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2011 Changes to the NEC – 250.30
The following is an instructional page from our 2011 Changes to the NEC Textbook/DVD Package complete with graphics and video where applicable. As part of our on-going effort to provide free resources to help the industry, we will be sending this content as part of a series of newsletters. Each newsletter will feature pages taken directly from our textbooks. This can be a great training resource for your organization!
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250.30 Grounding Separately Derived Systems
This section has been reorganized and includes many revisions and Notes to clarify the grounding and bonding requirements of separately derived systems.
250.30 Separately Derived Systems—Grounding and Bonding.
Note 1: An alternate alternating-current power source such as an on-site generator isn’t a separately derived system if the neutral conductor is solidly interconnected to a service-supplied system neutral conductor. An example would be a generator provided with a transfer switch that includes a neutral conductor that’s not switched. Figure 250–28
Figure 250-28 (Click on image to enlarge)
Author’s Comments:
- According to Article 100, a separately derived system is a wiring system whose power is derived from a source where there’s no direct electrical connection to the supply conductors of another system.
- Transformers are considered separately derived when the primary conductors have no direct electrical connection from circuit conductors of one sys-tem to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors. Figure 250–29
Figure 250-29 (Click on image to enlarge)
- A generator having transfer equipment that switches the neutral conductor, or one that has no neutral conductor at all, is a separately derived system and must be grounded and bonded in accordance with 250.30(A). Figure 250–30
Figure 250-30 (Click on image to enlarge)
Note 2: See 445.13 for the minimum size neutral conductors necessary to carry fault current.
(A) Grounded Systems. Separately derived systems must be grounded and bonded in accordance with (A)(1) through (A)(8).
A neutral-to-case connection must not be made on the load side of the system bonding jumper, except as permitted by 250.142(B).
CAUTION: Dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment as well as metal piping and structural steel, in violation of 250.6(A), if more than one system bonding jumper is installed, or if it’s not located where the grounding electrode conductor terminates to the neutral conductor. Figure 250–31
Figure 250-31 (Click on image to enlarge)
(1) System Bonding Jumper. An unspliced system bonding jumper must be installed at the same location where the grounding electrode conductor termi-nates to the neutral terminal of the separately derived system; either at the separately derived system or the system disconnecting means, but not at both locations [250.30(A)(5)].
(a) Installed at Source. Where the system bonding jumper is installed at the source of the separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the source (transformer case). Figure 250–32
(b) Installed at First Disconnecting Means. Where the system bonding jumper is installed at the first disconnecting means of a separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal disconnecting means enclosure. Figure 250–33
Figure 250-32 (Click on image to enlarge)
Figure 250-33 (Click on image to enlarge)
Author’s Comment: A system bonding jumper is a conductor, screw, or strap that bonds the metal parts of a separately derived system to the system neu-tral point [Article 100 Bonding Jumper, System], and it’s sized to Table 250.66 in accordance with 250.28(D).
DANGER: During a ground fault, metal parts of electrical equipment, as well as metal piping and structural steel, will become and remain energized providing the potential for electric shock and fire if the system bonding jumper isn’t installed. Figure 250–34
Figure 250-34 (Click on image to enlarge)
(2) Supply-Side Bonding Jumper. If the separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper must be run to the derived system disconnecting means. The supply-side bonding jumper can be a nonflexible metal raceway, a wire, or a bus.
(a) If the supply-side bonding jumper is of the wire type, it must be sized in accordance with Table 250.66, based on the area of the largest ungrounded derived system conductor in the raceway or cable.
Question: What size supply-side bonding jumper is required for flexible metal conduit containing 300 kcmil secondary conductors? Figure 250–35
Figure 250-35 (Click on image to enlarge)
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG
Answer: (b) 2 AWG [Table 250.66]
(b) If the supply side bonding jumper is a bus, it must have a cross sectional area no smaller than required by Table 250.66.
(3) System Neutral Conductor Size. If the system bonding jumper is installed at the disconnecting means instead of at the source, the following require-ments apply:
(a) Sizing for Single Raceway. Because the neutral conductor of a derived system serves as the effective ground-fault current path for ground-fault current, it must be routed with the ungrounded conductors of the derived system and be sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor of the derived system. Figure 250–36
Figure 250-36 (Click on image to enlarge)
(b) Parallel Conductors in Two or More Raceways. If the conductors from the derived system are installed in parallel in two or more raceways, the neutral conductor of the derived system in each raceway or cable must be sized not smaller than specified in Table 250.66, based on the area of the largest un-grounded conductor of the derived system in the raceway or cable. In no case is the neutral conductor of the derived system permitted to be smaller than 1/0 AWG [310.10(H)].
Author’s Comment: If the system bonding jumper is installed at the disconnecting means instead of at the source, an supply side system bonding conduc-tor must connect the metal parts of the separately derived system to the neutral conductor at the disconnecting means in accordance with 250.30(A)(2).
(4) Grounding Electrode. The grounding electrode must be as near as practicable, and preferably in the same area where the system bonding jumper is in-stalled and be one of the following: Figure 250–37
Figure 250-37 (Click on image to enlarge)
(1) Metal water pipe electrode, within 5 ft of the entry to the building [250.52(A)(1)].
(2) Metal building frame electrode [250.52(A)(2)].
Ex 1: If the electrodes listed specified in 250.30(A)(4) aren’t available, one of the following electrodes can be used:
- A concrete-encased electrode encased by not less than 2 in. of concrete, located horizontally near the bottom or vertically, and within that portion of concrete foundation or footing that’s in direct contact with the earth [250.52(A)(3)].
- A ground ring electrode encircling the building/structure, buried not less than 30 in. below grade, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4) and 250.53(F)].
- A ground rod electrode having not less than 8 ft of contact with the soil meeting the requirements of 250.52(A)(5) and 250.53(G)].
- Other metal underground systems, piping systems, or underground tanks [250.52(A)(8)].
Note 1: Interior metal water piping in the area served by separately derived systems must be bonded to the separately derived system in accordance with 250.104(D).
Note 2: See 250.50 and 250.58 for requirements of bonding all electrodes together if located at the same building/structure.
(5) Grounding Electrode Conductor, Single Separately Derived System. The grounding electrode conductor must be sized in accordance with 250.66, based on the area of the largest ungrounded conductor of the derived system. A grounding electrode conductor must connect the neutral terminal of a separately derived system to a grounding electrode of a type identified in 250.30(A)(4) at the same point on the separately derived system where the system bonding jumper is connected. Figure 250–38
Figure 250-38 (Click on image to enlarge)
Author’s Comments:
- System grounding is intended to reduce overvoltage caused by induction from indirect lightning, or restriking/intermittent ground faults. Induced voltage imposed from lightning can be reduced by short grounding conductors and eliminating unnecessary bends and loops [250.4(A)(1) Note]. Figure 250–39
- System grounding also helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components.
Figure 250-39 (Click on image to enlarge)
Ex 1: If the system bonding jumper is a wire or busbar, the grounding electrode conductor is permitted to terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1). Figure 250–40
Figure 250-40 (Click on image to enlarge)
Ex 3: Separately derived systems rated 1 kVA or less aren’t required to be grounded (connected to the earth).
(6) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where there are multiple separately derived systems, a grounding electrode con-ductor tap from each separately derived system to a common grounding electrode conductor is permitted. This connection is to be made at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)]. Figure 250–41
Figure 250-41 (Click on image to enlarge)
Ex 1: If the system bonding jumper is a wire or busbar, the grounding electrode conductor tap can terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1).
Ex 2: Separately derived systems rated 1 kVA or less aren’t required to be grounded (connected to the earth).
(a) Common Grounding Electrode Conductor. The common grounding electrode conductor can be one of the following:
(1) A conductor not smaller than 3/0 AWG copper or 250 kcmil aluminum.
(2) The metal frame of the building/structure that complies with 250.52(A)(2) or is connected to the grounding electrode system by a conductor not smaller than 3/0 AWG copper or 250 kcmil aluminum.
(b) Tap Conductor Size. Grounding electrode conductor taps must be sized in accordance with Table 250.66, based on the area of the largest ungrounded conductor of the given derived system.
(c) Connections. All tap connections to the common grounding electrode conductor must be made at an accessible location by one of the following meth-ods:
(1) A connector listed as grounding and bonding equipment.
(2) Listed connections to aluminum or copper busbars not less than ¼ x 2 in.
(3) Exothermic welding.
Grounding electrode conductor taps must be connected to the common grounding electrode conductor so the common grounding electrode conductor isn’t spliced.
(7) Installation. The grounding electrode conductor must comply with the following:
- Be of copper where within 18 in. of the earth [250.64(A)].
- Securely fastened to the surface on which it’s carried [250.64(B)].
- Adequately protected if exposed to physical damage [250.64(B)].
- Metal enclosures enclosing a grounding electrode conductor must be made electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode [250.64(E)].
(8) Structural Steel and Metal Piping. To ensure dangerous voltage from a ground fault is removed quickly, structural steel and metal piping in the area served by a separately derived system must be connected to the neutral conductor at the separately derived system in accordance with 250.104(D).
(C) Outdoor Source. If the separately derived system is located outside the building/structure, a connection to the grounding electrode must be made at the separately derived system location.
ANALYSIS: Considering the amount of changes that have occurred in this section, it wouldn’t be entirely inaccurate to say that the whole section has been rewritten. Taking the changes one at a time, they’re as follows:
A new opening paragraph and two Informational Notes have been added. These changes alert the NEC user to other sections in Part II of the article that must be complied with. The first Informational Note is relocated text from 250.20(D), which tells the Code user how to determine whether or not a generator is a separately derived system. The second note gives a reference to 445.13, which explains how to size the neutral conductor of a generator when that conductor is used for carrying fault current as well as neutral current.
Section 250.30(A) has been revised to use the correct term “system bonding jumper” instead of the undefined term “point of grounding.” This is in the context of prohibited neutral to ground connection locations.
Section 250.30(A)(1) has been revised to more clearly detail the options of where the system bonding jumper is installed. Although previous editions gave the same options, this NEC edition more clearly spells them out.
Section 250.30(A)(2) introduces the new term “supply side bonding jumper” to this section. When the source of the separately derived system doesn’t contain an overcurrent device, a supply side bonding jumper must be installed to the first overcurrent device (although the Code uses the term “disconnecting means” instead of overcurrent device). The ungrounded conductors in such an application don’t have overcurrent protection, so the bonding jumper that’s routed with these circuit conductors must be larger in size than a typical equipment grounding conductor. A supply side bonding jumper satisfies this requirement.
Section 250.30(A)(3) mainly borrows the text that was previously in 250.30(A)(8). It does, however, add new text to provide guidance on sizing the grounded conductor for a delta (corner grounded) system. In these applications, the grounded conductor must be the same size as the ungrounded conductors.
Section 250.30(A)(4) is mainly the language that was in 250.30(A)(7) of previous Code editions. New to this edition, however, is an Informational Note alerting the Code user to see 250.50 and 250.58 when multiple electrodes are present at a building. An example of where this makes sense is a transformer in a wood-framed building without an underground metal water piping system. In such a case, driving a ground rod for the transformer is a permitted way of earth grounding the separately derived system. In many instances, the installer will drive the ground rod and connect it to the transformer, without considering the fact that he or she has just created an electrode that isn’t connected to the other electrodes in the building. In an installation such as this, the installer will be required to connect this “new” electrode to the “other” electrodes in the building, such as, perhaps, a ground rod located outside near the service equipment. By adding this new Informational Note, both installers and inspectors will be reminded of this requirement.
In 250.30(A)(6), the grounding electrode conductor(s) for multiple separately derived systems, has been changed to clarify that structural metal can be used to ground multiple separately derived systems, provided that the structural metal complies with 250.52(A)(2) or is connected to the grounding electrode system by a conductor not smaller than 3/0 AWG CU or 250 kcmil AL.
Section 250.30(C) is new to the NEC. This subsection addresses separately derived systems that are installed outside of a building or other structure. When this is the case, a grounding electrode connection to the transformer must be provided. |
2011 NEC Changes DVD Package |
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