Hi Troy;
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.
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.
"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."
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.
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.
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.
I'm sure I missed something, so hopefully others might join in as
well. (motor circuit fusing?)
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 4/22/2013 4:58 PM, Troy Harvey wrote:
Ray,
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?
thanks,
Troy Harvey
---------------------
Principal Engineer
Heliocentric
801-453-9434
tahar...@heliocentric.org <mailto:tahar...@heliocentric.org>
On Apr 5, 2013, at 10:12 PM, Ray Walters <r...@solarray.com
<mailto:r...@solarray.com>> 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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