If one bonding jumper is installed at the source, what must be considered for sizing?

The correct answer focuses on the need to consider the largest sum of the areas of the corresponding conductors when sizing a bonding jumper installed at the source. This approach is crucial because the bonding jumper must be capable of carrying any fault current that could flow through it during a fault situation, ensuring proper grounding and bonding of the electrical system to maintain safety and equipment protection.

When sizing the bonding jumper, it is essential to account for the largest conductor size because this will help ensure that the jumper can handle the maximum possible fault current from all conductors being bonded. The total area of the corresponding conductors reflects the potential load and fault conditions that the bonding jumper may need to accommodate.

Other options do not properly address the requirements for sizing a bonding jumper. For example, simply looking at random conductor sizes or the smallest conductor size would not guarantee the necessary safety and performance standards, as they could lead to undersized jumpers that are inadequate for potential fault conditions. Additionally, considering only the conductor at the nearest enclosure ignores the overall system's bonding needs and may compromise the effectiveness of the grounding system. Thus, focusing on the largest sum of areas ensures comprehensive safety and compliance with the grounding and bonding requirements outlined in Article 250.