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
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.
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
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).
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
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
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
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
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.
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
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
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
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
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.
Michael J. Johnston is IAEI’s
director of education and an IAEI principal member on
CMP-5. Johnston was formerly employed as an electrical
field inspections supervisor for the city of Phoenix,
Arizona. He is fully certified in many areas. He is a
member of the IBEW. He achieved both journeyman E-2 and
master electrician E-1 licenses in the state of
Connecticut. Additionally, he holds all IAEI
certifications. He also holds ICC Electrical Inspections
Certification. He is a member of the UL Electrical
Council. |
