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Grounding Separately Derived Systems

Posted By Michael Johnston, Friday, July 01, 2005
Updated: Saturday, February 16, 2013

Separately derived systems can be grounded or ungrounded. The primary difference between a grounded derived system and an ungrounded derived system is that no intentionally grounded system conductor exists in an ungrounded system. All conductors derived from these systems are ungrounded conductors. Where the separately derived system is required to be grounded as provided in 250.20(A) or (B), it must be grounded in accordance with the rules in 250.30(A). Ungrounded separately derived systems must be grounded in accordance with 250.30(B). This article provides a concise review of the rules for grounded separately derived systems and some application examples.

Grounding and Bonding


Figure 1. System bonding jumper installed at the source enclosure

When applying grounding and bonding rules to systems, it is important to have a basic understanding of the concepts of the terms grounding and bonding. When taking an elementary approach to these words, one should view grounding as connecting something to the earth. Bonding should be thought of as connecting objects together. Grasping the concepts of these two words helps one understand the performance criteria specified for grounded systems in Section 250.4(A)(1) and (2). Where the term grounding is used, it should fall under the coverage of either or both of these subsections that describe the concept and purpose of grounding. Bonding as described in Section 250.4(A)(3) and (4) is the process of connecting together the normally non-current-carrying conductive materials in a manner that establishes an effective ground-fault current path. Other electrically conductive materials are also required to be bonded to the electrical supply source in a manner that establishes an effective ground-fault current path, as detailed in Section 250.4(A)(5). Three critical elements of this path are that it be of the lowest possible impedance, be permanent and continuous, and have adequate capacity for any fault current likely to be imposed on it. The earth must never be considered, or used, as an effective ground-fault current path because it is such a high-impedance path. These concepts should be understood to properly apply the rules for grounding and bonding separately derived systems.

Definitions

Grounded. Connected to earth or to some conducting body that serves in place of the earth.


Figure 2. System bonding jumper installed at the first enclosure supplied by the derived system

Bonding (Bonded).The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.

Bonding Jumper, System.The connection between the grounded conductor and the equipment grounding conductor at a separately derived system.

Bonding jumper.A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.

Six Basic Components


Figure 3. System bonding jumper sized based on the largest derived phase conductor size

Grounded separately derived systems include six basic components in the grounding and bonding scheme:(1) the system bonding jumper, (2) equipment grounding conductor [primary side of transformer-type derived systems], (3) the equipment bonding jumper, (4) the grounded conductor, (5) the grounding electrode conductor, and (6) the grounding electrode. Let’s take a more detailed look at each component and the rules that apply to them.

Derived System Bonding Jumper

The system bonding jumper connects the system grounded conductor to the equipment grounding conductor(s) of the separately derived system. The system bonding jumper can be installed at the source enclosure or at any point up to and including the first system overcurrent device enclosure (see figures 1 and 2).


Figure 4. System bonding jumper sized based on the total circular mil area of the derived phase conductors in parallel

If the derived system ungrounded phase conductors do not terminate in an overcurrent device, the system bonding jumper must be located at the source enclosure [250.30(A)(1)], and be sized based on the ungrounded derived phase conductor size as provided in 250.28(A) through (D). Section 250.28(D) references Table 250.66 for the minimum size. For example, if the ungrounded derived phase conductors are 600 kcmil for each phase, the system bonding jumper cannot be smaller than 1/0 copper or 3/0 aluminum or copper-clad aluminum. If the size of the ungrounded derived phase conductors exceeds 1100 kcmil copper or 1750 kcmil aluminum, the 12.5 percent rule must be applied. As an example, if the derived phase conductors include three 500-kcmil copper conductors per phase, the system bonding jumper is sized based on a total circular mil area of 1500 kcmil x 12.5% which equals 187,500 circular mils. The next higher value in Table 8 of chapter 9 in the Code is 211,600 circular mils or 4/0 minimum (see figures 3 and 4).

Equipment Grounding Conductor


Figure 5. The equipment grounding conductor with the primary and the equipment bonding jumper with the secondary will be connected together usually in the transformer source enclosure.

If the separately derived system is a transformer type, usually an equipment grounding conductor is installed with the primary supply conductors to the transformer. This equipment grounding conductor can be any of the types provided in 250.118(1). If the equipment grounding conductor is a wire type, it should be sized using Table 250.122 based on the rating of the overcurrent device ahead of the primary conductors of the transformer. For transformer separately derived systems, this is the only conductor in the grounding and bonding scheme that is sized using Table 250.122. All other grounding and bonding conductors of the separately derived system are sized using Table 250.66. The equipment grounding conductor with the primary and the equipment bonding jumper with the secondary will be connected together usually in the transformer source enclosure (see figure 5). Where the separately derived system is not a transformer, typically there is no primary and the system is grounded at the source. Solar photovoltaic systems, fuel cells, and generators are examples of systems that typically do not have a primary feeder in the supply side of the source.

Derived System Equipment Bonding Jumper


Figure 6. Bonding can be established between the source enclosure using conduit or tubing with suitable fittings. Where the bonding jumper between these enclosures is a wire type, it is required to be sized in accordance with 250.102(C)

Since effective bonding between the source enclosure and the first system overcurrent device enclosure is essential, a bonding jumper in some form or other is generally required. Bonding can be accomplished by conduit or tubing that qualifies as an effective ground-fault current path, or by use of a conductor of the wire type. This conductor is specifically identified in this section as an equipment bonding jumper. Transformer secondary conductors are typically not protected where they receive their supply, and can be routed to the first enclosure in a number of different methods. Bonding can be established between the source enclosure using conduit or tubing with suitable fittings. Where the bonding jumper between these enclosures is a wire type, it is required to be sized in accordance with 250.102(C) [see figure 6]. This sizing requirement using 250.66 or the 12.5 percent rule is necessary because these conductors are generally unprotected. Equipment bonding jumper sizing rules at separately derived systems are the same as equipment bonding jumpers installed on the line side of the service disconnect. Transformer secondary conductors usually terminate in an overcurrent device at the equipment supplied by the system. The rules in 240.21(C) must apply to these transformer secondary conductors. Usually the only overcurrent protection ahead of them is located on the primary side of the transformer. In some instances, the overcurrent protective device for the transformer can provide protection for the primary and secondary conductors as provided in 240.4(F).

Derived System Grounded Conductor


Figure 7. Grounding electrode conductor sizing for systems that use a metal water pipe grounding electrode

The grounded conductor of a separately derived system must meet some basic sizing requirements. It must be adequate to supply the load served (220.61), and be at least large enough to meet the minimum size specified in 250.30(A)(8), if applicable. Where the system bonding jumper is not located at the source enclosure, the grounded conductor, where installed with the derived phase conductors, must meet the routing and sizing rules in 250.30(A)(8)(a), (b), and (c). The primary reason is to ensure the effective ground-fault current path is established between the source enclosure and the first enclosure supplied by the derived system.

Grounding Electrode Conductor

Single System Grounding Electrode Conductor


Figure 8. The same system grounded to a rod type of electrode

The grounding electrode conductor is the connecting path to the earth from the separately derived system grounded conductor. This conductor is not expected to carry ground-fault current to facilitate overcurrent device operation, because the earth is not permitted to be considered an effective ground-fault current path. The grounding electrode conductor for separately derived systems is generally sized based on the largest ungrounded derived phase conductor supplied by the system using Table 250.66. The grounding electrode used for the separately derived system will also have an affect on the minimum size required. As an example, if the grounding electrode is a metal water pipe electrode that meets the criteria in 250.52(A)(1), and the size of the largest ungrounded derived phase conductor supplied by the system is a 750 kcmil copper, then the minimum size grounding electrode conductor required would be a 2/0 copper or 4/0 aluminum or copper-clad aluminum based on Table 250.66 (see figure 7). However, if the grounding electrode for the separately derived system is a ground rod, grounding electrode conductor can be a 6 AWG copper or 4 AWG aluminum [250.66(A) and see figure 8]. Remember the restrictions for installations of aluminum conductors as provided in 250.64(A). It also should be emphasized that the electrode used for the separately derived system is required be as specified in 250.30(A)(7) and will be covered later in this article. Often the choice of grounding electrode is determined by the electrodes available and closest to the system. An electrode is required.


Figure 9. The common grounding electrode conductor permitted by 250.30(A)(4) cannot be smaller than 3/0 copper or 250 kcmil aluminum or copper-clad aluminum

Multiple Systems to a Common Grounding Electrode Conductor
Where multiple separately derived systems are installed, the Code allows the common grounding electrode conductor tap concept to be used (see figure 9). A common grounding electrode conductor sized not smaller than 3/0 copper or 250 kcmil aluminum or copper-clad aluminum must be used for the common grounding electrode conductor.

Grounding electrode conductor taps are then permitted to be installed from each separately derived system to the common grounding electrode conductor. The grounding electrode conductor taps are required to be sized in accordance with Table 250.66 based on the size of the derived phase conductors of the separately derived system it serves (see figure 10). Grounding electrode conductor taps are required to be connected to the common grounding electrode conductor by using a listed connector, by connections to a copper or aluminum busbar not smaller than 1/4 in. x 2 in., or by the exothermic welding process. Grounding electrode conductors installed for separately derived systems must be in accordance with the installation rules provided in 250.64(A), (B), (C), and (E).

Derived System Grounding Electrode


Figure 10. Grounding electrode conductor taps are required to be connected to the common grounding electrode conductor by using a listed connector, by connections to a copper or aluminum busbar not smaller than 1/4 in. x 2 in., or by the exothermic welding

Grounded systems must be connected to a grounding electrode. Simply, this means separately derived systems that are grounded must have a grounded conductor and metal enclosures that are connected to the earth as provided in 250.30(A)(7). Grounding electrode is defined in Article 100 and the list of grounding electrodes permitted for grounding is provided in 250.52(A). The Code requires that the grounding electrode be as close as practicable and preferably in the same area as the grounding connection to the system and it must be either a metal frame grounding electrode or a metal water pipe electrode. Whichever electrodes are closer to the system predicate which one to use. If either of the above grounding electrodes is not available for use, any of the other grounding electrodes in 250.52(A) must be used (see figure 11). A grounding electrode for a separately derived system is not an option, it is a requirement.

Structural Metal Framing and Metal Water Pipe Bonding


Figure 11. A grounding electrode for a separately derived system is not an option, it is a requirement

Section 250.30(A)(6) now refers to 250.104(D) for structural metal framing member and metal water pipe bonding rules for separately derived systems. This is appropriate since Part V of Article 250 covers bonding. Metal water piping systems and structural metal members interconnected to form a building frame are both required to be bonded to separately derived systems in accordance with 250.104(D)(1) through (D)(3). The basic rule calls for metal water piping systems or structural metal building frames that exist in the area served by the separately derived system to be bonded to the system grounded conductor. This connection must be at the same point where the grounding electrode conductor is connected to the system. The bonding jumper is required to be sized using Table 250.66 based on the largest ungrounded conductor of the derived system. Where the common grounding electrode conductor concept is used for multiple separately derived systems as permitted in 250.30(A)(4), any metal water piping or structural metal building framing that exists in the area served by the system is required to be bonded to the common grounding electrode conductor.

Grounding Generator Derived Systems


Figure 12. Transfer equipment that does not switch the grounded conductor

Where a separately derived system is produced by a generator, how the generator is grounded is determined by the type of transfer equipment installed for use with that generator. Generators all produce a new voltage system, but if the grounded conductor of the generator is common to the service supplied grounded conductor, then the grounding rules for the generator must meet the provisions in 250.4(A)(5), which means that no grounding connection is permitted on the load side of the grounding connection at the service (see figure 12). If a transfer switch does not include a switching action in the grounded conductor, the connections at the generator should include a grounded conductor insulated from ground and grounded metal parts, and an equipment grounding conductor for grounding and bonding metal parts. On the other hand, where the transfer equipment does include a switching action in the grounded conductor, the generator must be grounded as a separately derived system in accordance with 250.30(A) (see figure 13). The Code currently only provides direction for such installations through the fine print note no. 1 to Section 250.20(D). Perhaps there is room for this information to be folded into positive text to provide clearer direction for users faced with these types of installations.

Ungrounded Derived Systems


Figure 13. Transfer equipment that does switch the grounded conductor

Where a separately derived system is ungrounded, it includes no system conductor that is intentionally grounded. This does not mean that grounding and bonding rules that apply to such systems can be disregarded. The metal enclosures for such systems must be connected to a grounding electrode as specified in 250.30(B)(2) using a grounding electrode conductor that is sized using Table 250.66 based on the size of the largest derived phase conductor of the system. The grounding electrode for an ungrounded system must be provided in accordance with 250.30(A)(7). So although there is no system grounded conductor with this type of system, the functions of grounding and bonding are still necessary. Any equipment grounding conductors on the load side of the first system overcurrent device are required to be sized using Table 250.122 based on the rating of the overcurrent protective device protecting the feeder or branch circuit supplied by the system. Section 250.21 has been revised to require ground detectors on ungrounded systems.

Summary

Grounding a system or metal equipment basically means connecting them to the earth. Bonding metal parts and equipment means connecting the normally non-current-carrying metal parts together in a manner that ensures electrical continuity and the capacity to conduct safely any fault current likely to be imposed on it. Safe electrical installations include both grounding and bonding. Separately derived systems are grounded to earth (the electrode) at the source and all normally non-current-carrying metal parts and equipment are bonded to the point of grounding of the derived system. As the metal equipment is bonded together for the effective ground-fault current path, they are bonded and grounded through this same path. Grounding (connecting to earth) and bonding (connection together) are both essential for safe electrical systems. Separately derived systems that are grounded must meet the minimum requirements provided in 250.30(A). As always consult the local authority having jurisdiction for any requirements that may supplement or modify the minimum requirements of the NEC.

For additional information on grounding separately derived systems, see Soares Book on Grounding, 9th edition. This book is available through IAEI Customer Service: 1-800-786-4234.


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Tags:  Featured  July-August 2005 

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