The National Electrical Code specifies that the proper method of grounding the electrical service of
a building or structure shall be in accordance with Section 250-24(a). Section 250-24(a) directs that
alternating current systems which are required to be grounded by Section 250-20(b), have a grounding
electrode conductor bonded from the service grounded (neutral) conductor to an acceptable grounding
electrode as listed in Sections 250-50 or 250-521 [250-24(a)(1)], Figure 1.
Note: Illustrations not available on the Internet.
1 An acceptable grounding
electrode system would include [250-50]:
- Underground metal piping electrically continuous for a minimum of 10 feet suplemented with ground
- Effectively grounded metal frame of a building.
- Bare, galvanized, or other electrically conductive coating, reinforcing steel rods not smaller
than ½ inch in diameter if the total length of the steel is not less than 20 feet.
- Ground ring encircling the building or structure in direct contact with the earth at a depth below
the earth’s surface of not less than 2½ feet consisting of at least 20 feet of bare coper conductor
not smaller than No. 2.
Neutral-to-Ground Connection – Section 250-24(a)(1)
The grounding electrode conductor, which is used to bond the service
grounded (neutral) conductor to the grounding electrode, can be installed at any accessible location
from the load end of the service-drop or service-lateral up to and including the service disconnecting
means, Figure 1.
CAUTION: Some inspectors insist the connection to the grounded (neutral) conductor originate from the meter
socket enclosure. Other inspectors prefer that the connection originates from the service disconnect
enclosure. The Code states that either location is acceptable.
Objectionable Neutral Current – Section 250-24(a)(5).
A neutral-to-ground connection shall not be made on the load side of
the service disconnecting means except as permitted for separately derived systems [250-30(a)(1)],
separate buildings or structures [250-32(b)(2)], or meter enclosures [250-142 Ex. 2]. A neutral-to-ground
connection at any other location will cause objectionable neutral current to flow on the equipment
grounding conductor (conductive metal parts of the electrical equipment). This violates Section 250-6(a)
and can create an electric shock or fires, as well as power quality problems, Figure
Author’s Comment:The dangers and power quality problems of improper neutral-to-ground connections
were covered in the previous issue of Power Quality magazine.
Grounded (neutral) Conductor Required - Section 250-24(b).
For the metal parts of the electrical equipment to be safe from electric shock, the
electrical equipment must be electrically connected (bonded) to the power suply grounded (neutral)
conductor, typically at the transformer. The low impedance path necessary to clear phase-to-ground
faults is accomplished by bonding the equipment grounding conductors (metal parts of the electrical
system) such as metal enclosures, rigid metal conduit, intermediate metal conduit, electrical metallic
tubing, armored cable, etc., to the grounded (neutral) service conductor [Sections 250-2(b) and 250-118].
At service equipment (the building disconnect), the low impedance fault return path is established
by bonding the metal service disconnect enclosure to the grounded (neutral) service conductor within
the service disconnecting means. This neutral-to-ground connection is accomplished by the installation
of a main bonding jumper (screw or strap) [250-28], which is suplied by the equipment manufacturer
[384-3(c)], Figure 3.
DANGER If the conductors from the electric utility do not include a grounded (neutral) conductor
[violation of Section 250-24(b)], then the earth must serve as the fault return path [violation of
Section 250-2(d)]. Under this condition, if there is a phase-to-ground fault, the high impedance path
of the earth will not allow sufficient fault current to flow to open the circuit overcurrent protection,
and the metal parts of the electrical system would remain energized practically at line voltage, Figure