Probable Effect Concentration Quotients (PECQS) Practice Test

For a THWN copper conductor rated at 30 A in free air, what happens to its ampacity when it is buried?

1. It remains the same

2. It decreases

3. It increases

4. It doubles

The correct answer is: It decreases

When a THWN copper conductor rated at 30 A is buried, its ampacity decreases due to several factors related to the thermal characteristics of the surrounding environment. In free air, the conductor has the benefit of being able to dissipate heat efficiently into the surrounding air, allowing it to carry its full rated current without overheating. However, when buried, the conductor is surrounded by soil, which has a much lower thermal conductivity than air. This results in reduced heat dissipation, as the soil retains heat more effectively and limits the ability of the conductor to cool down. Consequently, the temperature of the conductor rises more quickly when it is carrying current, potentially reaching unsafe operating conditions. To ensure safety and reliability, electrical codes and standards account for this decrease in ampacity when conductors are buried, typically providing a lower ampacity rating for the same conductor compared to when it is installed in free air. Thus, the ampacity of a THWN copper conductor decreases when it is buried.