[RE-wrenches] Generator bonding discussion
Drake Chamberlin
drake.chamberlin at redwoodalliance.org
Fri Nov 13 09:25:19 PST 2009
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 non–current-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 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.
>>>
>>>· 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:Allan at positiveenergysolar.com>Allan at 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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