Conductor Ampacity, Derating and the Termination Limit
You may start a derating calculation from a 90 °C conductor's ampacity. You may not finish there. Whatever number the calculation produces, the conductor's allowable ampacity is still capped by the temperature rating of the terminations it lands on — and that is almost always 60 °C or 75 °C.
This is the rule that makes THHN look better on paper than it is allowed to be in practice, and it is behind a large share of failed inspections on otherwise sound installations.
This page covers NEC 110.14(C), how correction and adjustment stack, and the termination traps that catch experienced installers. It is written for electricians, estimators and engineers sizing conductors for commercial and industrial work.
The rule in one line
Under NEC 110.14(C), the conductor's temperature rating "shall not exceed the lowest temperature rating of any connected termination, conductor or device."
Everything else follows from that. The conductor insulation can be rated higher; the ampacity you are allowed to use cannot exceed the column matching the weakest termination in the circuit.
Which column applies
| Equipment | Column | Section |
|---|---|---|
| Rated 100 A or less, or marked for 14 AWG through 1 AWG conductors | 60 °C | 110.14(C)(1)(a) |
| Rated over 100 A, or marked for conductors larger than 1 AWG | 75 °C | 110.14(C)(1)(b) |
Higher-rated conductors may be used in both cases — but their ampacity is taken from the column above, unless the equipment is specifically listed and identified for the higher temperature.
Schneider Electric states the practical position bluntly in its wire temperature bulletin: no distribution or utilization equipment is listed and identified for the use of 90 °C wire at its 90 °C ampacity. If you are counting on the 90 °C column at a termination, check the equipment label — not the lug.
The lug trap
Manufacturers commonly fit 90 °C-rated lugs to equipment rated only 60 °C or 75 °C. The lug marking is not the equipment rating. Terminations inside equipment follow the equipment label.
And check both ends. A 75 °C breaker at one end and a 60 °C receptacle at the other means the conductor must be sized on the 60 °C ampacity, because the lowest-rated termination in the circuit governs.
Where 90 °C conductors genuinely help
The code is explicit that the higher rating is usable for the calculation. In the words of 110.14(C): conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both.
So the 90 °C column is your starting point when you have heat to account for — a hot ambient, or a crowded raceway. You take the hit from that starting point rather than from the 75 °C figure, which often leaves you with a usable conductor where starting lower would not have.
Schneider puts the sequence plainly: begin derating at the 90 °C ampacity, compare the result against the ampacity based on the termination rating, and the smaller of the two numbers becomes the allowable ampacity.
The two factors that reduce ampacity
Ambient temperature correction
Ampacity tables are built on a 30 °C (86 °F) ambient. Anything hotter reduces capacity; anything cooler increases it. Read the correction factor from the column matching the conductor insulation rating, not the termination rating.
| Ambient | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 21–25 °C | 1.08 | 1.05 | 1.04 |
| 26–30 °C | 1.00 | 1.00 | 1.00 |
| 31–35 °C | 0.91 | 0.94 | 0.96 |
| 36–40 °C | 0.82 | 0.88 | 0.91 |
| 41–45 °C | 0.71 | 0.82 | 0.87 |
| 46–50 °C | 0.58 | 0.75 | 0.82 |
| 51–55 °C | 0.41 | 0.67 | 0.76 |
Notice how much flatter the 90 °C column is. At 46–50 °C a 60 °C conductor keeps 58% of its rating while a 90 °C conductor keeps 82%. That spread is the real argument for 90 °C insulation in hot spaces — not a higher number at the terminal.
Adjustment for more than three current-carrying conductors
| Current-carrying conductors | Factor |
|---|---|
| 1–3 | 100% |
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
The cliff between 7–9 conductors (70%) and 10–20 (50%) is steep. Adding a tenth current-carrying conductor to a raceway costs every conductor in it another 20 percentage points.
Which conductors count
- Ungrounded conductors — always count.
- Equipment grounding and bonding conductors — never count. (They do still count for conduit fill; the two calculations treat them differently.)
- A neutral carrying only the unbalanced current of a balanced 3-wire single-phase circuit — does not count.
- A neutral serving significant nonlinear load — counts.
- A neutral on a 3-wire circuit from a 4-wire 3-phase wye — counts.
- Energised spare conductors — count. Conductors not energised at the same time, such as 3-way travellers, do not.
Worked example
Schneider's example: a 300 kcmil copper conductor at the 75 °C column is rated 285 A. Put six current-carrying conductors in the raceway (80%) in a 40 °C ambient (0.88 at 75 °C):
285 × 0.80 × 0.88 = 201 A
And a motor example from EC&M that shows the termination cap biting: a motor needs 74 A. A 6 AWG THHN copper conductor is rated 75 A in the 90 °C column — it looks sufficient. But the equipment terminations are 75 °C, where 6 AWG is rated 65 A. 6 AWG is not permitted, despite the 90 °C number clearing the load.
The order of operations
- Start at the ampacity for the conductor's own insulation rating — usually the 90 °C column.
- Apply the ambient correction factor from the column matching that insulation.
- Apply the adjustment factor for the number of current-carrying conductors.
- Work out the ampacity for the same conductor in the termination's column (60 °C or 75 °C per 110.14(C)).
- The smaller of steps 3 and 4 is the allowable ampacity.
Then size the overcurrent protection to that allowable ampacity — not to the table value you started from.
Five ways this goes wrong
- Applying correction to the termination column. Correction and adjustment start from the insulation's own column. Starting at 75 °C throws away the benefit of 90 °C wire.
- Stopping at the derated number. Step 4 is not optional. The termination cap applies whether or not derating was needed.
- Reading the lug instead of the label. A 90 °C lug on 75 °C-rated equipment is still a 75 °C termination.
- Checking only one end of the run. The lowest-rated termination anywhere in the circuit governs.
- Miscounting the neutral. Whether it counts depends on the system and the load, and getting it wrong moves you across an adjustment-factor band.
Sourcing wire and cable
ATEK Distribution is an electrical distributor based in Minneapolis, Minnesota, supplying contractors, industrial facilities and government buyers nationwide. Send a conductor schedule or a bill of materials with the insulation type, size and quantities, and we will quote it with lead times — including the raceway and fittings on the same quote.
Frequently asked questions
Can I use the 90 °C column for conductor ampacity?
You can use the 90 °C column as the starting point for ampacity correction and adjustment calculations, but not as the final allowable ampacity at the termination. NEC 110.14(C) caps the result at the column matching the lowest-rated termination in the circuit, which is normally 60 °C or 75 °C.
What temperature column applies to my equipment?
The temperature column depends on the equipment rating. Equipment rated 100 A or less, or marked for 14 AWG through 1 AWG conductors, uses the 60 °C column under 110.14(C)(1)(a). Equipment rated over 100 A, or marked for conductors larger than 1 AWG, uses the 75 °C column under 110.14(C)(1)(b). Higher ratings apply only where the equipment is listed and identified for them.
Does the ground wire count as a current-carrying conductor?
No, equipment grounding and bonding conductors are never counted as current-carrying conductors for ampacity adjustment. They do count toward conduit fill, however — the two calculations treat them differently, which is a common source of confusion.
Does the neutral count as a current-carrying conductor?
The neutral counts in some configurations and not others. A neutral carrying only the unbalanced current of a balanced 3-wire single-phase circuit does not count. A neutral serving significant nonlinear load does count, as does a neutral on a 3-wire circuit taken from a 4-wire 3-phase wye system.
Do I apply correction or adjustment first?
Apply both to the conductor's own insulation-column ampacity — the order of the two multipliers does not change the result. Start at the 90 °C ampacity, multiply by the ambient correction factor and the adjustment factor, then compare the result with the termination-limited ampacity and take the smaller.
Why does my 90 °C wire not let me use a smaller conductor?
Your 90 °C wire does not reduce conductor size because terminations, not insulation, set the ceiling. Virtually no distribution or utilisation equipment is listed for 90 °C conductor ampacity, so the allowable ampacity falls back to the 60 °C or 75 °C column. The 90 °C rating earns its keep in hot ambients and crowded raceways, where it loses less to correction and adjustment.
Related guides
- NEC conduit fill — the companion calculation, with different rules for the grounding conductor.
- Wire, cords and cables — conductor and insulation types.
- Circuit breakers — sizing overcurrent protection to the allowable ampacity.
- Industrial fuse selection guide — the fuse side of overcurrent protection.
Sources and edition note
Facts on this page are drawn from Schneider Electric Data Bulletin 0110DB9901 (Wire Temperature Ratings and Terminations), EC&M's General Installation Requirements series, and published NEC study material. Table and section numbering has changed between NEC editions — the ampacity table has appeared as both Table 310.16 and Table 310.15(B)(16), and the correction and adjustment tables have been renumbered more than once. NEC editions are adopted state by state and amended locally. Confirm every value and section number against the edition adopted in your jurisdiction. This page is general guidance and does not replace the National Electrical Code or the judgement of the authority having jurisdiction.
Send a conductor schedule or a bill of materials. ATEK Distribution supplies wire, cable, raceway and overcurrent protection to contractors, industrial facilities and government buyers nationwide from Minneapolis, Minnesota. Quote responses go out within one business day, often the same day.