[RE-wrenches] Supply Side Connections: Grounding

[Ray]

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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