<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div>
<span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div>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.</div><br class="Apple-interchange-newline"></span>
</div><div>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.....</div><div><br></div><div>Ray</div>
<br><div><div>On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
<div bgcolor="#ffffff" text="#000000">
Ray,<br>
<br>
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.<br>
<br>
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. <br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
R Ray Walters wrote:
<blockquote cite="mid:422455DC-C204-4373-A4D3-5AD0C19FA72C@solarray.com" type="cite">I agree that we don't want to create the code first, and
try and develop the product after. On the other hand, if a DC AFI can
be developed that could stop some of the problems I've seen breakers
not help, I'm installing them, and pushing for code requirements.
<div>AC GFIs were gimmicky too at first, but now have gone on to save
countless lives; usually kids, but a few of us wet booted contractors
too.</div>
<div>As far as running things at 100%, I do agree with you, but I
also think that 156% over rating in many cases is too much. If the wire
and breaker are that oversized, it is less likely to trip when you want
it to. </div>
<div>My most recent damage I saw, the cable a few inches back was in
no way damaged, as it only saw array short circuit current, but the
connector that arced burned up a whole circuit board.</div>
<div>More oversizing would have only increased the arc potential, and
reduced the chances of a breaker tripping.</div>
<div>Proper sizing (not too big, not too small) is the way.</div>
<div><br>
<div><span class="Apple-style-span" style="border-collapse: separate; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-indent: 0px; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; ">
<div>R. Walters</div>
<div><a moz-do-not-send="true" href="mailto:ray@solarray.com">ray@solarray.com</a></div>
<div>Solar Engineer</div>
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</div>
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<br class="Apple-interchange-newline">
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<div>On Apr 6, 2010, at 9:16 AM, robert ellison wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
<div>I have seen info from independent tests that convinces me that
AFCI's probably don't work for AC and i hate to think what they would
do for DC, if anything. Besides drive the costs up.</div>
<div>This was a few years ago and maybe they have gotten it
together by now.</div>
<div> </div>
<div>Anyone remember the original ground faults form Trace
(?) after the code change requiring them in 96? Expensive and prone to
catching fire comes to mind, if i remember correctly. Lets not
encourage more if that type of experimentation in the industry.</div>
<div> </div>
<div>Just the same i am not a believer of running anything at 100%,
it will always have a higher failure rate than something run at a lower
capacity, be it a generator, lawn mower or a circuit breaker. </div>
<div> </div>
<div>Bob</div>
<div> </div>
<div> </div>
<div> </div>
<div> Mon, Apr 5, 2010 at 5:36 PM, William Miller <span dir="ltr"><<a moz-do-not-send="true" href="mailto:william@millersolar.com">william@millersolar.com</a>></span>
wrote:<br>
</div>
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<div><font size="3">Ray:<br>
<br>
It is my analysis that combiner breakers (if present) will protect only
wiring upstream of the combiner -- that is, the individual string
circuits. This protection would happen if there is a fault in one
individual string (in the wiring or the modules) that allows current
from other strings, in excess of the breaker rating, to be supplied
through the breaker feeding the faulted string.<br>
<br>
There are two scenarios at play here:<br>
<br>
1. Any fault between the combiner and the feeder destination will not
trip any circuit breakers. The breakers are sized such that the
current from each individual string is less than the breaker rating (by
more than 1.56 times) and they will not open.<br>
<br>
2. PV GFDI protection at the destination end of a feeder will not
help. PV GFDI circuits will not remove power from a feeder and they
will open the ground-to-grounding conductor bond.<br>
<br>
Analyzing this further: Fault conditions are made more likely given
that PV string circuits are no longer protected by conduit. Faults are
then more likely in individual string circuits (those circuits without
conduit protection). This is most problematic at installations with
two or fewer strings, where there is no combiner, i.e. residential
installations. Statistically, residential installations offer greater
exposure to electrical fires because: occupancy occurs for more hours
per year, fire alarms and sprinklers are often not installed, children
are more often present and standards are more lenient for residential
wiring systems.<br>
<br>
These two facts are PVs dirty little secrets. Further innovation is
needed...<br>
<font color="#888888"><br>
William Miller</font>
<div class="im"><br>
<br>
<br>
At 12:22 PM 4/5/2010, you wrote:<br>
<blockquote type="cite">I think the 100% rating exception is an
interpretation issue. I consider the assembly to be defined as the
breaker mounted in its listed enclosure.<br>
I agree that the AFIs would add cost, but they might actually offer
some protection too. (possibly one AFI unit could offer protection for
multiple circuits?)<br>
I've never had a PV circuit breaker actually trip, except some nuisance
tripping due to faulty breakers.<br>
PV breakers seem to only offer protection for very limited situations
ie, a short in a PV wire being backfed by enough other PV circuits to
trip the breaker.<br>
It could happen, but I've never actually seen it. Even completely
shattered modules still have enough internal resistance to limit the
short circuit current to<br>
a value below the breaker trip point.<br>
<br>
Ray<br>
<br>
On Apr 4, 2010, at 11:47 PM, Kent Osterberg wrote:</blockquote>
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