Mike Dolt Enterprises
Mike Dolt Enterprises
User: PaulEd, Location: United States
Asked on: Saturday, January 18, 2025 at 12:20 pm EST
Answered on: Thursday, March 6, 2025 at 7:29 am EST
Question: Having a brain (blank) here guysAccording to the national electrical if I need to derate conductors and for example, I have XHHW which is rated from 90°C, but I install it on a roof where XHHW is only rated at 75°C. (Outdoors) can I still derate from the 90° column or do I need to debate from the 75° column?
You must derate from the 75°C column in this scenario. Here's why: Termination Limitations
The NEC (National Electrical Code) specifies conductor ampacity based on termination temperature ratings. Even if the conductor itself is rated for 90°C, if it's terminated in equipment (breakers, panels, etc.) only rated for 75°C, you're bound by that lower temperature rating. Common termination temperature ratings are 60°C or 75°C.
Ambient Temperature Correction (Derating)
When ambient temperatures exceed the ambient temperature used in the NEC tables, you must apply temperature correction factors. This is essentially "derating" to account for the higher temperature.
Specific Application
You're installing the XHHW conductor outdoors on a rooftop where the temperature can be higher. The NEC recognizes that sunlight exposure and lack of airflow on rooftops can lead to higher conductor temperatures. Because of this, in many cases, XHHW can only be rated to 75°C.
How to Approach the Calculation
Determine the final ampacity
The NEC requires that you use the lower of the termination temperature or the conductor temperature rating. In your case, the lower is 75°C.2.
Derate from the 75°C column: You must derate using the ambient temperature correction factors from the 75°C column.
Key NEC Sections to Consider
NEC 310.15
This article covers conductor ampacity, ampacity tables, and correction factors.
NEC 110.14(C)
This section addresses the temperature limitations of conductors in equipment.
Important Considerations
Equipment Markings
Always check the equipment nameplates (breakers, panels, etc.) for their temperature rating. This is often printed clearly on the device.
In short: You derate from the lower of the conductor rating or the termination rating. In your case, you must derate from the 75°C column.Disclaimer: Electrical codes can be complex, and interpretations can vary. Always consult with a licensed electrical contractor or electrical engineer for specific applications to ensure compliance with local codes and safe practices. Flag for review