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Grounding versus Bonding Part 4 of 12 — 2005 NEC®

Grounding and Bonding of Separately Derived AC Systems

By Mike Holt

When you hear "separately derived systems (SDS), do you automatically think of generators? If you don't work with generators, you may think SDS is not relevant to you. However, that thought may conflict with the reality of your situation.

Here's a frequently overlooked fact: All transformers, except autotransformers, are separately derived. This is because the primary circuit conductors have no direct electrical connection to the secondary circuit conductors.

Here's another frequently overlooked fact: Not all generators are separately derived. A generator supplying a transfer switch that opens the grounded neutral conductor or if there is no grounded neutral conductor is separately derived [250.20(D) FPN 1]. See Figure 250-71. But often generator setups don't switch the grounded neutral conductor in the transfer switch.

To determine if you have an SDS, start with the definition given in Article 100. An SDS is a premises wiring system whose power derives from a source other than a service. It also has no direct electrical connection (including a solidly grounded circuit conductor) to supply conductors originating from another system (Figure 250-70).

System Bonding Jumper

In Part 3 of this series, we learned you can't make neutral-to case (load side) bonds [250.24 (A)(5)]. We also learned this rule provides an exception for SDS, if you follow the requirements of 250.30(A)(1).

  • Bonding the metal parts of the SDS to the secondary grounded neutral terminal-by installing a system bonding jumper-ensures quick removal of dangerous voltage from a secondary ground fault. Opening the overcurrent protection device of the secondary circuit [250.2(A)(3)] accomplishes this removal (Figure 250-30A1 01 NEW.cdr).
  • Connects the equipment grounding conductors of the SDS (metal case) to the grounded neutral conductor (typically the X0 terminal.
  • Connect the system bonding jumper, per 250.28(A) through (D), and size it based on the derived phase conductors.
  • Install the system bonding jumper at only one location. Either at the source or the first system disconnecting means or overcurrent device. See Figures 250-78 and 50-79.
  • Dangerous objectionable current will flow on conductive metal parts of electrical equipment, metal piping, and structural steel-in violation of 250.6(A)-if you improperly install the system bonding jumper at both the SDS and the secondary system disconnecting means (Figure 250-30A0 03 NEW.cdr).

Size matters

Size the system bonding jumper per Table 250.66 based on the area of the largest ungrounded secondary conductor [250.28(D)].

Question: What size system bonding jumper do you need for a 45 kVA transformer, where the secondary conductors are 3/0 AWG (See Figure 250-75)?

(a) 4 AWG (b) 3 AWG (c) 2 AWG (d) 1 AWG

Answer: (a) 4 AWG (Table 250.66).

Where you run an equipment bonding jumper to the secondary system disconnecting means, size it per Table 250.66 based on the area of the largest ungrounded secondary conductor.

Question: What size secondary equipment bonding jumper do you need for a nonmetallic raceway containing 350 kcmil secondary conductors (Figure 250-30A2 CC250-10.cdr)?

(a) 1 AWG (b) 1/0 AWG (c) 2/0 AWG (d) 3/0 AWG

Answer: (b) 1 AWG (Table 250.66).

Where you run SDS conductors in parallel, install a secondary equipment bonding jumper in each raceway (or cable) and size it per Table 250.6 based on the area of the largest ungrounded secondary conductor [250.102(C)].

Grounding Electrode Conductor (GEC)

For each SDS, ground the grounded neutral terminal (typically X0) to a suitable grounding (earthing) electrode of a type identified in 250.30(A)(7). Size the secondary system grounding (earthing) electrode conductor per 250.66 based on the total area of the largest ungrounded secondary conductor (Figure 250-81).

To prevent objectionable current flow in metal parts of electrical equipment, metal piping, and structural steel, terminate the GEC to the same point on the SDS where you installed the system bonding jumper (Figure 250-78).

Note the exceptions:

  1. Where the system bonding jumper [250.30(A)(1)] is a wire or busbar, you can terminate the GEC to the equipment grounding terminal bar or bus on the metal enclosure of the SDS (Figure 250-30A3x NEW).
  2. Where the SDS originates in listed equipment suitable as service equipment, you can use the GEC from the service or feeder equipment to the same grounding electrode as the GEC for the SDS-if it meets the size requirements.
  3. You do not need to ground (earth) SDS rated 1 kVA (1,000 VA) or less. But you must install a system bonding jumper per 250.30(A)(1), to ensure the clearing of ground-faults.

Connect grounding (earthing) electrode tap conductors to the GEC without splicing the common GEC. Install the GEC per 250.64-the basic requirements are that the GEC be:

  • 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)].

If you run the GEC in metal enclosures, make them electrically continuous from the point of attachment to cabinets (or equipment) to the GEC [250.64(E)]. Bond building structural steel and all metal piping to an effective ground-fault current path, per 250.104(D).

If you have multiple SDS, you can ground the grounded neutral terminal (X0) of each SDS to a common GEC. But, the GEC and grounding (earthing) electrode tap must comply with 250.30(A)(4)(a) through (c) (Figure 250-30A4 CC250-11.cdr). We can summarize those as:

  1. Common GEC Size. The common GEC must not be smaller than 3/0 AWG copper or 250 kcmil aluminum.
  2. Tap Conductor Size. Size each grounding (earthing) electrode tap per 250.66 based on the largest separately derived ungrounded conductor of the SDS.
  3. Connections. Make grounding (earthing) electrode tap connections at an accessible location with a listed connector or an exothermic weld. Another option: use listed connections to busbars not less than 1/4* × 2 in. If using aluminum busbars, comply with 250.64(A).

Grounding (Earthing) Electrode

Terminate the GEC to a grounding (earthing) electrode as close as possible-and preferably in the same area as-the system bonding jumper. The grounding (earthing) electrode must be the nearest:

  • Metal water pipe electrode, as specified in 250.52(A)(1), or
  • Structural metal electrode, as specified in 250.52(A)(2)

If neither of these electrodes is available, use one of the following:

  • Concrete-encased electrode encased by not less than 2 in. of concrete, located within (and near the bottom of) a concrete foundation (or footing) in direct contact with earth. This foundation must contain 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. It must contain 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)].

Observe the FPN in 250.30(7), to ensure quick removal of dangerous voltage from a ground fault. Bond metal water piping and structural steel in the area served by an SDS to the grounded neutral conductor at the SDS, per 250.104(D).

Grounded Neutral Conductor

If you install the system bonding jumper at the secondary system disconnecting means instead of at the source of the SDS, follow these requirements (Figure 250-84):

  1. Routing and Sizing. Route the grounded neutral conductor with the secondary conductors, and size it not smaller than specified in Table 250.66 based on the largest ungrounded conductor for the SDS.
  2. Parallel Conductors. If you install the secondary conductors in parallel, size the grounded neutral secondary conductor in each raceway or cable based on the area of the largest ungrounded secondary conductor in the raceway.

You cannot size the grounded neutral secondary conductor smaller than 1/0 AWG [310.4]. Remember, the grounded neutral conductor serves as the effective ground-fault current path.

Because SDS are pervasive, it's important to know-and correctly implement-SDS requirements. A mistake in the grounding and bonding of an SDS can prevent the clearing of a fault and it can allow dangerous potential to build on metal parts of the electrical system. Your new knowledge of SDS grounding and bonding requirements allows you to prevent both problems.

Mike Holt Comment: Summary from my best selling book, Understanding Grounding versus Bonding.

2005 Grounding versus Bonding Textbook — 2005

Grounding versus Bonding textbook is loaded with detailed color-coded graphics so you can easily differentiate between grounding and bonding. This text gets to the root of all problems associated with grounding and bonding. Subject includes: Circuit and System Grounding, Grounding Electrode System and Grounding Electrode Conductor, Enclosure, Raceway, and Service Cable Grounding, Bonding, Methods of Equipment Grounding, Direct-Current Systems, and Grounding of Systems and Circuits.

Product Code: 05NCT2
ISDN: 1-932685-22-7

Price: $30.00 each

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