Article 250

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Grounding and Bonding 250.30 Grounding and Bonding of Separately Derived AC Systems Text clarifies the method of sizing equipment bonding jumpers when the separately derived system conductors are run in parallel. And the text relating to the “common” grounding electrode conductor for multiple separately derived systems was changed to require this conductor not to be sized smaller than 3/0 AWG. (A) Grounded Systems. Separately derived systems must be bonded and grounded in accordance with (1) through (8). (2) Equipment Bonding Jumper Size. Where an equipment bonding jumper is run to the first disconnecting means, it must be sized based on the largest ungrounded secondary conductor, in accordance with Table 250.66. Question: What size equipment bonding jumper is required for a nonmetallic raceway containing 500 kcmil conductors from the source of a separately derived system? Figure 250-10 (a) 1 AWG    (b) 1/0 AWG    (c) 2/0 AWG    (d) 3/0 AWG Answer: (b) 1/0 AWG, Table 250.66 Where separately derived system conductors are run in parallel, the bonding jumper for each raceway or cable must be sized based on the largest ungrounded secondary conductor in each raceway or cable [250.102(C)]. (4) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where there are multiple separately derived systems, it’s permissible to connect a tap from the grounded neutral terminals of each separately derived system to a common grounding electrode conductor. Each tap must connect the grounded neutral terminal of a separately derived system to the common grounding electrode conductor. The grounding electrode conductor and grounding electrode taps must comply with (a) through (c). Figure 250-11 Ex 1: Where the system bonding jumper specified in 250.30(A)(1) is a wire or busbar, the grounding electrode conductor is permitted to terminate to the equipment grounding terminal bar or bus of the separately derived system. Ex 2: A grounding electrode conductor isn’t required for systems rated 1 kVA or less, however the system bonding jumper (neutral-to-case) connection 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 conductor 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 × 2 in. Where aluminum busbars are used, the installation must comply with 250.64(A). (3) By the exothermic welding process. Grounding electrode tap conductors must be connected to the common grounding electrode conductor in a manner that the common grounding electrode conductor isn’t spliced.

Figure 250-10 (Click on image to enlarge) Figure 250-11 (Click on image to enlarge)