Inverter Output Circuit and DC GEC requirements [RE-wrenches]

[Bill Brooks]

Drake,

If I understand your situation correctly, you are saying that the inspector
required you to run an Equipment Grounding Conductor (EGC) from the module
frames to the ground rod. This is not a code requirement, but it can have
some benefit to reduce the effects of lightning. This raises the question of
whether or not that ground rod should be tied to the main house ground. I
believe good practice is to tie that frame grounding electrode to the main
house ground with the size of conductor that runs from the main ac panel to
the existing grounding system (usually 6AWG or 4AWG). I would run that bare
copper wire in the trench outside the conduit that carries the plus, minus,
and dc equipment grounding conductors to the inverter.

I have heard of inspectors requiring a ground rod at the array and not
requiring that ground rod to be tied to the main house ground. This does not
make any logical sense to me given the reason for grounding electrodes and
why we have grounding electrode conductors (GEC) for SYSTEM grounding, and
equipment grounding conductors (EGC) for EQUIPMENT grounding.

Basic rule is that all grounding electrode be tied together to form a
grounding electrode system so that the dc and ac going in and out of the
inverter are connected to the same grounding electrode system. This is not a
code requirement, this is good practice and healthful for the inverter.

It is absolute wrong to take a grid-tied inverter, most of which have
internal ground fault protection, and tie the grounded conductor (usually
negative unless you have a SunPower array) at the array unless you employ
grounding of separate structures. To ground the grounded conductor at the
array (and by default it is always grounded at the inverter), there can be
no equipment grounding conductor run from the array to the inverter. Any
combiner box with overcurrent protection would need to be mounted at the
array, all grounded conductors grounded to the same ground rod as the
equipment ground, and then non-metallic conduit, no equipment ground, and
only positive and negative conductors run in the conduit to the inverter.
This essentially defeats the ground fault protection in the inverter. It is
arguable whether this is better for lightning protection, but it can save a
lot of copper if the wiring run is lengthy from the array to the inverter.
The problem with this method is that most inspectors will scratch their
heads and insist on an equipment grounding conductor in the conduit with the
plus and minus. This defeats the purpose of the separate electrode. If an
equipment grounding conductor is in the conduit, all electrodes need to be
tied together and the negative grounded conductor can ONLY be attached at
the inverter--no exceptions.

I know that probably was more than you were asking, but there is so much
confusion on this issue, we need to get this straightened out. Summarizing
the best approach--install a ground rod at the array, tie the frame grounds
to that ground rod, tie the array ground rod to the main house ground rod
with a bare conductor in the trench, and only tie the negative to ground (to
that grounding electrode conductor that is buried in the trench running by
the inverter) at the inverter. Both dc and ac sides must have an equipment
grounding conductor in their respective conduits--is that simple enough?
Good luck working with your inspectors, but no inspector worth their salt
should have anything negative to say about this approach. 

Bill.

Bill Brooks, PE
Brooks Engineering
873 Kells Circle
Vacaville, CA 95688
office and mobile phone: 707-332-0761
fax:707-451-7739
email: billbrooks7 at yahoo.com
web: www.brooksolar.com

Ecclesiastes 11:7 Light is sweet, and it pleases the eyes to see the sun.


> -----Original Message-----
> From: Drake Chamberlin [mailto:Drake.Chamberlin at redwoodalliance.org]
> Sent: Wednesday, June 20, 2007 12:41 PM
> To: RE-wrenches at topica.com
> Subject: RE: Inverter Output Circuit and DC GEC requirements [RE-wrenches]
> 
> 
> Hello Bill,
> 
> We recently had an inspector require us to run a GEC to a ground rod
> from a grid tie only system.  Usually, this has not been required in
> systems where we don't have batteries and the DC grounding point is
> internal to the inverter.   The conductor was only able to make an
> equipment ground, not a connection to the DC grounding.
> 
> Is it usually necessary to run a GEC to a rod with grid tie only inverter?
> 
> Thanks,
> 
> Drake 


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