NEC and IEEE references Grounding Rods in triangular configuration

Location
Houston
Occupation
Engineer
On a grounding restating rod system, what are the specific sections in the NEC (National Electrical Code) and IEEE standards that address the following:
  1. The required distance between grounding rods when installed in a triangular configuration,
  2. The recommended or required distance between the triangular grounding rod configuration and the generator sets (gensets).
I am looking for the exact NEC and IEEE references that govern these distances.
 
As Ron stated the NEC requires a minimum of 6' between rods but then give you an IN telling you that you would have a more effective GES if you doubled the length of the rod between them.

250.53(A)(2)Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.
 
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