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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.<br>
<br>
The older codes only allowed using the neutral to be grounded at
separate structures <i>if there was no other metallic path between the
buildings</i>. 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 "<i>no
other metallic path between the buildings</i>" 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.<br>
<br>
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.<br>
<br>
NEC 250.34(B) says the same thing for vehicle mounted generators that
are bonded to the vehicle.<br>
<br>
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. <br>
<br>
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.<br>
<br>
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 <i>solidly</i> 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.<br>
<br>
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.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar<br>
<br>
<br>
Drake Chamberlin wrote:
<blockquote
cite="mid:20091112150812.017B0181090@friskymail-a3.g.dreamhost.com"
type="cite">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. <br>
<br>
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 <br>
<br>
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. <br>
<br>
Drake <br>
<br>
<br>
<blockquote type="cite" class="cite" cite="">Content-Language: en-us<br>
<br>
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 can’t
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.
<br>
<br>
· 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.”<br>
<br>
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.<br>
<br>
<br>
Allan Sindelar<br>
<a moz-do-not-send="true"
href="mailto:Allan@positiveenergysolar.com">
Allan@positiveenergysolar.com</a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
<b>Positive Energy, Inc.<br>
</b>3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
505 424-1112<br>
<a moz-do-not-send="true" href="http://www.positiveenergysolar.com/">
www.PositiveEnergySolar.com</a><br>
<br>
Larry,<br>
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.<br>
<br>
Solutions (unfortunately none of these may be suitable and they all
have
potential problems):<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
See. I don't have any good solutions.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar</blockquote>
<br>
</blockquote>
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