517.14 Panelboard Bonding.
The equipment grounding terminal buses of the normal and essential branch-circuit panelboards serving the same individual patient care vicinity shall be connected together with an insulated continuous copper conductor not smaller than 10 AWG. Where two or more panelboards serving the same individual patient care vicinity are served from separate transfer switches on the emergency system, the equipment grounding terminal buses of those panelboards shall be connected together with an insulated continuous copper conductor not smaller than 10 AWG. This conductor shall be permitted to be broken in order to terminate on the equipment grounding terminal bus in each panelboard.
handbook commentary
Section 517.14 requires that where two or more panelboards supply the same patient vicinity and are supplied from different emergency transfer switches, the equipment grounding terminal bars must be bonded together with an insulated conductor not smaller than 10 AWG.
This is in addition to the EGC going back to the distribution equipment and is just to mitigate any ground shift along the way by bonding all panels in the patient care area together. It is similar to the bonding we do in a pool.
