a multifamily building has two (1200) amp services,one service on each in of the building. each service has two ground rods (6)ft apart as the only electode to ground the services, there is no firewall to separate the building. the service entrance conductor from the power compamy to connect to the supply side of the service will be (600)kcmil. my question is what size conductor is require to bond the ground rod electrodes together to comply with section 250.58 ? there is no building steel or metal water pipe to bond to, thanks,mj
2002 NEC: "250.53(E) Supplemental Electrode Bonding Connection Size. Where the supplemental electrode is a rod, pipe, or plate electrode, that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire."
Also: "250.66 Size of Alternating-Current Grounding Electrode Conductor. The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 250.66, except as permitted in 250.66(A) through (C). FPN: See 250.24(B) for size of ac system conductor brought to service equipment. (A) Connections to Rod, Pipe, or Plate Electrodes. Where the grounding electrode conductor is connected to rod, pipe, or plate electrodes as permitted in 250.52(A)(5) or 250.52(A)(6), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire."
The ground rod is only required to be #6; regardless of the service size. The MBJ on the water pipe comes from 250.66 which is based on the size of the service entrance conductors. But the real question is how do you get 1200 amps thru a 600mcm without burning it up? Should be 3 600s in parallel (which will change your MBJ size). I've run across that setup around here. Older wood frame apts, no bldg steel. They used to treat the long buildings as two separate and feed both ends. No fire walls; only reason we knew they were separate was to see the service drops on both ends and check out the electric rooms. They stopped doing this quite awhile back. Since these buildings are older, they almost always had copper (or steel) water pipe, plastic wasn't in use yet. We bonded to that.
To size to bonding jumper from rod to rod is not clear. 250.53(c) states to use 250.66 to select the size wire to bond electrodes together. The rule of 250.66(a) is for the sole connection to the rod is not required to be larger then #6 copper. My question is : to comply with 250.58 to have a common grounding electrode , if one service is on each in of a building and the only electrodes used are ground rods, the gec from rod to rod should be selected from table 250.66 and not 250.66(a) correct me if I am wrong...
I believe you are wrong. 250.58 states what to do, not how to do it. No mention of size. Multiple rods make up a ground rod system, be it 2 rods or 10. Running the wire from rod to rod is the same as going to one rod, i.e. it's all the same system. Take this (imaginary) situation: a building has a nice new service fed by parallel 600s. It has building steel, concrete encased and ground rods as electrodes, and has a lightining system. By your rule I'd take a #6 to the rods then a 3/0 to the lightining system rod because I'm bonding the 2 systems together. What good would it do? Point 2: One of the reasons that #6 is stipulated as minimum is because the code panels (long ago) determined that the earth could only absorb so much. That quantity of current could be carried on a #6 and transferred to earth on an 8' rod given a low enough resistance (25 ohm). Granted there are some arguments to change this but it hasn't yet happened.