I've searched the forum and haven't found an answer for this exact situation. Here's the configuration: Main 200A service panel at one end of house (exterior), and 100A sub panel at opposite end of house (interior access on exterior wall). EMT running in crawlspace to connect panels. Both panels in one building. I know that the ground bus and neutral bus are not bonded at the sub panel, and their respective conductors run back to the main service panel.
Question: What are the thoughts on adding additional grounding electrodes at the sub panel side of the house, and connecting those to the sup panel's ground bus (again, not bonded to neutral) as an additional redundant ground? Does this increase grounding safety, grounding performance? Does it interfere with the purpose of the ground back to the main panel? Is it a waste of money, since the main service panel has its own grounding electrodes? Most of all, what does the NEC say, if anything, on this situation?