When using multiple ground rods, the spacing should be at least the sum of their driven lengths. How far apart should two 10-foot ground rods be?

The correct answer is based on the principle that when using multiple ground rods, the spacing between them should be at least equal to the sum of their driven lengths. In this case, each ground rod is 10 feet long.

To determine the minimum distance between the two ground rods, you would add their lengths together: 10 feet + 10 feet equals 20 feet. This ensures effective grounding because it minimizes potential resistance and improves the overall performance of the grounding system by allowing for a better dissipation of lightning energy.

The requirement for spacing is critical in lightning protection design as it helps to prevent ground potential rise and ensures a more uniform grounding system. Thus, placing the two 10-foot ground rods 20 feet apart aligns perfectly with this guideline, making it the correct choice.