By Mike Holt for EC&M magazine
The number one NEC question that I am asked is: When are neutral-to-ground (I prefer
neutral-to-case) connections required, where are they prohibited, and why?
The purpose of the National Electrical Code is the practical safeguarding of persons and
property from hazards arising from the use of electricity [90.1(A)]. In addition, the
NEC contains provisions that are considered necessary for safety. Compliance with the
NEC, combined with proper maintenance, shall result in an installation that is essentially
free from hazard [90.1(B)].
To prevent a fire, electric shock, improper operation of circuit protection devices, as
well as improper operation of sensitive equipment, the grounding of electrical systems,
the bonding of equipment and circuit conductors must be done in a manner that prevents
objectionable current (neutral return current) from flowing on conductive materials, electrical
equipment, or on grounding and bonding paths [250.6].
This is accomplished by keeping the grounded (neutral) conductor separated from the metal
parts of electrical equipment, except as required for service equipment in 250.24(B) and
separately derived systems in 250.30(A)(1)] in accordance with 250.142.
Interestingly, the NEC allows the grounded-to-case connections for existing ranges, dryers,
and ovens as limited in 250.140 and at meter enclosures located in accordance with 250.142(B)
Required Neutral-to-Case Connections.
Service Disconnecting Means [250.24(B)]. Because electric utilities are not required to
install an equipment grounding conductor, alternating-current services supplied from a
grounded utility transformer shall have a grounded (neutral) conductor run from the electric
utility transformer to each service disconnecting means. The grounded (neutral) conductor
shall be bonded to each disconnecting means enclosure (neutral-to-case connection)
by a screw or strap supplied by the equipment manufacturer [250.28]. Figures 37 and 38
250-24B 01.cdr 250-24B 02.cdr
The grounded (neutral) service conductor shall be sized to safely carry the maximum ground-fault
current likely to be imposed on it from where a ground-fault may occur [110.10]. This
is accomplished by sizing the grounded (neutral) conductor in accordance with Table 250.66,
based on the total area of the largest ungrounded (hot) conductor. In addition, the grounded
(neutral) conductors shall have the capacity to carry the maximum unbalanced neutral current
in accordance with 220.22. Figure 41 250-24B1 01.cdr
Question. What is the minimum size grounded (neutral) service conductor required for a
200A, 3-phase, 480V service where the ungrounded service conductors are 3/0 AWG and the
maximum unbalanced load is 100A. Graphic?
(a) 4 AWG (b) 3 AWG (c) 2 AWG (d) 1 AWG
Answer. (a) 4 AWG, Table 250.66
Danger: If a grounded (neutral) service conductor, which serves as the effective ground-fault
current path is opened or not provided at all, then a ground fault cannot be cleared and
the metal parts of electrical equipment, as well as metal piping and structure steel will
become and remain energized providing the potential for electric shock as well as fires.
Transformers or Other Separately Derived Systems [250.30(A)]. To provide the low impedance
path necessary to clear a ground-fault from the separately derived system, the metal parts
of electrical equipment shall be bonded to the grounded (neutral) terminal (Xo) of the
derived system. The neutral-to-case bond can be made at the source of a separately derived
system or at the first system disconnecting means.
The bonding jumper used for this purpose shall be sized in accordance with Table 250.66,
based on the area of the largest ungrounded conductor. Figure 47 250-30A1 01.cdr
Question. What size bonding jumper is required for a 45 kVA transformer if the secondary
conductors are 3/0 AWG?
(a) 4 AWG (b) 3 AWG (c) 2 AWG (d) 1 AWG
Answer. (a) 4 AWG, Table 250.66
DANGER: If a bonding jumper is not installed from the equipment grounding conductor to
the grounded (neutral) terminal of the derived system, then a ground fault cannot be cleared
and the metal parts of electrical equipment, as well as metal piping and structure steel
will become and remain energized providing the potential for electric shock as well as
fires. Figure 49 250-30A1 03.cdr
Common Improper Neutral-to-Case Connections. The most common improper neutral-to-case
bonds occur in panelboards, separate building disconnects, transformers and generators.
Authors Comment: Neutral current will flow on metal underground water piping system
where the water service to building is metallic. But this only occurs if the underground
water pipe system is metallic and conductive to other buildings. This is not a NEC Code
violation, but I thought you should be aware of this. Figure 19 250-06A 06.cdr
Panelboards Bonding of the neutral terminal to the case of a panelboard, which
is not part of service equipment or separately derived systems, creates a parallel path
for return neutral current. The result is neutral current (net current) flowing on the
metal parts of electrical equipment as well as the grounding and bonding conductors. Figure
14 Figure 250-06A 01.cdr
Connection at Separate Buildings Where an equipment grounding conductor is run
with the feeder conductors to a separate building [250-32(B)(1)], a common and dangerous
mistake is to make a neutral-to-case bond in the separate building disconnect. This ties
the neutral and equipment grounding conductors together allowing objectionable neutral
current to flow on the feeder equipment grounding conductor. Figure 24 250-06A 11.cdr
Separately Derived Systems The neutral-to-case bond for a separately derived system
shall not be made at more than one location because doing so results in a parallel path
for neutral return current.
Transformers If a neutral-to-case bond is made at both the transformer and at the
secondary panelboard, then neutral current will flow through metal raceways and grounding
and bonding path on its return path to the power supply. Figure 15 Figure 250-06A 02.cdr
Generator If the grounded (neutral) conductor in a transfer switch is not opened,
then the grounded (neutral) from the generator will be solidly connected to the utilitys
service grounded (neutral) conductor. Under this condition, the generator is not a separately
derived system, and a neutral-to-case bond shall not be made at the generator or at the
generator disconnect [250.20(D) FPN 1].
If a neutral-to-case bond is made at both the generator and generator disconnect, then
objectionable neutral current will flow through metal raceways and grounding and bonding
path to the power supply.
What are the dangers associated with improper neutral-to-case connections [250.6]?
Improper neutral-to-case connections can create a fire hazard, electric shock and electrocution,
improper operation of protection devices, and power quality issues for sensitive electronic
equipment. Particularly when the neutral is open or it has a high impedance path.
Fire Hazard. Fire occurs when the temperature rises to a level sufficient to cause ignition
of adjacent combustible material in an area that contains sufficient oxygen. In an electrical
system, heat is generated whenever current flows. Improper wiring, resulting in the flow
of neutral current on grounding and bonding paths can cause the temperature at loose connections
to rise to a level that can cause a fire. In addition, arcing at loose connections is
particularly dangerous in areas that contain easily ignitable and explosive gases, vapors,
or dust. Figure 22 250-06A 09.cdr
Electrocution. Death from an electric shock (ventricular fibrillation*) can occur when
the touch voltage is above 30V RMS resulting in as little as 30 milliamperes of current
flowing though the body. This can occur when improper neutral-to-case connections are
made and the neutral is opened.
*Alternating current, particularly 60 Hz disrupts the hearts electrical circuitry, causing
it to go into ventricular fibrillation, which prevents the blood from circulating through
the brain, resulting in death in a matter of minutes.
Improper Operation of Circuit Protection Devices. Nuisance tripping of a protection device
equipped with ground-fault protection can occur if neutral current returns on the equipment
grounding conductor, instead of the neutral conductor because of multiple and illegal
neutral-to-case bonds. A circuit breaker with ground fault protection (480Y/277V, 3-phase
system over 1,000A) uses the residual current method to detect a ground fault. On a 3-phase,
4-wire system, the trip unit will sum the currents in the 3 phase conductors and in the
neutral. When no ground fault is present, the summation of currents flowing on A+B+C+N
will equal zero. Any current flow not equal to zero is considered a ground fault. Where
multiple neutral-to-case bonds have been made, neutral current will flow on the equipment
grounding path. Depending on the impedance of this path versus the neutral conductor path,
the ground fault protective relay may see current flow above its pickup point and cause
the protective device to open the circuit.
If this condition exits (multiple neutral-to case bonds) and a ground fault occurs, the
protection relay might not operate because some of the ground-fault current will not flow
on the equipment grounding conductor (some fault current returns on the neutral conductor
partially bypassing the ground fault protective device).
Power Quality Issues. When objectionable neutral current travels on the metal parts of
electrical equipment because of improper neutral-to-case connections, the electromagnetic
field generated from circuit conductors will not canceled. This uncanceled net current
flowing on metal parts of electrical equipment and conductive building parts causes elevated
electromagnetic fields in the building. These low frequency electromagnetic fields can
negatively impact sensitive electronic devices, particularly video monitors and medical
equipment. For more information about Power Quality as it relates to grounding and sensitive
electronic equipment, visit http://www.mikeholt.com/Powerquality/Powerquality.htm
To protect against electric shock, fires and the improper operation of electrical as well
as electronic equipment from objectionable current on metal parts. Neutral-to-case connections
must only be as permitted by the NEC as service equipment and separately derived systems
in accordance with 250.142.
Mike Holts Comment: If you have any
recommendations on how to improve this article, please let me know: Mike@MikeHolt.com