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<div class="moz-cite-prefix">Hi Troy;<br>
<br>
Many moons ago, I looked at the curves as you did, and special
ordered some 350 amp T fuses, but then I quit making extra work
for myself. In a dead short, the 350 is really not going to blow
that much faster, while the 400 amp blows much faster than a lower
rated breaker will trip.<br>
There are several reasons why we use 400s with 4/0 cable. First
and foremost is that that is (or was) the fuse and cable size
recommended by Trace back when we used a lot of class Ts. If you
stick with the manufacturer's recommendations, you are doing your
job by the NEC. <br>
"110.3(B) INSTALLATION AND USE: Listed or labeled equipment shall
be installed and used in accordance with any instructions included
in the listing or labeling."<br>
The 2nd point, is that cables within NEC are designed for
continuous use 3 hrs or more. An inverter is not going to exceed
the cable rating for more than a few seconds during a surge. If
something is wrong, it will almost always be a higher current
short, and the fuse will still blow. The idea of a short circuit
situation with an inverter in which a 350 amp fuse would blow, but
not a 400 amp is just going to be rare. What CAN happen though is
a normal surge that could nuisance trip a very expensive and hard
to replace 350 amp fuse.<br>
Third, I'm using class T fuses to protect battery cables that are
never over 10 ft long. They usually run in conduit for less than
24 in, so it is considered a nipple, and most of the fill and temp
adjustment factors don't apply: 310.15 (B)3. Also, since its for
short runs and most of the circuit is not in conduit, you can use
the free air rating of the cables. 310.15(A)2 exception. At least
that is how it was explained to me. I never argued this last
point with an AHJ, so others could debunk me on that, but I simply
would show the AHJ the sizing reference in the inverter manual,
and we're done.<br>
Finally, Outback still lists the class T fuse 400 amp for 4/0
cable but they add a note (for mobile only) since these days class
T fuses are mostly used in non UL listed fuse holders with no
disconnects. Bolt switch still makes UL listed class T
disconnects, though. BTW, I'm not advocating for going back to
the 90s as Robin mentioned and using class T fuse exclusively. I
thought the original thread was about adding supplemental
protection at the battery terminals or for each string, in
addition to regular breakers. In that case you can put in larger
fuses too, because you're really after the interrupt rating more
than the regular fuse rating. The Midnite breaker still takes
care of the primary OCPD requirements. <br>
<br>
I'm sure I missed something, so hopefully others might join in as
well. (motor circuit fusing?)<br>
<pre class="moz-signature" cols="72">R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre>
On 4/22/2013 4:58 PM, Troy Harvey wrote:<br>
</div>
<blockquote
cite="mid:0DE206DA-633D-41B9-B025-551D6CE24691@heliocentric.org"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
Ray,
<div><br>
</div>
<div>One thing I didn't understand is how you can pair a 400 amp
class-T fuse with a 4/0 cable. The 400 fuse curve looks like the
continuous rating is 350A in the graph supplied by Allan. Which
is well above the 4/0 ampacity of 250A (copper). Anyone want to
explain?</div>
<div><br>
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<div style="word-wrap: break-word; -webkit-nbsp-mode: space;
-webkit-line-break: after-white-space; "><span
style="font-size: 11px; ">thanks,<br>
<br>
Troy Harvey<br>
---------------------<br>
Principal Engineer<br>
Heliocentric<br>
801-453-9434<br>
<a moz-do-not-send="true"
href="mailto:taharvey@heliocentric.org">taharvey@heliocentric.org</a><br>
</span><br>
<br>
</div>
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<br>
<div>
<div>On Apr 5, 2013, at 10:12 PM, Ray Walters <<a
moz-do-not-send="true" href="mailto:ray@solarray.com">ray@solarray.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
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<div text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">I just finished a rewire and
we kept the Ananda power center, Allan <span
class="moz-smiley-s1"> :-) </span><br>
Back when we used class T fuses more they were always
over sized relative to Heinemann breakers.<br>
400 amp class T fuse = 250 amp breaker= 4/0 cable<br>
200 amp class T fuse = 175 amp breaker= 2/0 cable<br>
<br>
I still think class T fuses are superior in some ways to
breakers: they have better interrupt capacity and trip
faster in a short circuit condition.<br>
<br>
However, If you spend too much time thinking about and
looking at the trip curves, you'll break your brain and
special order some weird fuses and breakers. (anybody
need a 350 amp class T fuse?)<br>
Just follow the inverter manufacturer's recommendations
and all will be well.<br>
On the other hand, William has brought up a topic I have
harped on for years: Having the OCPD in the cabinet
doesn't protect the majority of the circuit. Class T
fuses at the battery terminals do, but they're not rated
for the corrosive environment. I have thrown a few
class T fuses away that had acid eating away at the
ends.<br>
I don't have the solution, but I will continue to point
out that this is a real problem. Dropping a wrench
across the battery terminals can lead to a spectacular
failure that not only can cause a fire, but might even
cause a battery explosion, yet NEC offers no
protection. <br>
We use insulated wrenches from experience, and hope for
the best. <br>
<pre class="moz-signature" cols="72">R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre>
On 4/5/2013 8:29 PM, William Miller wrote:<br>
</div>
<blockquote
cite="mid:6.0.1.1.2.20130405192312.03083f90@millersolar.com"
type="cite"> Friends:<br>
<br>
Good topic. Some questions:<br>
<br>
1. Most manufacturer's present an installation guide
that shows one OCPD in the battery circuit and that is
in the BOS cabinet. This means the battery leads are
unprotected. Do we need an OPCD at the battery
terminals?<br>
<br>
2. Class T fuses are generally recommended for this
application. The data shows them as "fast acting." Is
this a problem? Will they act too fast and open during
normal surge loads?<br>
<br>
Thanks in advance!<br>
<br>
William Miller<br>
<br>
<br>
<br>
<br>
<br>
<blockquote type="cite" class="cite"
cite="x-msg://1311/">Troy,<br>
<br>
Overcurrent device size is matched to the conductor
size. The inverse time constant nature of an
overcurrent device can typically handle the surge
currents as long as conductor sizing has truly been
done correctly for the conductor. Circuit breakers are
preferred to fuses because they can be reset. <br>
<br>
There has been volumes written on this issue. The
constant current at lowest battery voltage should be
used, plus the ac ripple content on the battery
circuit. This is usually a much larger conductor than
your average designer will plan for. The best thing is
to look at Midnight, Outback, and Schneider and see
what size overcurrent devices they require for their
products. That will give you a good clue as to how to
size the conductor and overcurrent device.<br>
<br>
Bill.<br>
<br>
<b>From:</b> <a moz-do-not-send="true"
class="moz-txt-link-abbreviated"
href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a moz-do-not-send="true"
href="mailto:re-wrenches-bounces@lists.re-wrenches.org"
eudora="autourl">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>]
<b>On Behalf Of </b>Troy Harvey<br>
<b>Sent:</b> Friday, April 05, 2013 3:38 PM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> [RE-wrenches] Fuse sizing in battery
circuits<br>
<br>
I've got a question about battery string fusing.
Typically we size the wire from the batteries to the
inverter based on continuous rating procedures (max
power/efficiency)*125%. <br>
<br>
However a 6kW inverter, can peak at 12kW for 5-10
seconds, doubling the source current. That is no big
deal for the wire, because it is a short time frame...
little heat will be generated. However, in fusing the
sub-strings, you need to account for that peak surge
current so you don't blow fuses all the time. But if
you put a 500-1000 amp fuse on a 4/0 wire, above the
max surge draw of the inverter, the wire will be
under-protected for its ampacity rating. Any thoughts
on the catch-22?<br>
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