Grounding Services
Presenter: Michael J. Johnston
NECA Executive Director,
Standards and Safety
Objectives:
• Review general requirements for grounding electrical
services and service equipment
• Review requirements specific to services supplied by
grounded utility systems (sources)
• Review requirements specific to services supplied by
ungrounded utility systems (sources)
• Review requirements for grounded conductors, main
bonding jumpers, and equipment bonding jumpers
• Review essential conductors and connections for
grounding electrical services.
General Grounding Provisions
• Electrical systems are grounded (connected to
ground) in a fashion that limits voltages imposed by
lightning, line surges, and contact with higher voltage
lines.
• Grounding electrical systems stabilizes to voltage to
ground during normal operation. [250.4(A)(1)]
• Grounding equipment limits the voltage potential to
ground by keeping the grounded conductor and
conductive parts of equipment at or as close to earth
potential as possible. [250.4(A)(2)]
Definition of Service
• The conductors and equipment for delivering electric
energy from the serving utility to the wiring system of
the premises served.
• The definition of service includes the statement that
electric energy to a service can be supplied only by
the serving utility.
• If electric energy is supplied by other than the serving
utility, the supplied conductors and equipment are
considered feeders, not a service.
Services Supplied by Grounded Source
• Grounded conductor is required to be brought to the
service equipment.
• The grounded conductor must be routed with the
ungrounded phase conductors.
• The grounded conductor must be connected to the
service equipment enclosure.
• The grounded conductor must be connected to the
earth by a grounding electrode conductor.
• The grounded conductor must be sized not smaller
that required by 250.24(C).
Services Supplied by Ungrounded Source
• The utility system or source is ungrounded.
• No grounded conductor is required to be brought to
the service equipment.
• The service equipment is required to be grounded.
• A grounding electrode conductor connects the
equipment enclosures to a grounding electrode
system meeting the requirements of Article 250,Part
III.
Grounding Electrode System Required
• All grounding electrodes specified in 250.52(A) that
are present at each building or structure service shall
be bonded together and used as the grounding
electrode system for the service.
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Metal water pipe
Structural metal frame electrodes
Concrete-encased electrodes
Ground rings
Other electrodes such as rods, pipes, or plates
Conductors at Grounded Services
• Four conductors must be present for proper grounding
at electrical services supplied by grounded utility
sources.
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Grounded conductor
Main bonding jumper
Grounding electrode conductor
Equipment grounding conductor
• Each of these conductors will be covered separately.
Grounded Conductor Requirements
• Required to be run to the service equipment
enclosure.
• Required to be bonded to the service equipment
enclosure.
• Required to meet the minimum sizing requirements in
250.24(C).
• Overcurrent devices generally not permitted in the
grounded conductor.
• Must be identified in accordance with 200.6.
Grounded (Neutral) Conductor Functions
• Carries the maximum unbalanced current on the
neutral of the systems served.
• In ground-fault conditions, the grounded (neutral)
conductor at each service functions as an effective
ground-fault current path.
• It must be run to each service disconnecting means
for this reason [250.24(C)]
Main Bonding Jumper
• The main bonding jumper connects the grounded
service conductor to the equipment grounding
conductor (enclosure) at the service equipment.
• Main bonding jumper can be a wire, bus, screw, or
other suitable conductor. [250.28(A)]
• Main bonding jumpers in listed equipment can be
used without calculating size.
• Wire-type main bonding jumpers shall be sized
according to the provisions in 250.28(D) which
indicates use of Table 250.66 or the 12.5% rule for
larger services.
Suitable for Use as Service Equipment
• Equipment is required to be suitable for use as service
equipment. [230.66]
• Service equipment includes grounding and bonding
provisions for use as service.
• Main bonding jumper is part of listed equipment and
can be used without calculation of size.
Grounding Electrode Conductor
• The grounding electrode conductor connections at the
service shall meet the provisions in 250.24(A)(1)
through (A)(5).
• Connect at an accessible location at any point from
the load end of the service drop or lateral to an
including the grounded conductor terminal bus at the
service disconnect. [250.24(A)(1)]
• Where the service transformer is outside the building,
an additional grounding connection shall be made at
the transformer or elsewhere outside the building.
[250.24(A)(2)]
Load-Side Grounding Connections
• The grounded conductor generally shall not be
connected to normally non-current-carrying metal
parts of equipment, to the equipment grounding
conductor, or be connected to ground (earth) on the
load side of the service disconnecting means.
[250.24(A)(5)]
• Note that the grounded conductor is permitted to be
used for grounded on the supply side of the service
disconnect as indicated in 250.142(A).
• Separation generally required beyond that point to
control normal current present in the grounded
conductor (often a neutral).
Equipment grounding Conductor
• The last conductor that will be present in the service
equipment is the equipment grounding conductor,
which could be the service equipment enclosure.
• All four conductors have to be installed and connected
properly within the service equipment enclosure.
Service Bonding Requirements
• Service bonding rules are provided in 250.92.
• The following conductive parts of equipment are
required to be bonded together:
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Service raceways
Cable trays
Cable bus framework
Auxiliary gutters
Service cable armor or sheath
• Note: The bonding on the supply side of the service
must be robust (strengthened method) due no
overcurrent protection ahead of the service.
Service Bonding Requirements
• Service bonding rules are provided in 250.92 (cont).
• The following conductive parts of equipment are
required to be bonded together:
– Service equipment enclosures
– Meter sockets or metering equipment enclosures
– Raceways that join metering equipment to the service
equipment enclosures
• Note: Some service equipment includes the meter and
service disconnect in the same enclosure.
Bonding Methods
• Electrical continuity of conductive parts at service
equipment shall be ensured by one of the following
methods:
– Using the grounded conductor
– Threaded couplings or threaded bosses on enclosures
– Threadless couplings and connectors for metal raceways
and metal-clad cables
– Other listed devices, such as bonding-type locknuts,
bushings, or bushings with bonding jumpers
Bonding Methods (cont.)
• Bonding jumpers shall be installed around concentric
or eccentric knockouts that are punched or otherwise
impairing effective bonding connections between
conductive parts.
• Note that standard locknuts are not permitted as to
sole bonding means required by Section 250.92(B).
Grounded Conductor for Bonding
• Permitted on the supply side of the service disconnect
in accordance with 250.142(A).
• Section 250.92(B)(1) indicates a method of bonding
at the service equipment using the grounded
conductor (usually a neutral conductor).
• The method of bonding is common between a
separate meter socket enclosure and the service
disconnecting means enclosure.
Supply-Side Equipment Bonding
• Metallic parts shall be bonded together using one of
the methods in 250.92(B).
• Equipment bonding jumpers (wire-types) on the
supply side of the service disconnecting means shall
be sized not smaller than required by 250.102(C).
• Use Table 250.66 or 12.5% rule for larger services.
• Connections shall be in accordance with Section
250.8.
Other Service Bonding Connections
• Metal water piping systems must be bonded in
accordance with 250.104(A).
• Other metal piping, including metal gas piping
systems are required to be bonded in accordance with
250.104(B).
• Structural metal building frames must be bonded in
accordance with 250.104(C).
Summary for Grounded Services
• Four conductors have to be installed and connected at
the service equipment for proper grounding and
bonding:
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Grounded conductor (usually a neutral)
Main bonding jumper
Grounding electrode conductor
Equipment grounding conductor
Ungrounded Services
• Rules for services supplied by ungrounded systems
(utility sources) are provided in 250.24(E).
• Grounding electrode conductor is required to connect
the equipment to a grounding electrode system
meeting the requirements of Part III of Article 250.
• This connection shall be made at any accessible point
from the load end of the service drop or lateral to the
service disconnecting means.
Ungrounded Services (cont.)
• A minimum of three conductors must be present in
the service disconnecting means enclosure as follows:
– Grounding electrode conductor
– Supply-Side Equipment bonding jumper(s)
– Equipment grounding conductor(s)
• Note: More bonding conductors may be required for
water pipe bonding, other pipe bonding and bonding
structural metal.
• Sizing requirements are the same as those for
grounded services.
Summary for Grounded Services (cont.)
• Grounded services include a grounded conductor that
must be:
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Routed to the service disconnect enclosure
Bonded to the service disconnect enclosure
Sized in accordance with 250.24(C)
Identified in accordance with 200.6
• Bonding at service equipment shall meet the
requirements in 250.92(A) and (B).
• Other metallic parts to be bonded include metal water
piping, other metal piping, and structural steel.
Summary for Ungrounded Services
• Ungrounded services do not include a grounded
conductor.
• Must have a grounding electrode conductor connected
to a grounding electrode system meeting the
requirements in Part III of Article 250.
• Bonding at service equipment shall meet the
requirements in 250.92(A) and (B).
• Other metallic parts to be bonded include metal water
piping, other metal piping, and structural steel.
Grounding Services
Presenter: Michael J. Johnston
NECA Executive Director,
Standards and Safety