- Adjustment factor from Table 310.15(C)(1) (your book), 1 if three or fewer current-carrying conductors
- Ampacity from Table 310.16, termination column (60 C or 75 C under 110.14(C)(1)), same size (your book)
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Texas Journeyman Electrician Exam Prep 2026 NEC (paperback)
Our own workbook has a full lesson on conductor ampacity among its 8 calculation method lessons, with worked calculations, code book lookup paths and 486 practice questions. Work it with your soft-bound NEC open to Table 310.16.
Do you derate from the 90 C or 75 C column on the journeyman exam?
Derate from the column for the insulation, which is 90 C for THHN and XHHW-2. Then check that the result does not exceed the termination column value for the same size. 110.14(C)(1) sets 60 C or 75 C depending on the circuit rating and the equipment marking, and most exam questions state it.
Does the ground wire count as a current-carrying conductor?
No. Counting the equipment grounding conductor as current-carrying is one of the traps our workbook flags. Count only current-carrying conductors when you read Table 310.15(C)(1).
What if both high ambient and more than three conductors apply?
Apply both factors. Multiply the Table 310.16 value by the ambient factor from Table 310.15(B)(1)(1) and by the adjustment factor from Table 310.15(C)(1), then compare the result to the termination column value.
