array grounding...that depends... [RE-wrenches]

[Marco Mangelsdorf]

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I don't know if there are many of you out there who have puzzled over the
sizing of the equipment grounding conductor on the DC side of PV systems.
I certainly have.  I had a most illuminating, pun intended, conversation
with John Wiles on this subject this AM that I wanted to share.  

So, how does one decide which size grounding conductor to use from the
array(s) to the inverter?  Well, that depends on the inverter: is it one
that has a ground-fault protection system that physically (either with a
fuse, breaker or contactor) opens the array circuit(s)?  (Which is the case
with the less than 10 kW largely residential-type inverters that I'm aware
of.)  Or is the ground-fault protection system one that simply turns off the
inverter in case of a ground fault detection while not disconnecting the
array circuit(s)? (As is the case with the Xantrex PV series and Ballard 30
kW and 75 kW inverters.)

If your inverter is in category no. 1 (which physically opens the array
circuit(s)), you would use good ole table NEC 250.122 to determine the
grounding conductor size.

If you're using a commercial inverter of the Xantrex PV series or Ballard
type, you would instead use the following protocol: short circuit current of
the array circuit(s) x 1.25 and take that sum and go to NEC table 310.16.

The long and short of it, as I understand it, is that using a residential
inverter, which was designed to be used primarily on "dwellings," allows you
to typically spec out a substantially smaller grounding conductor compared
to the current-carrying conductors.  Whereas working with a commercial
inverter requires a grounding conductor close to or the same size as the
current-carrying conductors.  

aloha,
marco

Marco Mangelsdorf
President
ProVision Technologies, Inc.
69 Railroad Avenue, Suite A-7
Hilo, Hawaii 96720 USA
(808) 969-3281, fax 934-7462
www.provisiontechnologies.com



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