Welcome to a NECA Webinar based on the National Electrical Code. This seminar
is the fourth in a series of electrical grounding and bonding Webinars presented by
NECA. This Webinar topic is grounding separately derived systems such as
transformers and generators. Introduction of speaker and subject.
1
Presenters should use this slide to provide and overview of what will be covered in
the presentation. This program covers the general requirements for grounding
separately derived systems on the load side of the service disconnecting means. A
complete review of the requirements in Section 250.30 are included in this
presentation. This program provides specific focus on transformer-type as well as
generator-type separately derived systems.
2
Presenters should review the requirements in 250.30(A)(1) through (8). These will
be covered in detail in this presentation.
3
This slide provides the definition of the term separately derived system from article
100 of the NEC-2008.
4
This slide shows two examples of systems that are not separately derived. In the
transfer switch, there is no switching action in the neutral conductor, thus the
generator is not to be grounded as a separately derived system because with the
transfer equipment in the normal position or the standby position, the grounded
conductor remains connected to ground (earth). The auto transformer is not
separately derived because one conductor (the grounded conductor) is common to
both primary and secondary. A luminaire (lightning fixture) ballast is a good example
of an auto transformer.
transformer
5
Photo shows a transformer separately derived system that is typical of many
commercial and industrial facilities. Note the primary and secondary feeder entries
on the lower right front of the enclosure. The grounding electrode conductor can be
seen routed through the masonry wall to an electrode according to 250.30(A)(7).
6
Photo shows an electrician making connections in a dry-type transformer. Note the
primary and secondary feeder conductors are connected and identified according
215.12(C). Equipment grounding conductors (with the primary) and equipment
bonding jumpers (with the secondary) can be seen exiting the left side of the
enclosure. The transformer is not energized and the worker is wearing suitable PPE
(hard hat, safety glases, and vest).
7
Section 250.20(A) and (B) provides the requirements for whether a
system must be grounded, or if grounding the system is optional.
If systems are required to be grounded based on the criteria in
250.20(A) or (B), then the system shall be grounded in accordance with
250.30(A).
Where a generator-type systems is provided with transfer equipment
that includes a switching action in the grounded (neutral) conductor, the
generator is required to be grounded in accordance with Section
250.30(A).
8
Presenters should emphasize how important the transfer switch is in determining
the grounding requirements for generators. If there is a switching action in grounded
conductor in the transfer equipment, The generator has to be grounded as a
separately derived system according to 250.30(A).
9
If there is a switching action in grounded conductor in the transfer equipment, The
generator has to be grounded as a separately derived system according to
250.30(A). Graphic shows a switch in the grounded conductor. When the transfer
switch is in the standby mode, the grounded conductor is not connected to the
service grounding electrode.
10
If there is no switching action in grounded conductor in the transfer equipment, the
generator is not grounded as a separately derived system according to 250.30(A).
Graphic shows solidly connected grounded conductor that remains whether the
transfer switch is in the normal mode or the standby mode. When the transfer
switch is in the standby mode, the grounded conductor remains connected to the
service grounding electrode. Note the separation and isolation between the
grounding conductor and the equipment bonding jumper/equipment grounding
conductor at the generator
generator. This meets the requirements in 250
250.24(A)(5).
24(A)(5)
11
This slide provides a review of the arrangement of Section 250.30. Presenters
should review the required grounding components for grounded systems as
compared to ungrounded systems. An ungrounded system includes only
ungrounded conductors and as no grounded conductor (a conductor that is
intentionally connected to ground).
12
This slide provides the arrangement of 250.30(B), for ungrounded systems. There is
no grounded conductor in an ungrounded system. Presenters should make the point
about requirements for ground detectors for ungrounded systems.
13
This slide is a graphic representation of a transformer-type separately derived
system showing all elements of 250.30(A). Instructors should use the Code book
and review with the students each of the elements addressed in 250.30(A)(1)
through (8). Make the point that the system bonding jumper connection can be
made in the source enclosure or the first system overcurrent device enclosure.
14
Photo of a transformer separately derived system showing system bonding jumpers,
grounding electrode conductor, grounded conductor, equipment grounding
conductors, and equipment bonding jumpers. This installation uses parallel
conductors according to 310.4.
15
This slide reviews the general provisions in Section 250.30(A). It is important to
review the restriction of grounding connections to the grounded conductor on the
load side of the grounding point. The grounding point can be either at the source
enclosure or at any point up to and including the first disconnecting means or
overcurrent device enclosure.
16
The system bonding jumper shall connect the equipment grounding conductors of
the derived system to the grounded conductor.
This connection shall be made at any single point on the derived system from the
source to the first system overcurrent device or
disconnecting means enclosure.
17
System bonding jumpers are permitted in the source enclosure and the first system
disconnecting means enclosure where doing so does not create a parallel path for
the grounded conductor (neutral current). See Exception No. 2 to 250.30(A)(1).
18
This graphic shows the system bonding jumper in the source enclosure and an
equipment bonding jumper between enclosures that serves as an effective groundfault current path to the source. Note that the grounding electrode conductor
connection to the system is made in the same enclosure as the system bonding
jumper.
19
The photo shows the source enclosure and the first system overcurrent device
enclosure. The transformer secondary connection have to meet the 10 or 25 foot
rules in 240.21.
20
The system bonding jumper is shown in the first system overcurrent device
enclosure. Note the equipment bonding jumper between enclosures serving as an
effective ground-fault current path. The grounding electrode conductor connection to
the system is made in the same enclosure where the system bonding jumper is
installed.
21
Review the definition of the term system bonding jumper in 250.2.
22
Size system bonding jumper according to 250.28(C). Use Table 250.66 or 12.5%
rule as necessary. Note that the system bonding jumper could be any of the types
provided in 250.28(A) where it is part of listed equipment. Where it is a wire-type
conductor, use Table 250.66 or the 12.5% rule where the cm area of the largest
derived phase conductor exceeds 1100 kcmil copper or 1750 kcmil aluminum.
23
The equipment bonding jumper (wire-type) run between the source enclosure and
the first system overcurrent device enclosure shall
be sized in accordance with 250.102(C). The equipment bonding jumper could be a
raceway of any type provided in 250.118.
Use Table 250.66 or the 12.5% rule for larger derived phase conductors supplied by
the derived system
system.
24
Bonding between the source enclosure and the first system overcurrent device
enclosure can be with any of the acceptable equipment grounding conductors
(raceway-types) listed in 250.118. This serves as an effective ground-fault current
path. Workmanship is important here. Fittings must be made up tight.
25
Equipment bonding jumpers of the wire-type must be sized according to
250.102(C). Note that from the secondary of a derived system to the first system
overcurrent device enclosure, the conductors are supply-side. Size the EBJ
according to Table 250.66 or use the 12.5% rule where the cm area of the largest
derived phase conductor exceeds 1100 kcmil copper or 1750 kcmil aluminum. Note
that flexible metal conduit and liquidtight flexible metal conduit have limitations as an
effective ground-fault current path per 250.118(5) and (6). Equipment bonding
jumpers of the wire type must be installed
installed.
26
Review the sizing requirements for the equipment grounding conductor (primary
side) using 250.122 and the equipment bonding jumper (secondary side) using
250.66.
27
Grounding electrode conductor rules in 250.30(A) address single grounding
electrode conductors and a common grounding electrode
conductor used with multiple separately derived systems.
Size the grounding electrode conductor using Table 250.66 based on the size of the
cm area of the largest derived phase conductor
supplied by the system.
system
28
Section 250.8 provides a complete list of methods for connecting grounding and
bonding conductors.
The type of grounding conductor connection or bonding conductor connection must
meet any specific requirements of either
250.30(A)(3) and (A)(4) and the general requirements in 250.8.
29
Presenters should provide a review of using Table 250.66 for sizing the grounding
electrode conductor for separately derived systems.
30
Instructors should review the provisions of 250.66(A), (B), and (C). These sizing
allowances only apply where the grounding electrode conductor is solely connected
to the electrode and no other electrodes are connected to the same grounding
electrode conductor. This slides reviews the provisions for rod, pipe, and plate
electrodes in (A) and concrete-encased electrodes in (B).
31
Section 250.66(C) indicates that a grounding electrode conductor connected to just
a ring electrode does not have to be any larger than the ring. The minimum size
required for a ground ring is 2 AWG in accordance with 250.52(A)(4).
32
A grounding electrode conductor for a single separately derived system shall be
sized in accordance with 250.66 based on the cm of
the largest derived phase conductor
Shall be used to connect the grounded conductor of the derived system to the
grounding electrode as specified in 250.30(A)(7).
33
The graphic shows the grounding electrode conductor connected to the grounded
conductor of the derived system. This connection is made in the source enclosure
with the system bonding jumper. Size the GEC using Table 250.66. The GEC does
not have to be larger than the maximum values provided in Table 250.66.
34
Where more than one separately derived system is installed, it shall be permissible
to connect a tap from each separately derived
system to a common grounding electrode conductor.
Each tap conductor shall connect the grounded conductor of the separately derived
system to the common grounding electrode
conductor.
conductor
35
The grounding electrode conductors and taps shall comply with 250.30(A)(4)(a)
th
through
h (A)(4)(
(A)(4)(c).
)
The common grounding electrode conductor tap concept is addressed in
250.30(A)(4).
The common grounding electrode conductor shall not be smaller than 3/0 AWG
copper or 250
2 0 kcmil
k il aluminum.
l i
Each tap conductor shall be sized in accordance with 250.66 based on the derived
phase conductors of the separately derived system
it serves.
36
Use this graphic to differentiate between the common grounding electrode
conductor and the grounding electrode conductor tap. Remind the students that the
common grounding electrode conductor shall be not smaller than the maximum
values in Table 250.66.
37
All tap connections to the common grounding electrode conductor shall be made at
an accessible location by one of the following
methods:
(1)
A listed connector
(2)
Listed connections to aluminum or copper busbars not less than 6 mm × 50
mm (1/4 in
in. × 2 in
in.))
(3)
The exothermic welding process.
38
Grounding electrode conductor connections required to be listed. Note that for water
pipe electrodes, the connection shall be made within 1.52 m (5 ft) of the point of
entry to the building. The purpose is to reduce the length of water pipe that has to
serve as a grounding electrode conductor.
39
There are also bonding requirements for metal piping systems. Section
250.30(A)(6) refers to 250.104(D) for the bonding requirements.
Metal water piping systems and structural metal that is interconnected to form a
building frame shall be bonded to separately derived
systems in accordance with (D)(1) through (D)(3).
The grounded conductor of the derived system shall be bonded to the nearest
available point of the metal water piping system(s) in
the area served by each separately derived system.
40
Where exposed structural metal that is interconnected to form the building frame
exists in the area served by the separately derived system, it shall be bonded to the
grounded conductor of each separately derived system.
41
The grounding electrode required for a separately derived system are required by
250.30(A)(7). The electrode has to be as near as practicable and preferably in the
same areas as the grounding electrode conductor connection to the system.
42
The grounding electrode has to be the nearest of either a water pipe electrode or a
building frame electrode. If neither is available for use, any of the other electrodes in
250.52(A) shall be used for grounding the separately derived system.
43
Article 100 of the NEC defines the term Grounding Electrode as a conducting object
through which a direct connection to
earth is established.
The grounding electrode establishes and maintains a direct connection to earth.
A list of grounding electrodes is provided in Section 250.52(A).
44
A metal underground water pipe is required to be in direct contact with the earth for
3.0 m (10 ft) or more.
Interior metal water piping located more than 1.52 m (5 ft) from the point of entrance
to the building shall not be used as a part of
the grounding electrode system or as a conductor to interconnect electrodes that
are part of the grounding electrode system
system.
45
Water pipe electrode connection made using a listed pipe clamp. Connection is
made within 1.52 m (5 ft) of the water pipe point of entry to the building.
46
The metal frame of the building or structure that is connected to the earth by any of
the methods in 250.52(A)(2) items (1) through (3).
47
Section 250.30(A)(8) provides sizing rules for the grounded conductor supplied by a
separately derived system.
These sizing rules are similar to the sizing requirements for grounded conductors of
services as provided in 250.24(C)(1).
48
Grounded conductors have to be identified according to 200.6.
Overcurrent devices are generally prohibited in any grounded conductor in
accordance with 240.22.
49
All generators are electrical power sources or systems. How they are grounded
depends on the transfer equipment employed.
50
The photo shows a utility transformer (normal power source for the building) and a
generator (standby power system for the building). There should be some form of
transfer equipment located on the premises for switching from normal to standby
power when the utility source is interrupted.
51
Where the transfer switch for the generator system includes a switching action in
the grounded (neutral) conductor, the generator (system) must be grounded as
required in 250.30(A).
52
If there is a switching action in grounded conductor in the transfer equipment, The
generator has to be grounded as a separately derived system according to
250.30(A). Graphic shows a switch in the grounded conductor. When the transfer
switch is in the standby mode, the grounded conductor is not connected to the
service grounding electrode.
53
Photo shows a transfer switch in an electric room of a building.
54
This photo shows a transfer switch that includes a switching action in the grounded
(neutral) conductor (far right side of switch). Note that the photo was taken with the
equipment shut off. See NFPA 70 for requirements for safety-related work practices.
55
Where the transfer switch for the generator system does not include a switching
action
ti in
i th
the grounded
d d ((neutral)
t l) conductor,
d t th
the generator
t ((system)
t ) iis nott
grounded as specified in 250.30(A).
The grounded (neutral) conductor and the equipment grounding
conductor/equipment bonding jumper must be isolated from each
other in accordance with 250.24(A)(5).
56
Generators are equipment that requires grounding. The frame of a generator is
usually connected to an equipment grounding conductor but can also be connected
to an auxiliary electrode, especially of the generator is located outside. There is a
terminal lug provided on most generators for this purpose.
57
If there is no switching action in grounded conductor in the transfer equipment, the
generator is not grounded as a separately derived system according to 250.30(A).
Graphic shows solidly connected grounded conductor that remains whether the
transfer switch is in the normal mode or the standby mode. When the transfer
switch is in the standby mode, the grounded conductor remains connected to the
service grounding electrode. Note the separation and isolation between the
grounding conductor and the equipment bonding jumper/equipment grounding
conductor at the generator
generator. This meets the requirements in 250
250.24(A)(5).
24(A)(5)
Note that the generator frame is connected to an auxiliary electrode in accordance
with 250.54.
58
This slide provides a high level review of the elements of the grounding and bonding
scheme for an ungrounded separately derived system. Note that there is no
grounded conductor.
59
This slide is a graphic illustration of the grounding scheme for an ungrounded
separately derived system. Note that there is no grounded (neutral) conductor and
system bonding jumper. All other grounding and bonding conductors are required
and must be sized the same way as if it were a grounded system.
60
For ungrounded systems, a grounding electrode conductor, sized in accordance
with 250.66 for the derived phase conductors, shall
be used to connect the metal enclosures of the derived system to the grounding
electrode.
The grounding electrode conductor connection shall be made at any point on the
separately derived system from the source to the
first system disconnecting means.
61
There is no grounded conductor in an ungrounded system. There is an equipment
grounding conductor (primary side), equipment bonding jumper (secondary side),
grounding electrode conductor, and a grounding electrode.
62
The grounding electrode for ungrounded separately derived systems shall comply
with 250.30(A)(7).
This requirement does not apply to portable and vehicle-mounted generators as
indicated in 250.34.
63
This slide serves as the summary of the presentation and a review of the learning
objectives. Separately derived systems are defined in Article 100 of the
NEC. Section 250.20(D) provides key requirements that determine how
and when separately derived system must comply with Section
250.30(A).
Section 250.20(D) outlines the general requirement for grounding a
separately derived system and provides a reference to Section
250.30(A).
Rules for separately derived systems that are not grounded are provided
in Section 250.30(B).
Transfer equipment used for generator systems determines how a
generator-type separately derived system is to be grounded.
64
Rules for separately derived systems that are not grounded are provided in Section
250.30(B).
Transfer equipment used for generator systems determines how a generator-type
separately derived system is to be grounded.
65
This NECA Webinar is based on the National Electrical Code (NFPA 70-2008). This
seminar is the fourth in a series of electrical grounding and bonding Webinars
presented by NECA. This Webinar topic is grounding separately derived systems
such as transformers and generators.
66