Kent,
Thanks for helping sort out this issue. At a
minimum, the majority of the portable generators
being connected to wiring systems today are not
code compliant. Fortunately, the connections
haven't created incidents, since these systems
have become very common. Off grid installations must be in the minority.
Over the past several years, many major storms
have left millions of people without
power. Small generator sales have
skyrocketed. Utility companies pleaded with
people not to backfeed breakers, but to use a
transfer switch. Now, as it turns out, the 4
wire connections to wiring systems via transfer
switches, provided by commercially available
generator inlet boxes, create a code violation do to parallel neutral paths.
Maybe portable generators should be arranged like
mobile generators, where available lugs make
bonding the neutral to frame (or not) very
simple. Hooking up a 220 KVA mobile generator, for a concert, was really easy.
If the chasis could be un-bonded easily,
permanent rod could be installed, that the
portable generator could hook to when moved to
its backup power location. Or, maybe the NEC
should be amended to allow people who have lost
power in emergency situations, to connect to a
portable generator. Maybe even extend this to
solar backup systems, if no real world hazards can be demonstrated.
I'm still not convinced that it is not legal to
hook into a grounded and bonded wiring system, so
long as the equipment grounding conductor is
omitted. Here is the code section on portable generators:
____________________________________________________________________________________________
250.34 Portable and Vehicle-Mounted Generators.
(A) Portable Generators. The frame of a portable
generator shall not be required to be connected
to a grounding electrode as defined in 250.52 for
a system supplied by the generator under the following conditions:
(1) The generator supplies only equipment
mounted on the generator, cord-and-plug-connected
equipment through receptacles mounted on the generator, or both, and
(2) The normally noncurrent-carrying metal
parts of equipment and the equipment grounding
conductor terminals of the receptacles are connected to the generator frame.
____________________________________________________________________________________________________
Nothing here says we can't connect to a wiring
system via cord and plug. If we omit the EGC,
which runs in parallel with the neutral, parallel
neutral paths don't exist. Maybe it is inferred
somewhere else, or enforced by
interpretation. RV and marine inverters do
switch the neutrals, creating separately derived systems.
Up until lately, it was permitted to ground the
frames of electric ranges and clothes dryers with
a neutral wire. Although this is not permitted
for new systems anymore, I don't know of any
incidents that have occurred. If there is no
hazard involved, maybe the NEC could accept 3
wire connections for 120/240 V portable generators.
It would be good to find a solution to this issue
that does not require hundreds of dollars of
additional equipment and installation time,
especially if no real dangers exist. If hazards
to exist, equipment needs to be developed to address them.
Drake
At 01:06 PM 11/12/2009, you wrote:
Drake, I agree with you that there may not be a
code compliant way to connect a portable
generator other than to use a transfer switch
that also switches the neutral. And
non-the-less many portable generators are being
used for backup power without a switched neutral -- without incident.
The older codes only allowed using the neutral
to be grounded at separate structures if there
was no other metallic path between the
buildings. It was safe, still would be safe and
I think it is a mistake for the code to be
changed to disallow it. But the "no other
metallic path between the buildings" requirement
was often ignored. It is pretty rare to see
metallic water pipes these days; but phone lines
(soon to be history too), gas lines, intercoms,
and so on are often added after word. And when
they are grounded at both buildings, they carry
a share (a small portion) of the neutral current.
NEC 250.34(A) says a grounding electrode isn't
required if 1) the portable generator supplies
loads connected by plug and cord and 2)
non-current carrying metallic parts and
equipment grounding terminals of the receptacles
are connected to the generator frame. Sorry,
Drake, the intent here is that an EGC is
used. Using the neutral to connect the equipment grounds doesn't comply.
NEC 250.34(B) says the same thing for vehicle
mounted generators that are bonded to the vehicle.
NEC 250.34(C) requires the generator neutral to
be bonded to the generator frame if the
generator is a separately derived system. If
the generator can be used as a portable power
source, it will be a standalone separately
derived system and the neutral and ground must
be bonded. Despite advise seen on this list
serve, the neutral ground connection on a
portable generator should not be disconnected.
There is a FPN in 250.34(C) refering to
250.20(D) for grounding portable generators
supplying fixed wiring systems. NEC 250.20(D)
covers system grounding for separately derived
systems, that is when the generator neutral
isn't solidly connected to the service-supplied
neutral and the load neutral is switched between
the two systems. This is the code compliant
method to connect a portable generator to a
fixed wiring system. Unfortunately the
inverters we use aren't set up to switch the
neutral; they aren't really set up to connect to
a portable generator at all. And yet many
portable generators are being connected to
inverter systems and also directly to buildings
for backup power -- without incident.
The FPN in 250.20(D) talks about on-site
generators that are not separately derived
systems and defines them as having a grounded
conductor that is solidly interconnected to the
service supplied grounded conductor (then the
generator neutral ground bond is omitted). Most
inverters are set up for this type of generator
connection: a fixed on-site generator than can
have a solid neutral bond between the different
power sources. Yet, often, the customer wants
to use portable generator. Hence our catch 22.
I like Alan's work around using the
autotransformer as a generator balancer thereby
forcing the neutral current to zero. It is an
expensive piece to add to a small
generator. But the Outback x-240 is only large
enough to use for a small generator.
Kent Osterberg
Blue Mountain Solar
Drake Chamberlin wrote:
There may be no totally compliant way of using
the internally-bonded-neutral generators, but
they are being used a lot in both solar
applications and emergency generator backup
systems. Inlet receptacles and transfer
switches are being sold in quantity for use
with cord and plug connected generators, most of which are internally bonded.
Using a 3 to 2 prong adaptor might be the best
solution where code rigidity is not the
norm. In the old code, the neutral could be
separately bonded at different structures under
the same management. Using the adaptor, the
neutral bonds to the frame of the generator, so
other receptacles on it will still be
legitimate to use in accordance with 250.34
The building will get its ground through the
grounding electrode system of the
building. There are no parallel neutral
paths. All will have normal safety feathers
and will function well. It may or may not be
"code." Check out 250.34. I think there is
some argument for this method being compliant.
Drake
Content-Language: en-us
The problem of the bonded chassis in most
generators has been with us for years, and
there is no real solution. With most cheap
(and ubiquitous) portables, the AC neutral is
internally bonded to the chassis and cant be
easily undone. When I wrote the HP generator
article I attempted to address this issue
without getting too technical. I ran this
specific issue by John Wiles for his advice,
and the final wording reflected his position
that the AC ground loop that is, maintaining
the neutral-ground bond in both the generator
and the inverter system and accepting that the
ground conductor would carry a portion of any
current otherwise being carried on the neutral was the best of the options.
· The final wording as submitted to HP
was: Quality generators allow the neutral
conductor to either connect to the generator
chassis (for prime power application) or
remain separate from it. Portable generators
are seldom properly grounded, so manufacturers
ground the neutral output conductor to the
chassis. When connected into a grounded power
system, however, this presents a safety
hazard, as the neutral is now bonded to the
ground at two placesone in the power system
AC and one in the generatorand the safety
ground wire becomes current-carrying. Some
units also include AC ground-fault protection,
which is incompatible with connection to a
grounded power system. There is no simple,
Code-compliant solution to this. The safest
noncompliant approach is to bundle an
insulated green ground conductor with the
power conductors between the generator and the
main ground buss, to both ground the chassis and minimize shock potential.
Inherent in this is that that grounding
conductor between gennie and ground buss be
bundled in conduit and boxes, rather than
being left bare or exposed, as would otherwise
be OK with any grounding conductor.
Allan Sindelar
<mailto:al...@positiveenergysolar.com>al...@positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
<http://www.positiveenergysolar.com/>www.PositiveEnergySolar.com
Larry,
Sounds like there are two neutral ground
bonds: one at the generator and one at the
house. This causes part of the neutral
current to flow on the ground - a guaranteed
way to trip the GFI. This is the catch 22 of
connecting a portable generator, or one with
an outlet, to a house system. The house panel
has to have a neutral ground bond. The
generator also has to have a neutral ground
bond (Although the small Honda and Yamaha
inverter generators don't. How do they get
away with that? - No exposed metal to touch I
guess.) or the GFI won't work and ground
faults won't trip the generator output breakers.
Solutions (unfortunately none of these may be
suitable and they all have potential problems):
Permanently wire the generator. On the
generator, remove the neutral/ground bond and
remove all of the electric outlets. In some
locations, you aren't allowed to permanently
wire a generator unless it is U/L listed. And
most (maybe all) portable generators aren't U/L listed.
Replace the GFI outlet with a non-GFI outlet
(so it will be like the old Homelight
generator). Part of the neutral current will
flow on the ground wire. That's a code
violation, but for 20 and 30-amp outlets the
ground wire has adequate ampacity so nothing
is going to overheat. In fact, even 6 AWG SO
cord has a ground wire the same size as the conductors.
Use a 2-prong cheater. This will prevent the
GFI from tripping. It also prevents any
ground fault past the cheater from returning
to the generator. It's probably best to
locate the cheater at the load end of the
cord; that way if the cord is damaged the GFI
will still trip. If there is ground fault
past the cheater, there will be one or
probably more than one locations where there
is shock hazard. If you touch the generator
frame, it'll tingle; if the ground is wet, it might be severe.
Use a mobile-style inverter that will switch
the neutral/ground bond. Most of these are
not U/L for house wiring. I think this is the
best solution but it doesn't cover the
circumstance of a grid backup inverter connected to a portable generator.
See. I don't have any good solutions.
Kent Osterberg
Blue Mountain Solar
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Drake Chamberlin
Athens Electric
OH License 44810
CO License 3773
NABCEP TM Certified PV Installer
Office - 740-448-7328
Mobile - 740-856-9648
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