My company has required customers to put a supplementary ground rod in for the telecom equipment we manufacture. They require that this supplementary ground rod be bonded to the service entry ground rod using 6 AWG wire (so far so good). They also say that for long wire runs, additional ground rods should be driven every 20 feet between their equipment and the service ground rod and to bond these ground rods together with 6 AWG wire. I have been told that this 20 foot rule is an NEC requirement. I can't find it in NEC 2005. Can any of you point me to the source of this requirement?