[RE-wrenches] Fwd: DC wire sizing

[Michael Welch]

Bob O Shultz writes:
> Is it just me being dense or are none of you folks advocating for higher
> VD looking at the savings over time?

I'm not going to touch that with a 10' pole!

> If we assume that Kent's wire costs are correct (and even assuming a 33%
> mark-up, he's paying WAY, WAY too much for wire) , the difference in
> delivered watts between #10 and # 4 wire in this situation is 91W. If
> I were installing this in Southern Oregon, which is pretty average as
> far as peak sun hours/day go, we'd be looking at 91 x 4.5 (peak sun
> hours) x 365days/yr x 25yrs = 3736 KW/H. Even at $0.10/KWH that's
> about $375 AT TODAY'S POWER RATES. Anyone think those rates are going
> to stay the same or go down over the next 25 years? Anybody think they
> won't go up by 5X? 10X? 20X?

That's irrelevant.  You're losing 91 watts, right?  What's the cost of
adding 91 additional watts of solar panels.  Is it more or less than
saving 91 watts in line losses with a larger wire?

You can make this same calculation with everything else, and that was
Ray's comment -- 180 degrees azimuth at latitude inclination is a nice
number.  What Ray says to do about the problem (91 watts of line loss)
is consider several different ways.

If you add panels to make up the 91 watts of line losses, there is a
cost and you make XXX watts.  If you upsize the wiring, you also make
up the losses, the output is the same XXX watts, and the cost is some
value.

Now, pick the lowest cost.

Once you've done that, all the issues you raised are irrelevant.  The
system has an as-built cost, and the exact same power as with the
higher as-built cost is produced.

To get back to Ray's comment about azimuth and inclination, it works
the exact same way.  You can build a fancy racking system to get a
better inclination ($XXX added cost) or you can add panels to make
up for the poor inclination ($YYY added cost).  Once again, pick the
lower of the two.

Each major design decision often has multiple solutions that all
lead to the same results, but have different costs, including real
(money you pay) and intangible (value of not having to run a genny,
value of not having to mess with FLAs, value of keeping running if
one of two charge controllers go out, and so on).

Don't let yourself get too married to a single solution.

Now, consider the comments in the "Voltage Degradation" thread.
Are you designing for the future?  What is the cost of the design
you implement today, and the cost of fixing that design in the future?
Put a dollar cost and value on that and sell the customer on the quality
and durability of the design.

That's the answer to your last question -- who and what (and when)
are you doing this for.
--
Julie Haugh
Senior Design Engineer
greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on
Skype