The NEC says you can use the grounded neutral conductor to ground the noncurrent-carrying metal parts of frames and enclosures making up an electrical system. As we discussed in Part 1 and 2 of this series, you must sizethe grounded conductor (when used as an equipment-grounding conductor and a current-carrying neutral) large enough to provide a low-impedance path for ground related fault-current to return over it and cause the circuit breakers or fuses to operate and de-energize the feeder-circuit in the event of a fault. However, when each of two or more buildings has a grounded service fed from a main AC service, you must separately ground the electrical supply to each building.
For example, if you run the electrical supply to one of the buildings, with a feeder used to serve another building from the main building, you must ground the feeder neutral conductor to an electrode at the other building, except: 1) When there is only one branch-circuit run to the "other" building, and an equipment grounding conductor is run with this circuit; and 2) When an equipment-grounding conductor is run with the feeder-circuit for grounding purposes.
Grounded neutral conductors serving another building from a main building: Sec. 250-32(b)(2). When a common AC grounded service supplies one or more buildings/structures, you must separately ground each panelboard at each building or structure. You must ground the grounded neutral conductor at the panelboard in the second building/structure, and size it based on the rating or setting of automatic overcurrent protection device (OCPD) in the feeder circuit, as specified in Table 250-122. As per Sec. 225-33(a), the disconnecting means for each supply shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, a group of separate enclosures, or in or on a switchboard. If necessary, Sec. 225-32 and 250-32(d) allows you to locate the disconnecting means elsewhere on the premises.
Grounded conductor used as a neutral and equipment-grounding conductor: Sec. 250-32(b)(2). When you use a feeder-circuit from the service equipment in the main building to serve a panelboard in another building, special requirements apply to the grounded neutral conductor. This is true if you use the grounded conductor as a neutral plus an equipment-grounding conductor. The same rules apply to the second/third buildings, or any buildings installed on the premises and supplied from one common service by a feeder-circuit.
When one service supplies two or more buildings or structures, treat the conductors leaving the first building as a feeder. When they arrive at the second building, treat these conductors as service-entrance conductors and terminate as in the service equipment.
Since the feeder is grounded when it arrives at the second building, you can treat the equipment it serves as a service. This feeder circuit must have the grounded neutral conductor run inside the same raceway or cable, containing the ungrounded phase conductors. Size this conductor based on the size of the OCPD in the main building ahead of the feeder-circuit conductors supplying building two. If you use metal conduit, it must not connect in parallel with the grounded neutral conductor in one building so currents can flow from building to building.
Bonding jumpers used as equipment bonding jumpers: Secs. 250-32(b)(2) and 250-102(d). You must size the equipment-bonding jumper on the load side of the service overcurrent devices in accordance with the requirements in Table 250-122. The equipment bonding jumper is not required to be larger than the circuit conductors supplying the equipment, and not smaller than No. 14. Note that a single common continuous equipment bonding jumper shall be permitted to bond two or more raceways or cables, where such bonding jumper is sized by the provisions listed in Table 250-122.
Grounding conductor used to ground equipment: Sec. 250-32(f). Size the grounding conductor according to Table 250-122, and install in accordance with Secs. 250-92, 260-64(a) and (b), and 250-120(a). If you use a metal conduit to protect a grounding conductor, you must bond it to the conductor at both ends. The part of the conductor that is the only connection between the electrode and the grounded or grounding conductoror the metal enclosure of the disconnecting means does not have to be larger than required per Table 250-122.