Hello Ray,
I agree with Bob-O. Smaller wires between the inverter and the grid may meet
ampacity requirements and your I2R design goal, but high grid-tie voltage
has been a problem. Also, 1% of lost power over the life of a PV system adds
up especially when grid power continues to go up in price. I agree your
reasoning, but I also agree with Feed-In Tariff PV system operators who want
every kWh possible and are willing to pay the up-front cost. I'm sticking
with less than less than 1% wire loss between the inverter and the grid and
less than 2% overall wire loss unless the customer explicitly states that he
wants more loss.
Joel Davidson
----- Original Message -----
From: "Bob-O Schultze" <[email protected]>
To: "RE-wrenches" <[email protected]>
Sent: Wednesday, April 07, 2010 1:50 PM
Subject: Re: [RE-wrenches] DC wire sizing
Ray,
Sorry, I'm not buying that argument over a 25+ year design lifespan.
Also, -and perhaps something many folks don't consider- the NEC requires a
MAX loss of ≤5% over the ENTIRE circuit. That means all the way to the
mains panel. Best practices would require no more than 1.5% VD between the
inverter and the mains so as to avoid any potential of overvoltage
disconnection by the inverter. As it tries to push the current into the
grid, it also pushes up the voltage on that line and on it's own sensors.
Obviously, the larger the current flow and the higher the VD, the worse
the situation could be. Bear in mind that the grid is not held to the same
over and under voltage specifications as your inverter is. 3% + 1.5% is
pushing it just a bit too close for comfort and I still think wasting
watts in wire losses is bad design. We agree that orienting PVs to an
azimuth other than 180° and a bit less than latitude elevation is less
than ideal, but you "run what you brung" in terms of the orientation and
roof pitch of a structure. Better to take a hit on production if we're
talking about the backasswards method of incentivizing by the installed
watt, than not installing PV at all. That said, we DO have control over
wire sizing. IMO, throwing away watts forever just to cheapen it up a very
little bit upfront is poor economy and as the price of gird supplied power
increases over the years, the waste and lost revenue is even more
acerbated.
Best, Bob-O
On Apr 6, 2010, at 6:04 PM, R Ray Walters wrote:
Just run the numbers sometime. Compare the cost difference of #6 vs. #4
wire say, and then look at how many more watts you're actually saving,
then multiply that additional wattage by the installed cost per watt.
Very simply, once you've satisfied code requirements, there is a point at
which it is cheaper to add more panels than use bigger wire.
Also, that point is a moving target that fluctuates with PV and wire
costs. I've found recently for our projects, that that point is falling at
about 3% loss.
I also include a cost factor for oversizing the conduit, extra labor
(bigger wire is harder to handle), and any connectors needed to land the
larger wire.
I've got very well designed systems working for decades, using this
method.
This is how large commercial systems are designed as well.
You can't simply pull 1%, and then call us bad designers because we
actually do an economic analysis for each wire run.
It used to be unheard of to install PV facing anything but due south at
latitude tilt, but now we know to add a few more modules. Same concept.
R. Walters
[email protected]
Solar Engineer
On Apr 6, 2010, at 1:37 PM, Bob-O Schultze wrote:
Ray,
A 2% wire loss is the generally accepted metric for battery based systems
with relatively low PV voltage input (<150Voc). It's just plain bad
design to accept more than a 1% VD on higher voltage systems. PVs ain't
THAT cheap.
Best, Bob-O
On Apr 6, 2010, at 11:44 AM, R Ray Walters wrote:
Once I have fulfilled NEC min. requirements, I use a spreadsheet to
analyze the cost of larger wire vs. the cost of power lost. Going under
2% is usually not worth it, if copper prices are high, and PV cost is low
enough (current market). Sizing for under 2% was good economics a few
years back, when PV was high, and copper was low, though.
For NEC 2011, I agree: while I readily welcome development of DC AFI,
implementing code before the technology is ready, is a bad idea. But that
may be the only way to get the technology in place.....
Ray
On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote:
Ray,
Considering that we design PV wiring to be efficient with voltage (and
power) loss typically less than 2%, the wire size is nearly irrelevant
to arcing issues. Essentially all the energy available from the PV
array can be dissipated in the dc arc. And since the current is
limited by the nature of the IV curve, breakers alone usually won't
clear the fault. The best combiner breakers can do (if you have enough
parallel circuits) is isolate the fault to one string in the PV array.
With one string being 1 or 2 kW in many systems there is still the
potential for a lot of heat.
With the 2011 code just around the corner and no dc arc fault protection
on the horizon, it looks like our industry is again going to have a code
requirement that no one can fulfill.
Kent Osterberg
Blue Mountain Solar, Inc.
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