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.
At 12:19 AM 4/9/2013, you wrote:
HI Kent;
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.
HUP does show their short circuit #s. The 845 AH battery is 9600
amps and for comparison, the 2490 AH is 29,100 amps.
So yes, we could have trouble.
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.
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.
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.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760On 4/6/2013 1:21 PM, Kent Osterberg wrote:
Phil,
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!
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
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.
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 lowest 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.
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.
Kent Osterberg
Blue Mountain Solar, Inc.
<http://www.bluemountainsolar.com>www.bluemountainsolar.com
On 4/6/2013 7:29 AM, Phil Undercuffler wrote:
Ray,
690 does offer the protection, it's just that most maufacturers
haven't made it easy.
690.71 Installation
(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.
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.
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.
Phil Undercuffler
OutBack Power
On Friday, April 5, 2013, Ray Walters wrote:
I just finished a rewire and we kept the Ananda power center, Allan :-)
Back when we used class T fuses more they were always over sized
relative to Heinemann breakers.
400 amp class T fuse = 250 amp breaker= 4/0 cable
200 amp class T fuse = 175 amp breaker= 2/0 cable
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.
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?)
Just follow the inverter manufacturer's recommendations and all
will be well.
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.
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.
We use insulated wrenches from experience, and hope for the best.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 4/5/2013 8:29 PM, William Miller wrote:
Friends:
Good topic. Some questions:
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?
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?
Thanks in advance!
William Miller
Troy,
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.
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.
Bill.
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Troy Harvey
Sent: Friday, April 05, 2013 3:38 PM
To: RE-wrenches
Subject: [RE-wrenches] Fuse sizing in battery circuits
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%.
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?
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