310.15(B)
Ambient Temperature Correction Factors
Table 310.16 assumes your conductors are sitting in a comfortable 86-degree-Fahrenheit environment. But wires on a rooftop in Phoenix, inside a boiler room, or running through an attic in July are dealing with much higher ambient temperatures. When the surrounding air is hotter, the wire cannot shed heat as efficiently, so its safe current capacity drops. The correction factor table gives you a decimal multiplier — for example, 0.87 for a 75-degree-C conductor at 40 degrees C ambient. You multiply the base ampacity by that factor to get the adjusted ampacity. On the flip side, if the ambient temperature is below 86 degrees F, the correction factor is greater than 1.0, meaning your wire can actually carry more current than the baseline table value. There is also a special rooftop adder: raceways exposed to direct sunlight on or above a rooftop must have up to 60 degrees F added to the outdoor temperature before you apply the correction factor.
When You Need This
- Running conductors through an attic space where summer temperatures regularly exceed 120 degrees F
- Installing wiring on or above a rooftop where direct sun exposure adds significant heat
- Designing circuits for boiler rooms, commercial kitchens, or industrial ovens
- Working in northern climates where below-freezing ambient actually increases allowable ampacity
- Any project where your inspector asks you to show your ampacity derating calculations
Key Points
Common Mistakes
Skipping the temperature correction entirely because the wire insulation is rated for high temperatures
Forgetting the rooftop sun-exposure temperature adder — you must add up to 60 degrees F to the ambient before looking up the factor
Applying the correction factor from the wrong insulation-temperature column (for example, using the 60-degree factor on a THHN 90-degree conductor)
Not stacking the temperature correction on top of the conduit-fill adjustment when both apply
Exam Tip
Exam questions love rooftop scenarios. Remember: if the raceway is on a roof exposed to sunlight and less than 7/8 inch above the surface, add 60 degrees F to the outdoor temperature, then look up the correction factor for that combined temperature.
Frequently Asked Questions
Yes, if the soil or conduit environment is at a temperature other than 30 degrees C, you apply the correction factor. Underground conduit in hot climates can easily exceed 30 degrees C.
Yes. If the ambient temperature is below 30 degrees C (86 degrees F), the correction factor is above 1.0, meaning you can carry more current than the base table value. This can be helpful in cold-climate installations.
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Related Code Sections
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