What Other Conductors Can Be Ran With Service Conductors Per Nec
Grounding Service Equipment
[ Please Note: No illustrations are included in this article ]
Because utilities provide grounded ac services and almost facilities have at least one utility service connection, a grounded air-conditioning service probably provides power to your bounds wiring organisation. When yous have a grounded air-conditioning service, your bounds wiring system must have a grounding electrode conductor connected to the grounded service usher [250.24(A)].
This brings up the question of how to comply with grounding electrode usher requirements. Because a grounding electrode usher must connect the grounded conductor to the grounding (earthing) electrode, the question of how expands to include where. Can you make this connection just whatever identify?
Location, location, location
Some inspectors require the grounding electrode conductor to terminate to the grounded conductor concluding at the meter enclosure. Other inspectors require the grounding electrode conductor to terminate to the grounded final at the service disconnect.
The Code says y'all tin brand this connection at any accessible location-from the load end of the service driblet or service lateral, upwardly to and including the service disconnecting means [250.24(A)(one)]. See Figure 250-53. The choice then becomes an engineering conclusion that balances such factors as installation costs, available infinite, and maintenance bug.
Connections
In this day of increased demand for uninterrupted power, many facilities are dual-fed. This ways they have separate lines coming to the aforementioned service-nosotros refer to such services equally "double-concluded." If the dual feeds are in a common enclosure (or grouped together in divide enclosures) and they employ a secondary tie, y'all can utilise a single grounding electrode connectedness to the necktie point of the grounded conductors from each power source [250.24(A)(3)].
Whether your service is double-ended or non, you must install an unspliced main bonding jumper between the grounded last and the metal parts of the service disconnecting means enclosure. Ensure the bonding jumper complies with 250.28 and the installation complies with 250.24(C).
Your main bonding jumper is probably wire or busbar. Permit's say it is. And, permit's say y'all've installed this jumper from the grounded conductor terminal (or bus) to the equipment grounding terminal (or bus) in the service equipment. In this example, the NEC allows you to connect the grounding electrode to the same equipment grounding terminal (or motorbus or bar) to which you lot connected the primary bonding jumper [250.24(A)(4)].
My name is Bond. Load (side) Bond.
A load side neutral-footing bond is a common cause of power quality problems. Such a bond creates basis loops, which permit undesired current to circulate in the system. Power quality problems often atomic number 82 to the discovery and removal of such a bond. Merely, don't look for power quality problems to reveal the bond. Another business makes cosmetic activeness imperative. Load side neutral-ground bonds allow objectionable current to menses on conductive metallic parts of electrical equipment-thereby violating 250.6(A). This objectionable current can cause lethal electric daze (Effigy 250-55). And, it sets the phase for inadvertent flashovers, overheating of equipment, and other problems stemming from electricity in the wrong place.
So, don't make (or permit) a neutral-ground connection on the load side of the service disconnect [250.24(A)(v)]. There are two exceptions to this [250.142]. You can make this connection (Figure 250-five) for:
Separately derived systems if you follow the requirements of 250.30(A)(ane).
Divide buildings, if you lot follow the requirements of 250.32(B)(2).
Grounded Conductor
Electric utilities typically do non provide an equipment grounding (bonding) conductor to service equipment, and aren't required to do so. Thus, y'all must run a grounded conductor from the electric utility transformer to each service disconnecting means [250.24(B) and 250.130(A)]. See Figures 250-56 and 250-57.
Why is this important? Considering the grounded service usher provides the effective basis-fault current path to the ability source winding. This path ensures that opening the excursion protection device volition rapidly remove dangerous ground fault voltage from the circuit [250.4(A)(iii) and 250.four(A)(v)]. See Figure 250-58
Don't try to use the earth as a bonding jumper. The resistance of the earth is always too peachy for the earth to be an effective bonding jumper. Very lilliputian error current returns to the power source winding if earth is the only mistake-current return path.
Merely, permit's suppose the globe is your only mistake-current return path. What would be the consequences? For i affair, the excursion overcurrent protection device will not open up and clear the footing fault. Consequently, metal parts such every bit metal pipage and structural building steel will become-and remain-energized to circuit voltage (Figure 250-59 and/or Figure 250-24C2 02 250-28.cdr). The organization then poses a loftier risk of shock, arc blast, and fire.
If you are and so inclined, yous can combine phasor or polar form network assay techniques with Kirchoff'south Constabulary and Ohm'southward Law to quantify how much hazard such a configuration imposes at a given betoken. You can mathematically decide, for example, the voltage on a metal enclosure due to an open service grounded usher. Forensic engineers often creepo out these kinds of numbers when investigating why someone died or why a facility blew up. Information technology's easier just to comply with NEC requirements-to eliminate such a voltage in the first place.
So, you encounter the need for a grounded conductor-just how large should it be? Remember, this grounded service usher serves as the effective ground-fault electric current path. Thus, y'all must size information technology so it tin safely carry the maximum mistake current likely to be imposed on information technology [110.10 and 250.four(A)(5)]. To attain that, size the grounded usher per Table 250.66-based on the full area of the largest ungrounded conductor (Figure 250-lx). The grounded usher must as well have the capacity to deport the maximum unbalanced current, per 220.61.
Examination your understanding with a quick quiz. Here'due south the question:
What is the minimum size grounded service conductor required for a 480V, three-phase service, where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A (Effigy 250-61)?
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG
At present, here'due south the explanation. The unbalanced load requires a three AWG grounded service conductor-rated for 100A at 75?C per Table 310.16 [220.61]. However, the grounded service conductor cannot be smaller than ane/0 AWG (Tabular array 250.66). This minimum size requirement ensures the conductor will accommodate the maximum mistake current likely to be imposed on it. Thus, the reply is (d) ane/0 AWG.
What if you parallel your service conductors? Does that mean y'all employ but the one conductor, or practise you parallel your grounded conductor the way yous parallel the current-conveying conductors? Answer: No to both.
First, you must install a grounded conductor in each raceway whenever you parallel your service conductors.
Second, yous cannot simply divide your grounded usher into two smaller equal conductors. Yes, doing then would satisfy the requirement that the grounded conductor must accept the capacity to carry the maximum unbalanced electric current per 220.61. But, information technology could also effect in a grounded conductor that is too small for a given raceway.
To eliminate such a trouble, size each grounded conductor per Table 250.66-based on the full surface area of the largest ungrounded conductor in the raceway. Note that regardless of the number you come up with, the grounded usher in each parallel service raceway tin can never be less than i/0 AWG [310.four].
Let's review this with a quick quiz. Here's the question:
What is the minimum size grounded service usher required for a 480V, three-stage service installed in two raceways, where the ungrounded service conductors in each of the raceways is 350 kcmil and the maximum unbalanced load is 100A (Figure 250-62)?
(a) 3 AWG (b) two AWG (c) i AWG (d) 1/0 AWG
Now, here'southward the explanation:
The unbalanced load requires merely a 3 AWG grounded service usher, per Table 310.16 [220.61]. Nevertheless, the grounded service usher in each raceway cannot exist smaller than 1 AWG (Tabular array 250.66). As earlier, this is to ensure it will suit the maximum fault electric current probable to be imposed on information technology. But, ungrounded service conductors run in parallel cannot exist smaller than i/0 AWG. Thus, the answer is (d) 1/0 AWG per raceway.
Properly grounding and bonding service equipment improves condom while eliminating a mutual cause of power quality problems. You just have to make the right connections in the right places. If you think in terms of providing a low impedance ground-fault path back to the source, you will accept no problem doing then.
| 2005 Grounding versus Bonding Textbook — 2005 | ||||
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