2.4 Conductor Ampacity & Termination Workshop (0103)
Key Takeaways
- Every conductor question is answered twice: once for the raceway (correction and adjustment factors on the insulation-rating column) and once for the terminations (110.14(C)), and the smaller of the two answers governs.
- Ambient correction factors come from Table 310.15(B)(1) and adjustment factors for more than three current-carrying conductors from 310.15(C)(1); both multiply the ampacity column matching the conductor insulation.
- A neutral carrying only the unbalanced current of a 3-wire or 4-wire circuit is not a current-carrying conductor, but the neutral of a 4-wire wye circuit carrying a major portion of nonlinear load is (310.15(E)).
- Equipment grounding conductors and bonding conductors are never counted as current-carrying conductors for adjustment purposes (310.15(E)(3)).
- Conductors in a raceway or cable exposed to sunlight on or above a rooftop take the ambient adder of 310.15(B)(2) — 33°C (60°F) added to the outdoor ambient where the raceway is less than 7/8 in. above the roof surface — and Type XHHW-2 is excepted.
The Two-Answer Method
Almost every ampacity question on the E2 can be answered by the same four-step routine. Write it on the inside cover of your NEC:
- Read the base ampacity from Table 310.16 in the column matching the conductor insulation — 60°C for TW and UF, 75°C for THW and XHHW, 90°C for THHN, THWN-2, and XHHW-2.
- Correct for ambient using Table 310.15(B)(1), in the same insulation column.
- Adjust for conductor count using 310.15(C)(1) if more than three current-carrying conductors share the raceway or cable.
- Cap at the termination using 110.14(C): the 60°C column for equipment rated 100 A or less (unless marked 75°C), the 75°C column for equipment rated over 100 A.
Steps 1 through 3 give the raceway answer. Step 4 gives the termination answer. The smaller number governs. Candidates lose points by stopping after step 3, or by starting steps 2 and 3 in the 75°C column because that is where the terminations live.
Correction Factors — Table 310.15(B)(1)
| Ambient (°C) | 60°C column | 75°C column | 90°C column |
|---|---|---|---|
| 21–25 | 1.08 | 1.05 | 1.04 |
| 26–30 | 1.00 | 1.00 | 1.00 |
| 31–35 | 0.91 | 0.94 | 0.96 |
| 36–40 | 0.82 | 0.88 | 0.91 |
| 41–45 | 0.71 | 0.82 | 0.87 |
| 46–50 | 0.58 | 0.75 | 0.82 |
| 51–55 | 0.41 | 0.67 | 0.76 |
Notice how much flatter the 90°C column is: at 45°C a 90°C conductor keeps 87% of its ampacity while a 60°C conductor keeps 71%. That is the whole practical argument for specifying THWN-2 in a hot mechanical room.
Adjustment Factors — 310.15(C)(1)
| Number of current-carrying conductors | Percent of value in Table 310.16 |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
What counts as current-carrying
- Ungrounded (phase) conductors always count.
- A neutral carrying only the unbalanced current from other conductors of a 3-wire or 4-wire circuit does not count.
- The neutral of a 4-wire, 3-phase wye circuit where a major portion of the load is nonlinear (LED drivers, computer power supplies, variable-frequency drives) does count, because triplen harmonics add rather than cancel in it.
- Equipment grounding and bonding conductors never count.
The adjustment factors do not apply to conductors in nipples not exceeding 24 in. in length, or to underground conductors entering or leaving an outdoor trench where they have earthen or concrete cover.
Worked Problem 1 — Correction Only
A 6 AWG THWN-2 copper conductor runs through a boiler room where the ambient reaches 45°C. Three current-carrying conductors are in the raceway. The equipment is rated 100 A or less with 75°C terminations.
- Base ampacity, 90°C column: 75 A
- Correction at 45°C, 90°C column: 75 × 0.87 = 65.25 A
- No adjustment — only three conductors
- Termination limit, 75°C column for 6 AWG Cu: 65 A
Answer: 65 A. The raceway math (65.25 A) and the termination limit (65 A) land almost on top of each other, and the termination wins by a quarter of an amp.
Worked Problem 2 — Correction Plus Adjustment
Six 8 AWG THHN copper current-carrying conductors share a conduit in a 40°C space, feeding equipment rated over 100 A with 75°C terminations.
- Base ampacity, 90°C column, 8 AWG Cu: 55 A
- Correction at 40°C, 90°C column: 55 × 0.91 = 50.05 A
- Adjustment for 6 current-carrying conductors: 50.05 × 0.80 = 40.04 A
- Termination limit, 75°C column, 8 AWG Cu: 50 A
Answer: about 40 A. The derated raceway value governs. Note that the overcurrent device must also satisfy 240.4 — a 40 A device protects this conductor, and the 240.4(D) 10 AWG limit is irrelevant here because the conductor is 8 AWG.
Worked Problem 3 — Rooftop Adder
A conduit is supported 1/2 in. above a flat roof in full sun. The outdoor design ambient is 35°C. It contains four current-carrying 10 AWG THWN-2 copper conductors feeding a rooftop unit disconnect rated 100 A or less with 75°C terminations.
- Rooftop adder, 310.15(B)(2): where a raceway or cable is exposed to sunlight on or above a roof and is less than 7/8 in. above the roof surface, add 33°C (60°F) to the outdoor ambient. Effective ambient = 35 + 33 = 68°C.
- Base ampacity, 90°C column, 10 AWG Cu: 40 A
- Correction at 68°C, 90°C column: roughly 0.5, giving about 20 A
- Adjustment for 4 current-carrying conductors: 20 × 0.80 = 16 A
- Termination limit, 75°C column, 10 AWG Cu: 35 A — and 240.4(D) separately caps the device at 30 A for 10 AWG copper
Answer: about 16 A of usable ampacity, which will not carry a typical rooftop unit. The fix on the job is either to raise the raceway to 7/8 in. or more above the roof, or to use Type XHHW-2, which the exception to 310.15(B)(2) exempts from the rooftop adder entirely.
Inspector's field check. When you see conduit lying flat on a roof membrane feeding a packaged unit, ask two questions: what is the conductor insulation, and how far off the roof is the raceway? Those two answers decide whether the installation is compliant or is running a 40 A conductor at 16 A of capacity.
Six current-carrying 8 AWG THHN copper conductors share a raceway in a 40°C ambient, feeding equipment rated over 100 A with 75°C terminations. Table 310.16 gives 8 AWG Cu as 40/50/55 A. What is the final allowable ampacity?
A 4-wire, 3-phase wye feeder supplies a bank of LED drivers and computer power supplies. How is the neutral treated when counting current-carrying conductors for 310.15(C)(1)?
A raceway containing THWN-2 conductors is supported 1/2 in. above a sunlit roof where the outdoor ambient is 35°C. What does 310.15(B)(2) require?