400 amp service requires 500 mcm =380 amps at 75%. go to 250.66 a 500 requires a 1/0 copper grounding electrode conductor. you can run a 1/0 to the ist panel then (TAP)a #4 from (OUTSIDE) the panel to the second panel.You cannot Splice (unless Cadweld or Listed compression ) and going in one panel to the Neutral bar then to the other would be like a splice , But you can Tap.(250.64d)Page 214 of the NEC 2005 Handbook shows a nice detail of this.Since a #6 copper) per 250.66(A) is the largest size wire required to a Ground rod no matter what the size service. I feel you could run one #6 from the Ground Rod or Rods to the Water line (Detail on page 215 NEC Code Handbook) and you;d be done.All of theses are also shown well in Soares Grounding book.
As long as the neutrals between the panels and the meter are 1/0 copper or 3/0 aluminum or larger, you don't need to run a ground tap between panels (but you can if you want to).
Just run a 1/0 copper (or maybe a #2 copper -- depends on the size of service entrance conductors actually used) from one panel to the water pipe. Ideally, from that same panel, run a #6 copper to your first ground rod and then a #6 bonding jumper from that rod to the next.
[This message has been edited by markp (edited 03-15-2006).]
Trick440, what you are doing, is kinda allowed depending on how you look at it... This has been argued here before, contensiously.....
It is allowed by 250.64D depending on how you look at it... The way I see it there should only be one conductor, and full sized on the electrode, unless both are exothermically welded to it. As it says the conductor shoud be "without a splice". Otherwise one could be removed, and you would have a single 1/2 sized conductor serving as the GEC. 2 connections at the rod or water serve as a splice in what should be a single full sized conductor IMO. It says conductor, singlular....
Mark Heller "Well - I oughta....." -Jackie Gleason
I think not having all those smaller parallel grounds makes more sense when you realize you could connect your ground electrode in the meter can (if the POCO permits that) or anyplace along the service neutral (at the weatherhead even). NEC 250.142 allows the service neutral to be the wire you ground all your service equipment with as long as the neutral is sized per 250.66 or larger.
Having ground taps clouds the issue for me, but you would need to do that if you have a panel with no neutral.