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All questions and answers are based on the 2020 NEC.
Q. What are the Code rules for ambient temperature correction to conductor ampacity?
A. Ampacities for ambient temperatures other than those shown in the ampacity tables must be corrected in accordance with Table 310.15(B)(1).
The conductor ampacities contained in Table 310.16 are based on the condition where the ambient temperature is between 78°F and 86°F and no more than three current-carrying conductors are bundled together; such ampacity must be corrected in accordance with Table 310.15(B)(1) if the ambient temperature is lower than 78°C or greater than 86°C (Figure).
Example No. 1 – ambient temperature below 30°C (86°F)
What is the ampacity of a 12 THHN conductor when installed in an ambient temperature of 50°F?
Solution: Corrected Conductor Ampacity = Table 310.16 Ampacity × Ambient Temperature Correction Factor from Table 310.15(B)(1)
The conductor ampacity for 12 THHN is 30A at 90°C [Table 310.16].
The correction factor for a 90°C conductor installed in an ambient temperature of 50°F is 1.15 [Table 310.15(B)(1)].
Corrected Ampacity = 30A × 115% = 34.50A; round to 35A
Note: Ampacity increases when the ambient temperature is less than 30°C (86°F).
Example No. 2 – ambient temperature above 30°C (86°F)What is the ampacity of a 6 THWN-2 conductor installed in an ambient temperature of 50°C?
Solution: The conductor ampacity for 6 THWN-2 is 75A at 90°C [Table 310.16].
The correction factor for a 90°C conductor installed in an ambient temperature of 50°C is 0.82 [Table 310.15(B)(1)].
Corrected ampacity = 75A × 82% = 61.50A; round to 62A
Where raceways or cables are exposed to direct sunlight and located less than 7⁄8 in. above the roof, a temperature adder of 60°F (33°C) must be added to the outdoor ambient temperature to determine the ambient temperature correction in accordance with Table 310.15(B)(1) [Sec. 310.15(B)(2)]. The reason for the temperature adder is because the air inside raceways and cables that are in direct sunlight is significantly hotter than the surrounding air.
Example No. 3
What is the ampacity of 6 THWN-2 in a raceway located ½ in. above the roof, where the ambient temperature is 90°F?
Solution: Corrected Temperature = 90°F + 60°F adder = 150°F [Sec. 310.15(B)(2)]
The temperature correction factor for 150°F = 0.58 [Table 310.15(B)(1)]
6 THWN-2 is rated 75A at 90°C [Table 310.16]
Corrected Ampacity = 75A × 58% = 43.50A, round to 44A
Exception: Type XHHW-2 insulated conductors are not subject to the rooftop temperature adder.
Informational Note 1: See the ASHRAE Handbook—Fundamentals (www.ashrae.org) as a source for the ambient temperatures in various locations.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
