Ground Grid Resistance of Padmounted Transformers

[bilalk]

The NEC states that the resistance of man-made electrode shall be 25 ohms or less. Most literature out there discuss the ground resistance values in distribution substations to be under 5 ohms or better under 1 ohm. I could not find any document that discuss the ground resistance value of ground grids of outdoor padmounted transformers and/or switchgears. Does anyone know what could be a reasonable practical value to achieve when designing the ground grid of a pad mounted transformer?

 

[bacon4life]

NEC just requires a second ground if the first is more than 25 ohms, it doesn't actually require under 25 ohms. In rocky areas, even 100 ohms might require many ground rods.

http://ecmweb.com/content/who-cares-about-25-ohms-or-less