New York State Master Electrician Practice Exam

How should copper bus bars be rated for a service switchboard sized over 5000A with a 100% rating?

1. 400 ft per square inch

2. 500 ft per square inch

3. 600 ft per square inch

4. 700 ft per square inch

The correct answer is: 700 ft per square inch

When rating copper bus bars for a service switchboard sized over 5000A, the correct approach involves determining the ampacity based on several factors, including the installation environment, the temperature rating, and the specific application requirements. For a service switchboard with a 100% rating, the ratings provided by the NEC must be closely followed. Copper has a well-established ampacity rating, and for a switchboard of this size, a higher rating is required to maintain efficiency and safety standards. The choice of 700 ft per square inch aligns with industry standards and gives an appropriate allowance for the heat generated at such high currents. This rating also provides a margin to accommodate potential current spikes, ensuring the bus bars can handle the continuous load without overheating or suffering damage. Using a lower rating might not meet the necessary safety and operational guidelines, which could lead to overheating and raise significant risk factors for both the equipment and personnel. Overall, selecting the 700 ft per square inch rating reflects a thorough understanding of the requirements for high-demand electrical systems and the appropriate safety margins needed in electrical design and installation practices.