The bonding jumper from xo to the frame of the transformer is an equipment bonding jumper. The system bonding jumper is the wire from xo to the equipment grounding bar in the panel shown. 250.30A1 states that an unspliced SBJ must be installed between the equipment grounding conductors of the sep. derived system and the grounded conductor. The equipment grounding conductors originate at the first overcurrent device. Any grounding/bonding conductor on the supply side of this overcurrent device is always an equipment bonding jumper not an equipment grounding conductor. One end of the SBJ will always connect to the grounding bar in the enclosure with the first overcurrent device as this is the source of the equipment grounding conductors. See NEC Handbook 2005 or 2008 exhibit 250.13 and exhibit 250.14 for correct connections (in article 250.30) thanks - Harry

Why do you ask for the difference betwwen a 'derived system' and 'separately derived system?'

In your figure 100-1, there does not seem to be a grounding wire on the primary side. I know this is the way it is done but why?

Just wanted to add a few more comments related to this issue:

1. The system bonding jumper should be sized based on Table 250.66 vs. Table 250.122; and, the size is not limited to a max of 3/0 AWG as it is for a grounding electrode conductor.

2. The system bonding jumper is larger (normally) than an equipment grounding conductor since it must carry the return fault current of the unprotected phase conductors between the source (e.g., transformer secondary) and the first means of disconnect/OCDP on the SECONDARY, sufficiently long and without failure, to allow the OCPD on the PRIMARY side of the source to open and clear the fault.