Ecmweb 8010 Nec Code Quandaries 2016 Pr
Ecmweb 8010 Nec Code Quandaries 2016 Pr
Ecmweb 8010 Nec Code Quandaries 2016 Pr
Ecmweb 8010 Nec Code Quandaries 2016 Pr
Ecmweb 8010 Nec Code Quandaries 2016 Pr

Stumped by the Code? NEC Rules for Grounding and Bonding Transformers

Jan. 22, 2016
Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2014 NEC.

Q. What are the rules for grounding and bonding transformers?

A. Separately derived systems must be grounded and bonded in accordance with Sec. 250.30(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 Sec. 250.142(B).

Fig. 1. Size the system bonding jumper per Table 250.102(C)(1).

A system bonding jumper must be installed at the same location where the grounding electrode conductor (GEC) terminates 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)(1) and (5)]. Where the system bonding jumper is installed at the source of the separately derived system, the system bonding jumper must connect the neutral conductor of the derived system to the metal enclosure of the derived system [250.30(A)(1)(a)] (Fig. 1). Where the system bonding jumper is installed at the first disconnecting means of a separately derived system, the system bonding jumper must connect the neutral conductor of the derived system to the metal disconnecting means enclosure [250.30(A)(1)(b)].

A system bonding jumper is a conductor, screw, or strap that bonds the metal parts of a separately derived system to the system neutral point [Art. 100 Bonding Jumper, System], and it’s sized to Table 250.102(C)(1) in accordance with Sec. 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.

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 Sec. 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.

A supply-side bonding jumper (nonflexible metal raceway or wire) must be run from the derived system to the derived system disconnecting means [250.30(A)(2)]. If the supply-side bonding jumper is of the wire type, it must be sized in accordance with Table 250.102(C)(1), based on the area of the largest ungrounded derived system conductor in the raceway or cable [250.30(A)(2)(a)].

If the system bonding jumper is installed at the disconnecting means instead of at the source, the following requirements apply [250.30(A)(3)]:

a) The neutral conductor must be routed with the ungrounded conductors of the derived system to the disconnecting means and be sized not smaller than specified in Table 250.102(C)(1), based on the area of the ungrounded conductor of the derived system.

b) 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 250.102(C)(1), based on the area of the largest ungrounded 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)].

If the system bonding jumper is installed at the disconnecting means instead of at the source, a supply side bonding jumper must connect the metal parts of the separately derived system to the neutral conductor at the disconnecting means [250.30(A)(2)].

The grounding electrode for a separately derived system must be as near as practicable, and preferably in the same area where the system bonding jumper is installed and be one of the following [250.30(A)(4)]:

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)].

Exception No. 1: If the water pipe or structural metal electrode isn’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 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, 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 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 Sec. 250.104(D).

When sizing the GEC for a single separately derived system, it must be sized in accordance with Sec. 250.66, based on the area of the largest ungrounded conductor of the derived system. A GEC must terminate to the neutral at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(5)].

System grounding 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.

To prevent objectionable neutral current from flowing [250.6] onto metal parts, the GEC must originate at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)].

Exception No. 1: The GEC is permitted to terminate to the equipment grounding terminal at the derived system or first system disconnecting means in accordance with Sec. 250.30(A)(1).

Exception No. 3: Separately derived systems rated 1kVA or less aren’t required to be grounded (connected to the earth).

Fig. 2. Size GEC taps per Table 250.66.

Where there are multiple separately derived systems, a GEC tap from each separately derived system to a common GEC is permitted [250.30(A)(6)]. 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)] (Fig. 2).

Exception No. 1: If the system bonding jumper is a wire or bus bar, the GEC tap can terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with Sec. 250.30(A)(1).

Exception No. 2: Separately derived systems rated 1kVA or less aren’t required to be grounded (connected to the earth).

The common GEC can be one of the following [250.30(A)(6)(a)]:

1) A conductor not smaller than 3/0 AWG copper or 250kcmil aluminum.

2) The metal frame of the buildings that complies with Sec. 250.52(A)(2) or is connected to the grounding electrode system by a conductor not smaller than 3/0 AWG copper or 250kcmil aluminum.

GEC taps must be sized in accordance with Table 250.66, based on the area of the largest ungrounded conductor of the given derived system [250.30(A)(6)(b)]. All tap connections to the common GEC must be made at an accessible location by one of the following methods [250.30(A)(6)(c)]:

1) A connector listed as grounding and bonding equipment.

2) Listed connections to aluminum or copper bus bars not less than ¼ in. in depth by 2 in. in width

3) Exothermic welding.

GEC taps must be connected to the common GEC so the common GEC isn’t spliced.

The GEC must comply with the following [250.30(A)(7)]:

•  Be of copper where within 18 in. of the earth [250.64(A)].

•  Be securely fastened to the surface on which it’s carried [250.64(B)].

•  Be adequately protected if exposed to physical damage [250.64(B)].

•  Metal enclosures enclosing a GEC must be made electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode [250.64(E)].

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 Sec. 250.104(D) [250.30(A)(8)].

Separately derived systems located outside the building must have the grounding electrode connection made at the separately derived system location [250.30(C)].                                                                                       

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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