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Remote Buildings and Structures
We inadvertently got a little out of sync with this article series and sent Part 7 of 12 out before Parts 5 and 6, so here is Part 5 of 12. We will send Part 6 of 12 out in April and then 7 of 12 out again after that to get us back on track. If you want to see all the articles in this series, use this link and scroll down the page to the first article: Grounding versus Bonding Part 1 of 12. Sorry for the confusion.
By Mike Holt for EC&M Magazine
What's the right way to ground and bond buildings or structures supplied by feeders or branch circuits?
That little outbuilding was a badly-needed addition. But now you must supply power to it from the service of another building. You've sized the disconnecting means (subpanel) and breakers correctly, but you are unsure about the grounding and bonding. Crossing your mind are questions like these:
To answer those questions, we must first ask "what purpose would grounding and bonding serve?" Electricity is always trying to get back to its source-that's how an electric circuit works. We bond metal parts together so that we provide a low impedance path for fault current back to its source. When we ground (earth) to a grounding electrode, we are attempting to provide a low-impedance path to the earth for lightning.
The benefits of proper bonding include clearing faults, preventing shock and reducing fires. Grounding provides a low-impedance path for lightning. Properly grounded systems operate as intended. Improperly grounded systems create a fertile ground for power quality problems.
So, how do we provide these benefits to a building or structure that does not have its own service? The answer lies in complying with 250.32, and for that purpose we first turn our attention to the 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 from lightning [250.4(A)(1)]. See Figure 250-85
NOTE: Figures [illustrations] are not included in this newsletter.
Keep in mind that grounding does not:
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 (Figure 250-32Ax CC250-12.cdr).
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). See Figure 250-87.
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 is 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
Now, here's a caution. Don't bond the grounded conductor (neutral) to the disconnecting means or to the building. Doing this violates 250.6(A), because the resulting ground loop (parallel neutral current paths) will allow dangerous objectionable current to flow onto metal parts of the electrical installation (and onto as metal piping and structural steel). See Figures 250-88 and 250-89.
What if there's no equipment grounding (bonding) conductor 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 only 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:
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 poses potentially dangerous consequences.
Even if the initial installation doesn't result in dangerous objectionable current on metal parts, there remains the possibility that a future installation of metal piping or cables between the buildings or structures could create unwanted parallel neutral current paths (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, but to install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure in accordance with 250.32(B)(1).
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.
Can you answer this practice question? What size grounding (earthing) electrode conductor is required for a building disconnect that is supplied with 3/0 AWG? See Figure 250-92.
The answer is (a) 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, we'd look at Table 250.66. The answer, in this case, would be 1/0.
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." In the 2004 edition, you can sum up Chapter 2 by simply saying, "Refer to the NEC." Much of what you need to know is in Article 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 us 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)]:
Such electrodes are usually available, but not always. For example, you might not have access to a metal underground water pipe anywhere near your installation. Or, you would 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 one or more of the following, instead:
Now you have answers to those questions that were crossing your mind when you were trying to figure out the grounding and bonding for that little outbuilding. You now 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 is to think what purpose grounding and bonding serve, and what kind of a path you are providing for the electricity. Remember that electrons leaving a power supply are trying to get back to its 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 path for lightning.
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