# Ampacity correction and adjustment (NEC 310.15)

> To derate a conductor, take its Table 310.16 ampacity (the 90 °C column for 90 °C wire), multiply by the ambient temperature correction factor from Table 310.15(B)(1)(1) and by the Table 310.15(C)(1) adjustment factor for more than three current-carrying conductors. The result may not exceed the ampacity in the termination column.

Exams: Journeyman Electrician, Master Electrician. Conductors and overcurrent.

## Formula

    Adjusted ampacity (A) = Table ampacity (A) × Temperature correction factor × Adjustment factor
    Required table ampacity (A) = Load (A) ÷ (Correction factor × Adjustment factor)

## Variables

- **A**: Allowable ampacity from Table 310.16 (A)
- **CF**: Ambient temperature correction factor, Table 310.15(B)(1)(1)
- **AF**: Adjustment factor for the number of current-carrying conductors, Table 310.15(C)(1)
- **n**: Current-carrying conductors in the raceway or cable (conductors)

## Open your 2026 NEC to

- Table 310.16: Starting ampacity; derate from the 90 °C column for 90 °C insulation
- Table 310.15(B)(1)(1): Ambient temperature correction factors based on 30 °C (86 °F)
- Table 310.15(C)(1): Adjustment factors for more than three current-carrying conductors
- 110.14(C): The final ampacity may not exceed the termination temperature column

Table 310.16 ampacities assume an ambient of 30 °C (86 °F) and no more than three current-carrying conductors together. Hotter air, or more conductors heating each other in one raceway, means the conductor can safely carry less current. Correction handles temperature; adjustment handles bundling.

With 90 °C insulation such as THHN/THWN-2 or XHHW-2, start from the 90 °C column and read the correction factor from the 90 °C column too. That is why 90 °C wire is so common: it has more ampacity to lose before it falls below what the terminals allow.

After derating, compare the result with the ampacity in the termination column (60 °C or 75 °C under 110.14(C)). The conductor's usable ampacity is the smaller of the two. When you size a conductor, check both: the derated 90 °C ampacity must carry the load, and so must the termination column with no derating.

Neutrals that carry only unbalanced current and equipment grounding conductors are not counted as current-carrying. Counting neutrals is its own topic; these problems give you the count.

## Worked example

An EMT raceway holds 5 current-carrying 3/0 AWG copper THHN/THWN-2 conductors and runs through an area where the ambient temperature reaches 115 °F. Starting from the 90 °C column of Table 310.16, what is the ampacity of each conductor after correction and adjustment?

1. Look up Table 310.16, 90 °C copper column, for 3/0 AWG: 225 A.
2. Look up the correction factor for 115 °F in the 90 °C column of Table 310.15(B)(1)(1): 0.82.
3. Look up the adjustment for 5 current-carrying conductors in Table 310.15(C)(1): 80%, or 0.80.
4. Ampacity after correction and adjustment: 225 × 0.82 × 0.80 = 147.60 A.

**Answer: 147.60 A**

## How to solve it on the exam

1. Count the current-carrying conductors in the raceway or cable. Three or fewer means no adjustment factor.
2. Look up the ambient correction factor in the column that matches the insulation (90 °C for THHN), using the °C or °F side of the table.
3. Look up the adjustment percentage in Table 310.15(C)(1) and change it to a decimal (80% = 0.80).
4. To find ampacity: multiply the table ampacity by both factors. To find a size: divide the load by both factors, then find the first size in the 90 °C column that is at least that large.
5. Check the termination column: the conductor must also carry the load at 60 °C or 75 °C with no derating. Use the larger size, or the smaller ampacity.

## Common mistakes

- Derating from the 75 °C column when the insulation is rated 90 °C.
- Reading the temperature correction factor from the wrong column.
- Multiplying the load by the factors when sizing, instead of dividing.
- Forgetting the termination check, so the answer uses more ampacity than a 75 °C lug allows.
- Applying an adjustment factor to three or fewer current-carrying conductors.

## Exam tips

- Write the three numbers on your notepad before you multiply: table ampacity, CF, AF. Most wrong answers skip one of them.
- Texas attics and rooftops make temperature questions common. The table has a °F column, so you do not need to convert.

## Questions people ask

### What is the difference between correction and adjustment factors?

Correction factors account for ambient temperature above or below 30 °C (86 °F). Adjustment factors account for more than three current-carrying conductors in one raceway or cable.

### Can I derate THHN from the 90 °C column?

Yes. With 90 °C insulation you may apply correction and adjustment to the 90 °C ampacity, as long as the final result does not exceed the 60 °C or 75 °C termination ampacity.

### Do I apply both factors at the same time?

Yes. When the ambient is above 30 °C and there are more than three current-carrying conductors, multiply the table ampacity by both factors.

### Does the neutral count as a current-carrying conductor?

Only sometimes. A neutral that carries only the unbalanced current of a circuit does not count, but a neutral on a 4-wire wye feeder with mostly nonlinear loads does. Equipment grounding conductors never count.

## Sources

- [TDLR: Electrician examination information](https://www.tdlr.texas.gov/electricians/elecexam.htm)
- [EC&M: NEC requirements for ambient temperature correction](https://ecmweb.com/national-electrical-code/article/21173523/stumped-by-the-code-nec-requirements-for-ambient-temperature-correction-to-conductor-ampacity)

Interactive practice: https://texaselectricianexam.com/calculations/ampacity-correction/