Sections
250.30 Grounding and Bonding of Separately Derived AC Systems Continued
To prevent objectionable current from flowing onto metal parts of electrical equipment, as well as metal piping and structural steel, the grounding electrode conductor must terminate at the same point on the separately derived system where the system bonding jumper is installed.
Exception 1: Where the system bonding jumper [250.30(A)(1)] is a wire or busbar, the grounding electrode conductor can terminate to the equipment grounding terminal, bar, or bus on the metal enclosure of the separately derived system. Figure 250–77
Exception 3: Separately derived systems rated 1 kVA (1,000 VA) or less are not required to be grounded (earthed); however, to ensure ground faults can be cleared, a system bonding jumper must be installed in accordance with 250.30(A)(1).
(4) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where there are multiple separately derived systems, the grounded neutral terminal of each derived system can be grounded (earthed) to a common grounding electrode conductor. The grounding electrode conductor and grounding electrode tap must comply with (a) through (c). Figure 250–78
Exception 1: Where the system bonding jumper [250.30(A)(1)] is a wire or busbar, the grounding electrode tap can terminate to the equipment grounding terminal, bar, or bus on the metal enclosure of the separately derived system.
Exception 2: Separately derived systems rated 1 kVA (1,000 VA) or less are not required to be grounded (earthed); however, to ensure ground faults can be cleared, a system bonding jumper must be installed in accordance with 250.30(A)(1).
(a) Common Grounding Electrode Conductor Size. The common grounding electrode conductor must not be smaller than 3/0 AWG copper or 250 kcmil aluminum.
(b) Tap Conductor Size. Each grounding electrode tap must be sized in accordance with 250.66, based on the largest separately derived ungrounded conductor of the separately derived system.
(c) Connections. All grounding electrode tap connections must be made at an accessible location by:
(1) Listed connector.
(2) Listed connections to aluminum or copper busbars not less than 1/4 in. x 2 in. Where aluminum busbars are used, the installation must comply with 250.64(A).
(3) By the exothermic welding process.
Author’s Comment: See Article 100 for the definition of “Accessible” as it applies to wiring methods. Grounding electrode tap conductors must be connected to the common grounding electrode conductor so that the common grounding electrode conductor isn’t spliced.
(5) Installation. The grounding electrode conductor must be installed in accordance with 250.64.
Author’s Comment: The grounding electrode conductor must comply with the following:
- Be of copper where within 18 in. of 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)].
(6) Bonding. To ensure that dangerous voltage from a ground fault is removed quickly, structural metal and metal piping in the area served by a separately derived system must be bonded to the grounded neutral conductor at the separately derived system in accordance with 250.104(D).
(7) Grounding (Earthing) Electrode. The grounding electrode conductor must terminate to a grounding electrode that is located as close as possible, and preferably in the same area as, the system bonding jumper. The grounding electrode must be the nearest one of the following: Figure 250–79
(1) Metal water pipe electrode as specified in 250.52(A)(1).
(2) Structural metal electrode as specified in 250.52(A)(2).
Exception 1: Where none of the electrodes listed in (1) or (2) is available, one of the following is permitted:
- Concrete-encased electrode encased by not less than 2 in. of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with earth, consisting of not less than 20 ft of electrically conductive steel reinforcing bars or rods not less than 1/2 in. in diameter [250.52(A)(3)].
- A ground ring encircling the building or structure, buried not less than 30 in. below grade, consisting of not less than 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4) and 250.53(F)].
- A ground rod having not less than 8 ft of contact with the soil [250.52(A)(5) and 250.53(G)].
- Other metal underground systems, piping systems, or underground tanks [250.52(A)(7)].
FPN: To ensure that dangerous voltage from a ground fault is quickly removed, metal water piping (including structural metal) in the area served by a separately derived system must be bonded to the grounded neutral conductor at the separately derived system in accordance with 250.104(D).
Author’s Comment: This FPN makes no sense, since the requirement is contained in 250.30(A)(6).8) Grounded Neutral Conductor. Where the system bonding jumper is installed at the secondary system disconnecting means instead of at the source of the separately derived system, the following requirements apply: Figure 250–80
(a) Routing and Sizing. Because the grounded neutral conductor serves as the effective ground-fault current path, the grounded neutral conductor must be routed with the secondary conductors, and it must be sized not smaller than specified in Table 250.66, based on the largest ungrounded conductor for the separately derived system.
(b) Parallel Conductors. If the secondary conductors are installed in parallel, the grounded neutral secondary conductor in each raceway or cable must be sized based on the area of the largest ungrounded secondary conductor in the raceway. But the grounded neutral secondary conductor can not be smaller than 1/0 AWG [310.4].
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Figure 250–77
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Figure 250–78
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Figure 250–79
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Figure 250–80
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