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:* [email protected]
[mailto:[email protected]] *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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