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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