My reading of 2005NEC 250.32 is:

If only a single branch ckt (or multiwire branch ckt) is run to the
building, and the circuit includes a ground wire, a ground rod is not
required at the building. In all other cases (including the feeder
described) ground rod(s) or other grounding electrode is required at the
building.

There are then 2 options :
1. A grounding conductor IS run to the building: the grounding conductor
is connected to a ground bar and ground rod. The neutral bar is not
connected to the panel ground. There is a parallel path with (neutral)
and (ground electrodes at both ends with earth) but the earth path will
have a significantly higher resistance.

2. A grounding conductor IS NOT run to the building: the neutral is
connected (through a main bonding jumper) to the ground system and
ground rod. This is like a service. There can be no other continuous
metal paths that are bonded the ground conductor at both ends, and the
feeder shall not be fed with a GFCI (which would cause an immediate trip).

(I have seen electricians argue over this section.)

bud--


wrote:

If I am wiring a 100 amp subpanel in my detached garage with 240 using
2-2-2-4 AL, I am carrying the ground from my house's main panel (60 amp
breaker). Does NEC require that I have a grounding rod at my detached
garage? If that is the case, should I only be running 2-2-2AL?
thanks. Any arguments for using copper? The AL seems like it may be
overrated, but cheaper in the end.