As a general rule, you should never have two ground bonds of any kind at different places connected together.
The obvious apparent exception is at the entry panel, where the ground is bonded to both the neutral and the system earth ground. Note that this is done at a single location, and that is significant.
As stated, if you run a ground conductor from the main panel to the sub panel, you must not bond the neutral and ground at the sub panel. Doing so will split the neutral return current between the neutral and ground conductors. Besides causing there to be continuous current on the ground, this arrangement is potentially disastrous /forums/images/graemlins/shocked.gif- if the neutral conductor were disconnected, all of the neutral return current would flow through the ground conductor, which is typically not sized for the this capacity.
Also, if you have two earth grounds at different locations connected together by a ground conductor path, you will see stray currents due to variation in the grounding potential variations between the two earth grounds. This is particularly true if the entry panel earth ground and the sub panel earth grounds are of different types, or installed in different soil types. (I had an amusing experience where the GFI outlets in an outbuilding would trip whenever the automatic sprinklers would come on. The moisture would dramatically the efficiency of the local ground rod, causing enough stray current through an otherwise extremely effective grounding system. I will admit here and now that it was my own doing, /forums/images/graemlins/blush.gif and defend myself only by saying I was much younger, and a different kind of foolish, than I am now. /forums/images/graemlins/tongue.gif )
Again, as stated, if you don’t run a ground conductor from the entry panel to the sub panel, then you have to install a separate system earth ground for the sub panel and ground the neutral to the ground at the sub panel. At that point, the sub panel is now configured in the same manner as an entry panel with respect to grounding and neutral bonding.
Given the apparent close physical proximity of the entry panel and sub panel, and assuming that the outlets, fixtures, and devices fed from the sub panel will be in close proximity with those fed from the entry panel, a ground conductor should be run from the entry panel to the sub panel, and there should be no separate earth ground or neutral bond at the sub panel. This will avoid the possibility that the two grounds, potentially with different potential, will come in contact.
One of the big problems in trying to apply any commercial building techniques, be they electrical or structural, is that residential inspectors are often unfamiliar with anything other than the plain vanilla tract home methods. On the one hand they will reject extremely well thought out, well engineered applications simply because they haven't seen it before. Even worse they cannot be counted on to catch errors, their primary role, if they do not understand what is being done.
If you are going to do anything more than install a breaker and a couple of outlets, I would recommend getting a copy of the NEC or one of the annotated guides on the code. It is far too easy to kill someone or burn down your house with a well-intentioned misconnection. (Or well meaning advice from some guy on the internet. /forums/images/graemlins/cool.gif)
