All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600V, such as 120/240V, 120/208V, and 277/480V.

If you don't work with generators, you may think the requirements for separately derived systems (SDSs) aren't relevant to you. However, that thought may conflict with the reality of your situation.

People often forget that all transformers, except autotransformers, are separately derived. This is because the primary circuit conductors have no direct electrical connection to the secondary circuit conductors.

People also frequently forget that not all generators are separately derived. A generator that supplies a transfer switch that opens the grounded neutral conductor is separately derived [250.20(D) FPN 1] (Fig. 1 at right). This is also the case if there's no grounded neutral conductor. 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 Art. 100: “A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly grounded circuit conductor, to supply conductors originating in another system.” (Fig. 2).

System bonding jumper. Part 3 of this series explained that you can't make neutral-to case (load side) bonds [250.24 (A)(5)]. It also discussed the exception it provides for SDSs 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. You can also accomplish this by opening the overcurrent protection device of the secondary circuit [250.2(A)(3)].

Overall, the system bonding jumper:

  • Connects the equipment grounding conductors of the SDS (metal case) to the grounded neutral conductor (typically the X0 terminal).

  • Is sized based on the derived phase conductors. Connect the system bonding jumper, per 250.28(A) through (D).

  • Should be installed at only one location — either at the source or the first system disconnecting means or overcurrent device.

  • Shouldn't be installed at both the SDS and the secondary system disconnecting means because it will allow dangerous objectionable current to flow on conductive metal parts of electrical equipment, metal piping, and structural steel in violation of 250.6(A) (Fig. 3).

Size the system bonding jumper per Table 250.66 based on the area of the largest ungrounded secondary conductor [250.28(D)]. Where you run an equipment-bonding jumper to the secondary system disconnecting means, you must also size it per Table 250.66 based on the area of the largest ungrounded secondary conductor.

Where you run SDS conductors in parallel, install a secondary equipment bonding jumper in each raceway (or cable) and once again size it per Table 250.66 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.

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. However, note the following exceptions:

  • 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 (Fig. 4).

  • 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.

  • You don't need to ground (earth) SDSs rated 1kVA (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 mandate that the GEC is:

  • Copper, where within 18 inches 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 SDSs, 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) (Fig. 5). Those requirements can be summarized as follows:

  • (a) Common GEC size. The common GEC must not be smaller than 3/0 AWG copper or 250 kcmil aluminum.

  • (b) Tap conductor size. Size each grounding (earthing) electrode tap per 250.66 based on the largest separately derived ungrounded conductor of the SDS.

  • (c) Connections. Make grounding (earthing) electrode tap connections at an accessible location with a listed connector or an exothermic weld. You can also use listed connections to busbars not less than ¼ inch × 2 inch. If using aluminum busbars, comply with 250.64(A).

Grounding (earthing) electrode. Terminate the GEC to a grounding (earthing) electrode as close as possible to — 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:

  • A concrete-encased electrode encased in not less than 2 inches 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 feet of electrically conductive steel reinforcing bars (or rods) not less than ½ inches in diameter [250.52(A)(3)].

  • A ground ring encircling the building or structure, buried not less than 30 inches below grade. It must contain not less than 20 feet of bare copper conductor not smaller than 2 AWG [250.52(A)(4) and 250.53(F)].

  • A ground rod with not less than 8 feet 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:

  • 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.

  • 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 can't 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.

SDSs are common, so it's important to know — and correctly implement — SDS requirements. A mistake in grounding and bonding can prevent a fault from clearing, 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.