wire sizing at Higher voltages [RE-wrenches]

Matt Lafferty mlafferty at universalenergies.com
Tue Apr 20 19:00:51 PDT 2004


 

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Ray W wrote:

> Seems like the terminals should go up to a #4 Ga, if SMA's recommendation 
> is to size for less than 1%.

Ray:  (Ya'll are invited, too.)

First of all, I must say that I agree with "Watts loss is more important
than voltage-drop".  I hope everybody here understands the importance of
that.  I also hope all your efforts invested in studying for NABCEP were
fruitful.  On that note, Congrats to everybody who took and passed that
thing this weekend!  Cheers!

With all due respect, in the hundreds of SMA Sunny-Boy applications I've
been involved with, I have never crossed paths with a system configuration
in the real world that even comes close to needing anything over a #10 to
meet the <1% recommendation.  This is not to say that it isn't possible.
Maybe you have an example of one you can share so I'm not afflicted with the
always-possible "tunnel-vision" (I certainly don't know and haven't seen "it
all").  I must qualify my statement with the understanding that I am in a
region of CA that doesn't see those "Premium" low-temp / high-insolation
conditions experienced in other regions so I haven't had the opportunity to
apply those parameters.

I do use the word "need" in lieu of "could use"... Due to the multiple input
terminals on the SB and the versatile mounting capabilities, I haven't seen
a configuration that "needs" it.  This may require bringing 3 Circuits
"Home", but in every case I've seen that's a cheaper and more serviceable
from the ground at the Inverter approach than buying and installing a
Combiner.  SMA has also made sure that at least one Disco Manu has 3-Pole
UL.  Thanks John B!

Look at it from this perspective: Even using high-voltage modules (almost
exclusively high-wattage) which get you into the Input Voltage Window
quicker, you should be able to configure the system to get 3000 Watts STC-DC
at 250 Vmpp into the supplied #10 Terminals and still meet the <1%
recommendation.  

Let's assume a really BAD configuration (Module Volts / Watts / Amps
irrelevant) and an SB-2500... We'll design our Source Circuit for the Input
Voltage Window Floor... 250 VDC-mpp.  This also happens to be a site with a
200 foot Home Run.  (Yikes!!!!!  Look for a different location possibility!
What are you doing designing on the floor?)

At 3000 Watts / 250 V, that's a 12 A circuit no matter how you slice it.
Use XHHW-2 or THWN-2 as a conductor.  3 or less in a Raceway (Table 310.16).

#10 AWG = 40 A.  Derate for 70C = 58%.  Conductor Ampacity = 23.2, nearly
double the required ampacity of the circuit.  OK.  That's initial conductor
"ampacity value".  My calcs show a 2971 Watts effective at the Inverter...
99.0409% of the 3KW input.  My calcs also show a .9684% VD (Did I say
VD???).  Both are <1%.  I haven't found a combination of glass voltage,
wattage, and amps that exceeds this picture and is even remotely viable.

If that doesn't work for your application, I would have to guess that you
are too far from the array or should add at least one module to your strings
to stay off the MPP Floor (I think we all understand this principle... If
not, you really should come to one of my classes...).  You may also be
trying to combine multiple string home-runs in a single raceway, which is
common and totally acceptable to a point... Once you're into 1-1/4" & above,
costs start to exponentially increase.

Another point to consider is the application and intent of a purely
grid-connected inverter... To generate as many kWH as possible whenever the
Sun is shining and the Grid is hot.  There must be a balance between
"textbook" and reality.  Our basing designs on "Manu Spec Sheets" is one
part of that balanced equation (Tongue firmly clenched between teeth
here)... How frequently do we ever see STC?  When we do, how long does it
last?  What is the time-weighted proportional production value of that
compared to an overall time period of say, a year?  (If anybody comes up
with this one, I would recommend a career change to Meteorologist.  Pays way
more and you get to be on TV and nobody calls you on it when you're wrong!
LOL!)  

We should care about Cost per Delivered kWH... I believe you agree with that
principle.

We can come up with and argue about all kinds of "what if" scenarios to
infinity, but that's my girlfriend's role in my life.  (Yeah, strange but
true, she's a PV Engineer and likes to do that to me all the time... What
kind of a match does that sound like?  What else would we do?  LOL)  I'd
just as soon not have to do it with all of you, too... 

Hey, the bottom line is that we must all be responsible and reasonable in
what we do.  There is "book value" and there is "reality".  Everybody's
balancing point is different.  May they always be Honest and To The Best Of
Our Ability And Knowledge.

Two Palms and a Hammock for Everybody!

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

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