ICC Residential Electrical Inspectors Practice Exam

For ampacity adjustments, which temperature rating can be used for NM cable?

1. 60 degrees Celsius

2. 75 degrees Celsius

3. 90 degrees Celsius

4. 100 degrees Celsius

The correct answer is: 90 degrees Celsius

The correct choice regarding the temperature rating that can be used for NM (non-metallic) cable is 90 degrees Celsius. This rating is significant because it allows for higher ampacity, meaning that NM cable can carry more current without overheating at elevated temperatures. In practice, when working with NM cable, the ampacity must be adjusted based on the temperature rating of the conductors and the conditions under which the cable operates. The National Electrical Code (NEC) allows NM cable to be rated at 90 degrees Celsius for ampacity calculations, which means that when determining the allowable current-carrying capacity, one can use the ampacity values associated with that higher temperature rating. The other temperature ratings mentioned, such as 60 degrees Celsius and 75 degrees Celsius, are commonly seen in other types of cables, but they do not apply to NM cable in terms of the highest permissible ampacity. The 90 degrees Celsius rating allows for more flexibility in wiring configurations and enables compliance with the required electrical loads in residential settings. Thus, utilizing the 90 degrees Celsius temperature rating is essential for ensuring safety and efficacy when adjusting ampacity for NM cables.