I've seen all 3 options. This question actually comes up in the wiring
of sub panels, and out buildings as well. I find improperly bonded
wiring all the time that allows current to flow on the ground, because
Neutral was bonded at both ends, with an EGC also run in parallel(option
2 done incorrectly). This is a big No No, and can cause voltages on
exposed metal. Next, my understanding is that option 1 is no longer
allowed on a single service, although that is how all grid connected
houses are wired to the grid and each other with separate meters.
(sorry I'm out of the office, so I don't have a code reference)
As far as different opinions from different AHJs, I've had inspectors
switch mid project, with the first inspector saying option 2 was ok,
then the 2nd inspector required us to later run a GEC back to the GE
after. On option 3, you would always want an EGC running with the
conductors. I have occasionally then not connected that EGC at one end,
if we also had the separate GEC running back to the GE, to avoid a
"lightning loop". Having 2 ground conductors running to the same point
along different paths allows lightning to induce a current in the wiring
through the loop that is created.
It really does get more complicated, when you also consider the
grounding system's secondary purpose of providing lightning
protection. I have a great book from Polyphasor on grounding for
lightning protection that is a whole nother world beyond NEC. They had
grounding systems with dozens of GEs (for communications towers) but
everything was attached in a star pattern to avoid any lightning loops.
Its actually possible to meet both NEC and lightning protection
requirements, but it takes more consideration, and hence the varied
opinions. Not having current running on the ground system however is a
serious safety issue.
Ray Walters
Remote Solar
303 505-8760
On 9/7/19 7:05 AM, Jason Szumlanski wrote:
There doesn't seem to be a definitive and detailed consensus
article/resource out there that addresses this topic. There are
articles that dance around the subject, but little in terms of
practical ways to accomplish grounding/binding with a supply side
interconnection. AHJ's and engineers have all kinds of differing
opinions. As an industry, it seems like we should have a definitive
guide on the subject.
Let's take a simple/common ungrounded inverter (or microinverter)
system that is interconnected using supply side taps inside an
existing load center with main disconnect. From the roof to the PV AC
Combiner you have an appropriately sized EGC, and no GEC because a GEC
is not required in this ungrounded system other than the existing
premises GEC (and GE system). Let's assume the AHJ agrees that no GEC
is required from the roof, which is a whole other subject that drive
me nuts). Coming from the PV AC Combiner panel (if present) or
Inverter you have two current carrying conductors L1 and L2 that are
tapped on the service conductors. The neutral from the PV AC Combiner
or inverter is landed on the existing neutral bus of the same main
load center where the tap is made.
Now what happens with the EGC that comes from the roof to the PV AC
Combiner (or inverter)?
What I have run into, and there may be other opinions, are the
following assertions by AHJs:
* Land the EGC on the PV AC Combiner and stop there. Bond neutral to
ground. The neutral carries the fault current to the system
bonding point in the main service panel.
* Land the EGC in the PV AC Combiner and continue the EGC to the
ground bar in the main service panel. Do not bond neutral to
ground in the PC AC Combiner.
* From the PV AC Combiner, install a GEC directly to the existing
grounding electrode, and do not run an EGC to the main service panel.
* Some combination of the above.
Can anyone point to a clear and concise 3-line diagram that explains
the acceptable options? Hopefully something that and AHJ will
consider, like an IAEI article or NEC Handbook graphic.
Jason Szumlanski
Florida Solar Desing Group
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