I do it the hard way. Especially in commercial, where we often use parallel sets of conductors for a lateral service. Find the cross-sectional area times how many parallel runs (say, 6 * 600 kcm) and whatever that equivalent is, you can find it in 250.66. 1100 kcm is the largest size listed with 3/0 for a GEC. If the total equivalent size is over 1100 kcm, you can size the GEC at 12.5% of the hot. Say, the total was greater than the cross-sectional area of 1100. Find 12.5 % of that and use the table to find what wire size matches that figure and that's your GEC.
Around here, in a house, cities want nothing less than #4 for cold water ground and you can use #6 ground to a ground rod or UFER for a typical 200 amp service. Some cities require at least a 5/8 ground rod, I normally just buy a 3/4. That never fails inspection.
Also, all bonding must take place at the first means of disconnect, whether that is a disconnect switch or the panel itself. That means that all grounding conductors meet at the point where neutral is bonded to ground. This is to insure an effect ground fault current path that allows a grounding current to find it's way back to neutral, back through the feeding transformer and back through the suppling breaker at a high enough current to make it trip. Otherwise, the breaker wouldn't trip and something or someone would continue to get electrocuted and the effect of grounding thing to another rather than back that system bonding point creates alternate paths for ground fault current, sometimes referred to as a ground loop.
[This message has been edited by rws (edited 12-12-2006).]
