NEC 310.15(B)(1) Temperature Correction Table
Use this table when the ambient temperature is different from the base temperature used by the applicable ampacity table. Start with base ampacity, apply the temperature correction factor, then check conductor-count adjustment and terminal limits separately.
| Ambient Temperature | 60°C Column | 75°C Column | 90°C Column | Field Meaning |
|---|---|---|---|---|
| 10°C or less / 50°F or less | 1.29 | 1.20 | 1.15 | Cooler-than-base ambient can increase the correction factor, but final ampacity still has to stay inside terminal and equipment limits. |
| 11–15°C / 51–59°F | 1.22 | 1.15 | 1.12 | Use the column that matches the conductor insulation temperature rating used for the correction step. |
| 16–20°C / 60–68°F | 1.15 | 1.11 | 1.08 | Confirm the base ampacity table before applying this factor. |
| 21–25°C / 69–77°F | 1.08 | 1.05 | 1.04 | Slightly cooler conditions may allow a factor above 1.00 before final limiting checks. |
| 26–30°C / 78–86°F | 1.00 | 1.00 | 1.00 | This is the normal 30°C base condition for common NEC ampacity tables such as NEC 310.16. |
| 31–35°C / 87–95°F | 0.91 | 0.94 | 0.96 | Higher ambient temperature starts reducing usable ampacity. |
| 36–40°C / 96–104°F | 0.82 | 0.88 | 0.91 | Common high-ambient range for attics, rooftops, and hot equipment areas. |
| 41–45°C / 105–113°F | 0.71 | 0.82 | 0.87 | This range can force a larger conductor or a revised wiring method. |
| 46–50°C / 114–122°F | 0.58 | 0.75 | 0.82 | The 60°C column becomes restrictive quickly in high ambient conditions. |
| 51–55°C / 123–131°F | 0.41 | 0.67 | 0.76 | Do not trust normal ampacity without checking the correction factor. |
| 56–60°C / 132–140°F | Not permitted by table | 0.58 | 0.71 | The 60°C column is no longer available in this range. |
| 61–65°C / 141–149°F | Not permitted by table | 0.47 | 0.65 | High ambient conditions need close review before relying on conductor ampacity. |
| 66–70°C / 150–158°F | Not permitted by table | 0.33 | 0.58 | This is a severe temperature condition for many wiring methods. |
| 71–75°C / 159–167°F | Not permitted by table | Not permitted by table | 0.50 | Only the 90°C column remains available in this range. |
| 76–80°C / 168–176°F | Not permitted by table | Not permitted by table | 0.41 | Final usable ampacity still cannot exceed the applicable terminal limit. |
| 81–85°C / 177–185°F | Not permitted by table | Not permitted by table | 0.29 | Use only when the conductor insulation rating, wiring method, equipment, and field conditions support the review. |
Where Temperature Correction Fits
NEC 310.15(B)(1) is not the base ampacity table. Use the applicable ampacity table first, such as the NEC 310.16 Ampacity Table, then apply temperature correction when the ambient condition requires it.
Field Boundary
Temperature correction is not the whole ampacity check.
Confirm base ampacity, conductor-count adjustment, terminal temperature limits, raceway fill, box fill, and equipment instructions separately.
- Start with base ampacity. Use the conductor material, size, insulation type, and wiring condition before applying any correction factor.
- Apply the right temperature column. Match the correction step to the conductor insulation rating allowed for the calculation.
- Limit the final result. The corrected ampacity still has to respect NEC 110.14(C) Terminal Temperature limits and equipment markings.
Insulation Rating and Terminal Limits
A conductor with 90°C insulation may allow correction and adjustment calculations from the 90°C column where permitted. That does not mean the equipment termination can be loaded to the 90°C ampacity. The final usable ampacity is limited by the applicable terminal rating, equipment instructions, and conductor type.
| Field Condition | Check First | Field Meaning |
|---|---|---|
| THHN/THWN-2 conductors | Insulation and terminal ratings | The insulation rating may support 90°C correction steps, but the final ampacity often cannot exceed the equipment terminal limit. |
| NM-B cable | NEC 334.80 boundary | NM cable has special ampacity rules. Review NEC 334.80 NM Cable Ampacity before relying on a temperature-corrected result. |
| Equipment terminals | NEC 110.14(C) | Final conductor ampacity must stay within the temperature rating of the terminals used for the installation. |
| Manufacturer markings | Listed equipment instructions | Equipment instructions and markings can be more specific than a general table lookup. Review NEC 110.3(B) Listed Equipment when equipment instructions control the installation. |
Temperature Correction vs Conductor-Count Adjustment
Ambient temperature correction and current-carrying conductor adjustment are separate checks. If both conditions apply, the field result may need both factors before the final terminal-limit check.
- Temperature correction responds to the ambient condition around the conductor.
- Conductor-count adjustment responds to heat from multiple current-carrying conductors installed together. Use the NEC 310.15(C)(1) Ampacity Adjustment Table for that check.
- Raceway fill is separate. A temperature-corrected conductor still needs raceway-fill review under NEC Chapter 9 Conduit Fill Tables when conductors are installed in raceway.
Rooftop and High-Ambient Conditions
Hot locations can change the temperature used for ampacity correction. Rooftop raceways, attic spaces, sun-exposed areas, and equipment rooms may need a higher ambient-temperature input than a normal room-temperature assumption.
Field Boundary
Do not assume outdoor temperature is the same as conductor ambient temperature.
Rooftops, attics, raceways near heat sources, and equipment enclosures can create higher conductor ambient temperatures. Confirm the field condition before using the corrected ampacity.
Field Example
A conductor starts with a 75A base ampacity and the ambient-temperature correction factor for the installed condition is 0.88. Apply the temperature correction before trusting the ampacity result.
Example Check
75A × 0.88 = 66A corrected ampacity before any other required adjustment.
The corrected value still has to be checked against terminal temperature limits, conductor-count adjustment, wiring method limits, equipment instructions, and the actual overcurrent protection used on the job.
TradeHub Calculator Application
TradeHub uses this topic as the source-reference page for ambient temperature correction inside the ampacity workflow. Confirm the ambient condition, then review conductor count, terminal limits, wiring method, and raceway fill as separate field checks.
- Primary calculator: Use the Ampacity Calculator when ambient temperature, conductor count, terminal limits, and wiring method need to be checked together.
- Separate physical checks: Raceway fill, box fill, and pull-box sizing are separate from the temperature-correction factor.
Checks Before Trusting the Corrected Ampacity
- Confirm the applicable base ampacity table before applying a correction factor.
- Use the correct conductor insulation temperature column for the correction step.
- Apply NEC 310.15(C)(1) Ampacity Adjustment separately when current-carrying conductor count requires it.
- Limit the final result by NEC 110.14(C) Terminal Temperature and equipment markings.
- Check raceway fill, box fill, and manufacturer instructions separately before treating the installation as field-ready.
Related NEC Field References
NEC 310.16 Ampacity Table
Use this before temperature correction to identify base conductor ampacity.
NEC 310.15(C)(1) Ampacity Adjustment
Use this for conductor-count adjustment after the current-carrying conductors are identified.
NEC 110.14(C) Terminal Temperature
Use this to keep the final conductor ampacity inside the terminal temperature boundary.
NEC 334.80 NM Cable Ampacity
Use this when NM cable ampacity and correction boundaries affect the field result.
Source Alignment and Use Scope
This TradeHub field reference is based on NEC 310.15(B)(1), related 310.15 ampacity source alignment records, and TradeHub calculator source-scope records. It is intended for screening and planning only. It does not reproduce the NEC, replace the adopted Code text, approve a field installation, or override AHJ direction, manufacturer instructions, equipment markings, conductor listings, terminal ratings, or licensed electrical judgment.
Review the Code Citation & Source Log and TradeHub Methodology for source alignment, calculator scope, and professional-use boundaries.
Temperature Correction FAQ
Is NEC 310.15(B)(1) the same as the ampacity table?
No. Use the applicable ampacity table first, then apply NEC 310.15(B)(1) when the ambient temperature requires correction.
Do I use the 60°C, 75°C, or 90°C correction column?
Use the conductor insulation temperature rating allowed for the correction step, then limit the final usable ampacity by the applicable terminal temperature and equipment markings.
Does temperature correction replace conductor-count derating?
No. Ambient temperature correction and conductor-count adjustment are separate checks. A field condition may require both before terminal limits are applied.
Should attic or rooftop conductors use normal room temperature?
Not automatically. Attics, rooftops, sun-exposed raceways, and hot equipment areas can have higher conductor ambient temperatures than normal room conditions.