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I just went through this with SMA tech support on the Sunny
Islands. The breakers built in to the SIs have a 10,000 AIC
rating. The battery bank we are using, one string of Concord PVX
3050 T, 305 AH clearly won't surge to 10,000 Amps. We decided that
a series fuse isn't necessary on the battery terminal. <br><br>
<br>
At 12:19 AM 4/9/2013, you wrote:<br>
<blockquote type=cite class=cite cite="">HI Kent;<br>
<br>
I actually used to have info from Trojan on the short circuit current
available from the T105 and L16. I just don't remember it exactly,
but I recall it was around 2000 amps for the T105. I agree its
really shameful how lame most battery manus are at providing info
anymore. Its hard to even get good cycle life tables.<br>
HUP does show their short circuit #s. The 845 AH battery is 9600
amps and for comparison, the 2490 AH is 29,100 amps.<br>
So yes, we could have trouble.<br>
B&B a manufacturer of sealed deep cycle batteries shows their 12 v 90
AH battery with a whopping short circuit current of 2300 amps but only
for 100 milliseconds.<br>
Midnite shows their breaker AIC ratings and its 10,000 amps for the PV
breakers up to 50,000 amps for the 175 and 250 amp breakers. The
class T fuses I checked are 20,000 amps AIC. Remember that this is
all rated at 125 to 150 vdc, so all of these OCPDs will have even better
AIC at lower voltages. (I wish we could get a chart that showed AIC
relative to voltage.) The other factor in our favor is that the
batteries might be able to exceed the AIC for tenths of seconds, but it
can't sustain it like the grid can.<br>
Sorry that this is not enough info to do a definitive design, but at
least it can get us all thinking about AIC ratings relative to bigger
battery banks.<br><br>
<br>
<pre>R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre><font face="Courier New, Courier"></font>On 4/6/2013
1:21 PM, Kent Osterberg wrote:<br>
<blockquote type=cite class=cite cite="">Phil,<br><br>
Thanks for the code references, they are very useful and point out an
area where I fear many installations of battery based inverters are
deficient. You are right the manufacturers have not made this
easy!<br><br>
First, data on the short circuit current available from most batteries
isn't easily available, if available at all. Concorde publishes short
circuit current numbers for the SunExtender batteries. Neither Trojan,
Rolls/Surrette, nor Outback publishes short circuit current data for
their batteries. Full River publishes an internal resistance number that
can be used to estimate the short circuit current. Taken as a whole,
battery manufacturer's certainly haven't made it easy to comply with
690.71<br><br>
Second, sometimes interrupting capacity data on the breakers being used
in BOS equipment is also missing. Take a look at Outback's catalog and
try to find the AIC for the breakers used to protect the inverter wiring.
Is it published anywhere else? Should a system designer have to buy the
Outback breaker, find out who manufactured it, and then search for the
correct ratings in the in the breaker manufacturer's catalog? Obviously
not. <br><br>
Third, BOS manufacturers often connect circuit breakers for charge
controllers or dc utilization circuits that have AIC ratings of only 5000
amps directly to the battery. So while the inverter may be protected with
a breaker having an AIC rating 25,000 or 50,000, other devices don't. BOS
manufacturer's should include on the dc breaker panel data showing the
<font size=4>lowest</font> AIC breaker that is connected directly to the
battery. Then installers would have at least half of the information
needed to decide when a fuse is needed to backup the dc breakers. And
shouldn't the instructions for BOS equipment point out that a fuse may be
necessary at the battery.<br><br>
The short circuit current from a single string of golf cart batteries or
L16 batteries is probably less than 5000 amps. Fuse(s) are probably
needed in most installations with larger batteries or battery systems
with multiple strings of batteries. 2011 NEC 240.21(H) allows overcurrent
protection to be installed as close as practical to battery terminals -
even in classified locations. <br><br>
<br>
<pre>Kent Osterberg
Blue Mountain Solar, Inc.
<a href="http://www.bluemountainsolar.com">www.bluemountainsolar.com</a>
</pre><font face="Courier New, Courier"></font><br>
On 4/6/2013 7:29 AM, Phil Undercuffler wrote:<br>
<blockquote type=cite class=cite cite="">Ray, <br><br>
690 does offer the protection, it's just that most maufacturers haven't
made it easy.<br><br>
690.71 Installation<br><br>
(C) Current Limiting. A listed, current-limiting, overcurrent device
shall be installed in each circuit adjacent to the batteries where the
available short-circuit current from a battery or battery bank exceeds
the interrupting or withstand ratings of other equipment in that circuit.
The installation of current-limiting fuses shall comply with
690.16.<br><br>
The telcom industry has been doing this for years. Their norm is to
have each string of batteries on a shelf, and terminate at a
breaker. Each string then is combined at a bus bar or plate, to
help ensure equal current on each string. The loads and charging
sources all come to that same bus bar. It has some advantages, as
individual strings can be monitored and serviced without taking the
entire system down.<br><br>
FWIW, OutBack makes a battery rack for AGM batteries with series string
overcurrent and disconnects on every string. It's also to my
knowledge the first UL1741 Listed device available -- the other stuff
I've seen is either not listed, or is trading on old 508a
standards. AGMs might not be for every installation, but for jobs
where they are appropriate this could be a good, code-compliant
solution.<br><br>
<br>
Phil Undercuffler<br>
OutBack Power<br><br>
<br><br>
<br>
On Friday, April 5, 2013, Ray Walters wrote:<br>
<dl>
<dd>I just finished a rewire and we kept the Ananda power center, Allan
:-) <br>
<dd>Back when we used class T fuses more they were always over sized
relative to Heinemann breakers.<br>
<dd>400 amp class T fuse = 250 amp breaker= 4/0 cable<br>
<dd>200 amp class T fuse = 175 amp breaker= 2/0 cable<br><br>
<dd>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>
<dd>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>
<dd> Just follow the inverter manufacturer's recommendations and all
will be well.<br>
<dd>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>
<dd>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>
<dd>We use insulated wrenches from experience, and hope for the
best. <br>
<br>
<dd><pre>R.Ray Walters
<dd>CTO, Solarray, Inc
<dd>Nabcep Certified PV Installer,
<dd>Licensed Master Electrician
<dd>Solar Design Engineer
<dd>303 505-8760</pre><font face="Courier New, Courier"></font>
<dd>On 4/5/2013 8:29 PM, William Miller wrote:<br>
<blockquote type=cite class=cite cite="">
<dd>Friends:<br><br>
<dd>Good topic. Some questions:<br><br>
<dd>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>
<dd>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>
<dd>Thanks in advance!<br><br>
<dd>William Miller<br><br>
<br><br>
<br><br>
<blockquote type=cite class=cite cite="">
<dd>Troy,<br>
<dd> <br>
<dd>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>
<dd> <br>
<dd>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>
<dd> <br>
<dd>Bill.<br>
<dd> <br>
<dd><b>From:</b> re-wrenches-bounces@lists.re-wrenches.org
[<a 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>
<dd><b>Sent:</b> Friday, April 05, 2013 3:38 PM<br>
<dd><b>To:</b> RE-wrenches<br>
<dd><b>Subject:</b> [RE-wrenches] Fuse sizing in battery circuits<br>
<dd> <br>
<dd>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>
<dd> <br>
<dd>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>
<dd> </blockquote></blockquote></blockquote>
</dl><br><br>
<br><br>
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