All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.
What little outbuilding was a badly-needed addition, but now you need to supply power to it from the service of another building. You've sized the disconnecting means (subpanel) and breakers correctly, but you're unsure about the grounding. If you're smart, you'll ask yourself the following questions:
What are the grounding and bonding requirements for that subpanel?
What are the grounding and bonding requirements for the building itself?
Do you ground and bond the equipment in this building any differently based on the fact that the building disconnect is powered from the service of another building?
The benefits of proper bonding include clearing faults, preventing shock, and reducing fires. Grounding provides a low-impedance path for lighting, and properly grounded systems operate as intended. Improperly grounded systems create fertile ground for power quality problems.
So how do you provide these benefits to a building or structure that doesn't have its own service? The answer lies in complying with 250.32, and for that purpose let's first look at the disconnecting means.
Disconnecting means. The purpose of grounding (earthing) the building or structure disconnecting means to the earth is to limit elevated voltages on the metal parts caused by lightning [250.4(A)(1)] (Fig. 1). Keep in mind that grounding doesn't:
Provide a low-impedance fault-current path to clear ground faults. In fact, the Code prohibits the use of the earth as an effective ground-fault current path since it's such a poor conductor of current [250.4(A)(5) and 250.45(B)(4)].
Protect electrical or electronic equipment from lightning voltage transients.
You don't need a grounding (earthing) electrode where only one branch circuit serves the building or structure [250.32(A)]. For the purposes of 250.32(A), you can consider a multiwire branch circuit to be a single branch circuit.
To quickly clear a ground fault and remove dangerous voltage from metal parts, the building or structure disconnecting means must be bonded to an effective ground-fault current path [250.4(A)(3)]. To establish this path when your disconnecting means is supplied by the service of another building, your installation must comply with either 250.32(B)(1) or 250.32(B)(2) (Fig. 2).
Equipment grounding (bonding) conductor. You can bond the building or structure disconnecting means to an equipment-grounding (bonding) conductor (as described in 250.118) that's installed with the circuit conductors. Size this equipment-grounding (bonding) conductor per 250.122, based on the rating of the feeder protection device [250.32(B)(1)].
Grounded conductor neutral. Be sure not to bond the grounded conductor (neutral) to the disconnecting means or to the building because doing this violates 250.6(A), and the resulting ground loop (parallel neutral current paths) will allow dangerous objectionable current to flow onto metal parts of the electrical installation (and onto metal piping and structural steel) (Fig. 3 and Fig. 4).
What if an equipment grounding (bonding) conductor isn't run to the building or structure disconnecting means? In such a case, you can bond the disconnecting means to the circuit grounded conductor (neutral). But this is only permitted where there's no continuous metallic path between buildings and structures and ground-fault protection of equipment isn't installed.
Neutral as effective ground path. Where the grounded conductor (neutral) serves as the effective ground-fault current path, you must size it no smaller than the larger of:
The maximum unbalanced neutral load, per 220.61.
The available fault current, per 250.122.
But maybe the size of this conductor should be your last concern because using the grounded conductor (neutral) as the effective ground-fault current path can have dangerous consequences.
Even if the initial installation doesn't result in dangerous objectionable current on metal parts, the possibility remains for a future installation of metal piping or cables between the buildings or structures that could create ground loops. Thus, you should allow this only after careful consideration, and only as a last resort.
The preferred practice is to not use the grounded conductor (neutral) as the effective ground-fault current path. Instead, you should install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure in accordance with 250.32(B)(1).
Grounding electrode conductor. The grounding (earthing) electrode conductor (GEC) for a separate building or structure disconnecting means must terminate to the grounding terminal of the disconnecting means. And you must size it per Table 250.66, based on the largest ungrounded feeder conductor.
Try the following practice question to reinforce these concepts. What size grounding (earthing) electrode conductor is required for a building disconnect that's supplied with 3/0 AWG?
The answer is 4 AWG. If you look at Table 250.66, you'll see this clearly. And what if you supplied the disconnect with 500 kcmil cable? Then once again, you'd look at Table 250.66. The answer, in this case, would be 1/0. However:
Where the GEC is connected to a ground rod (or two, in a typical installation), that portion of the conductor that's the sole connection to the ground rod isn't required to be larger than 6 AWG copper [250.66(A)].
Where the GEC is connected to a concrete-encased electrode, that portion of the conductor that's the sole connection to the concrete-encased electrode isn't required to be larger than 4 AWG copper [250.66(B)].
Lightning. With its vast power and potential for destruction, lightning demands special attention. This is one reason for the existence of NFPA 780 “Standard for the Installation of Lightning Protection Systems.” You can sum up Chapter 2 of the 2004 edition by simply saying, “Refer to the NEC.” Much of what you need to know is in Art. 250. Another applicable standard is IEEE-142 “Grounding of Industrial and Commercial Power Systems” — also known as “the Green Book.”
Lightning is a high-frequency, multi-million volt electrical discharge that obligates you to provide a path to the earth so it can dissipate there rather than damage property or endanger people. To provide this path, you must ground (earth) the building or structure disconnecting means to one of the following electrodes [250.50 and 250.52(A)]:
Metal underground water pipe [250.52(A)(1)]
Metal frame of the building or structure electrode [250.52(A)(2)]
Concrete-encased grounding (earthing) electrode [250.52(A)(3)]
Ground ring [250.52(A)(4)]
Although such electrodes are usually available, that's not always the case. For example, you might not have access to a metal underground water pipe anywhere near your installation. Or you might need to do extensive damage to finished surfaces to access one of these electrodes. So what are you supposed to do? Fortunately, the NEC makes an allowance for this. Where you don't have access to one of these other grounding (earthing) electrodes — or they simply don't exist on your site — use a ground rod [250.52(A)(5)] or a metal underground systems electrode [250.52(A)(7)]
Now you have answers to those questions you had when you were trying to figure out the grounding and bonding for that little outbuilding. And you should understand how to properly ground and bond any structure when the power for that structure comes from the service of a separate building or structure.
The key to keeping a clear head about this concept is to think about the purpose grounding and bonding serves, and what kind of a path you're providing for the electricity. Remember that electrons that leave a power supply are trying to get back to their source, not back to the earth. However, lightning is trying to get to earth. Drawing the return path on paper will help you see if your installation plan allows for what electricity is trying to do. Ensuring your installation conforms to 250.32 will allow it to clear faults, prevent shock, and provide a safe path for lightning.