ERICO Practical Guide to Electrical Grounding - Manual (Page 89)

81
SELECTION OF GROUNDING
SYSTEM COMPONENTS
The overall effectiveness of any grounding system will be
determined by the individual components that are used to
construct the system and the manner in which the
components are connected. The purpose of this chapter will
be to review the selection of these components and the
methods by which they should be interconnected. Great
care must be exercised in selecting all of the following
grounding components:
• The Grounding Conductors
• The Grounding Electrodes
• The Connectors
THE GROUNDING CONDUCTORS
The NEC contains requirements for both the equipment
grounding conductors (EGC) and the grounding electrode
conductor (GEC). Recall that the EGC is used to connect
the noncurrent-carrying metal parts of equipment,
enclosures, raceways, etc., to the system grounded
conductor and/or the grounding electrode conductor at the
service or source of a separately derived system. The GEC,
on the other hand, is used to connect the grounding
electrode to the EGC and/or grounded conductor at the
service or source of a separately derived system.
EQUIPMENT GROUNDING
CONDUCTORS
Materials:
Section 250-91 (b) lists 11 components which are permitted
to serve as the equipment grounding conductor for both
branch-circuits and feeders. The permissible items are a
copper or other corrosion-resistant conductor. EGC’s are
permitted to be either solid or stranded; insulated, covered,
or bare; and in the form of a wire or a busbar of any shape,
rigid metal conduit, intermediate metal conduit, electrical
metallic tubing, flexible metal conduit where both the
conduit and fittings are listed for grounding, armor of Type
AC cable, the copper sheath of mineral-insulated,
metal-sheathed cable, the metallic sheath or the combined
metallic sheath and grounding conductors of Type MC
cable, cable trays as permitted in Sections 318-3(c) and
318-7 of the NEC, cablebus framework as permitted in
Section 365-2(a) of the NEC, other electrically continuous
metal raceways listed for grounding.
Installation:
No matter what type of EGC is selected, the NEC requires
in Section 300-3 (b) that in general, all conductors of the
circuit, including the EGC must be contained within the
same raceway, cable tray, trench, cable or cord. The purpose
of this requirement is to ensure the impedance of the EGC
remains at the lowest possible value. When the circuit
conductors are run in parallel, as permitted by Section 310-
4 of the NEC, the equipment grounding conductors are also
required to be run in parallel. In these parallel installations
the EGC must be a full sized conductor based on the
ampere rating of the overcurrent protective device
protecting the circuit conductors. The NEC further requires
in Section 250-92 (c) that the EGC shall be installed with
all of the applicable provisions in the Code for the type of
EGC which is selected. In other words, if rigid metal
conduit (RMC) is used as the EGC, as permitted in Section
250-91 (b) (2), the RMC must be installed in a manner that
meets all of the requirements for RMC contained in Article
346 of the NEC. Installers of electrical systems should
understand that when they install a raceway system, such
as RMC, and it is used as an EGC, each length of conduit
is part of the overall equipment grounding system. For this
reason, any terminations at boxes or couplings must be
made up wrenchtight to ensure a low impedance ground
path.
Size:
When the equipment grounding conductor is a separate
conductor, as permitted by 250-91 (b) (1), the size of the
EGC is determined by the rating or the setting of the
overcurrent protective device (fuse or circuit breaker)
which is ahead of the equipment, conduit, etc. Table 250-95
of the NEC contains the minimum size for aluminum,
copper-clad aluminum and copper equipment grounding
conductors. The table includes sizes for circuits from 15-
amperes up to 6000-amperes. The values listed in the table
are based on a maximum circuit conductor length of 100
feet. For conductor lengths longer than 100 ft, an
adjustment in the EGC size may be necessary. Section 250-
95 requires that where the circuit ungrounded conductors
are increased in size to allow for voltage drop, the circuit
equipment grounding conductors must be adjusted propor-
tionately as well.
GROUNDING ELECTRODE
CONDUCTORS
Materials:
The grounding electrode conductor is permitted to be
constructed of copper, aluminum, or copper-clad
Chapter 5: Selection of Components Used In Grounding
Grounding Book 4/14/99 10/5/99 6:02 PM Page 81 (Black plate)