Long distance DC feeders [RE-wrenches]

[Matt Lafferty]

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William M asked:

>  Have any of you fed DC for Sunny Boy inverters distances on the order 
> of
800 feet?  I suppose one would need a DC disconnect at both ends for safety 
purposes (to service feeder, for example).

And, the answers are.... Yes, and..... You are correct sir.

(Actually only 600' Underground, but I'd have to say the Canal Crossing part
of that one has to count for something!  Best advice? Stay away from
canals!)

Operation has been fine (since we got PG&E to clean up their act!!!!!!), AC
kWH output within a penny of pre-install projections at the 1-year mark even
with about 3 months of PG&E line issues.  Operational for 17+ months at this
point.  Our application required #6 conductors to stay just under 2%
Vdrop... A little high for my comfort, but I wasn't the final call....  (6
Sunny-Boys with all the DC in the same conduit for the underground).
Nevertheless, the kWH #'s might suggest that I'm a little on the
conservative (did I say that?) side.

Your wire sizing will be different depending on # current carrying
conductors in the raceway, exposure of the raceway &/or conductors, Impp &
Vmpp of your circuits.  Of course...

As far as disconnects go.... Put 'em at both ends!  Oh, I'd be sure and
install a supplemental grounding electrode at the Array.  Carry a full-size
grounding conductor between array and the SB / Service Electrode... If the
Service Electrode is questionable, marginal, or un-verifiable, I'd put in
another Supplemental at the equipment and bond to that as well.... Full-Size
Grounding Conductors throughout!

IMHO, as long as you size it all correctly and install it well, you will end
up with a reliable system.  As always, build the safety into your design. 

I would strongly consider locating the inverter(s) local to the array if at
all possible.  This recommendation grows stronger, the more inverters you
will be installing for practicality and safety  It's generally less expesive
all the way around to run three AC Wires, 2 Hots & a Ground, than it is to
run multiple circuits of DC & the Ground when you start adding length to a
run.  Primary to the safety issue is the GFP (bonded system conductor)
relationship when servicing...  

I have concerns about "safety" with this kind of application from a
"post-install service / maintenance" perspective.  Just because the DC
Disco(s) are open, does not remove the GFP link between Neg & Gr.... If a
service-person (whoever that happens to be some time down the road) does not
understand and follow the correct procedure for servicing arrays of this
nature, i.e. remove GFP after opening DC Disco and waiting the recommended 5
minutes.... Well, that same person will essentially become a minimum 1 amp
fuse if they screw up....  In the case of a multiple inverter system, even
if they remember to remove the GFP from the "system I thought I was working
on", the parallel relationship of the other inverter GFP's to the equipment
grounds creates a higher amp potential and creates some interesting
"acidental series connection" possibilities between the "system being
serviced" and the other systems... Not to mention the "well, I had voltage
on that end but it's not really VOC" head-scratching situations that helped
me learn all this in the first place years ago.  (This whole principle
applies no matter where or how far the glass is from the inverter...
Sunny-Boy or otherwise.  Your application's distance relationship adds some
additional factors)

Even if this person is qualified and knows but just "forgot to do it before
going out to the array", they aren't necessarily gonna be inclined to take
an 800',,,, errrrrrr 1600' hike back & forth and will just "chance it"....
Bet me!  If there's current flowing and they don't see it before it bites
'em, well, you know how that feels, I'm sure.  I would ALMOST recommend a
straight-up "cheat" and run Pos & Neg through the Array-End DC Disco(s)....
For safety's sake... The NEC is one thing... Reality in practice is another!
I'd even be inclined to put it right in my one-lines and permit stuff that
way and argue it on principle and effectiveness if called on it... Dollars
to doughnuts, I'd win that one with an Inspector  (Note the doughnuts!)   

Which leads to the question of "why don't we just do it all the time?"
Which leads to the "Gee... I dunno... Maybe 'cause this is the US and we
just have to be different from the rest of the world..." answer.  Which
leads to... Ahhhh, never mind!

This provides a strong case for non-grounded DC systems in my opinion.
Especially in this type of example!

Just some thoughts to consider... Good luck William... I know you will come
up with the rightest answer for this application!

Peace & Two Cents to All!

Matt Lafferty
mlafferty at universalenergies.com
(916) 422-9772 Office
(916) 914-2247 Fax
(916) 628-7694 Cell
www.universalenergies.com

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