[RE-wrenches] Square D QO breaker DC question

[Dan Fink]

Esteemed Wrenches;

A weird troubleshooting call today for my business partner DanB. I wish I
had been along if only for shock and awe and lessons learned.

A nicely installed vintage 12VDC PV system that someone else designed and
installed over 10 years ago, we replaced a dead/frozen battery bank a month
ago. DanB went back today because customer reported low battery bank
voltage.

He found the PV input breaker tripped, a Square D QO 2-pole 60 amp. About
600 watts of PV 12vdc nominal on the roof. Reset the breaker, and a few
minutes later it was too hot to touch.

So our working theory -- the 60a QO on *DC* has a 1.2x ampacity derate,
which puts 600w PV right at 60 amps instead of 50. There's also a bit of a
high altitude derate, which I don't understand at all. And during the
battery disaster last year bank voltage was certainly 11vdc or less, and
was a bit over 11vdc on DanB's arrival since the PV had not been charging.
That puts us at possibly 65.5 amps. The breaker was maybe just doing its
job correctly, though the heat was disturbing I am told.Or maybe it got
damaged during the battery disaster......both + and - were connected thru
the 2 poles.

In any case, our question --- due to my fuzzy memory and poor shopping
skills, we have a 100 amp QO 2 pole breaker in stock that needs a home, and
it'll plug right in. I have a nice tech bulletin from Schneider Electric
about DC ratings of their breakers that doesn't show that a 100a QO 2 pole
even exists, and a Schneider FAQ that says it'll work fine:
http://www.schneider-electric.us/sites/us/en/support/faq/faq_main.page?page=content&country=US&lang=en&id=FA125383&redirect=true

Any advice on this? We'd sure love to just drop by and pop in the new 100a
amp QO breaker instead of installing a whole new DC rated Midnite box. But
we don't want to leave a fire hazard either. With 15 years service on our
local volunteer fire department, It would be  rather embarrassing if a
"fix" we made to this old off-grid system caused a fire. ;-) We are aware
of the 5k AIR issue with these breakers, but the battery bank is small
enough I don't think it's quite up there, and information on AIR
interruption from battery manufacturers is hard to find......and of course
it's a weird curve.

Thanks in advance;

Dan Fink,
Executive Director;
Otherpower
Buckville Energy Consulting
Buckville Publications LLC
NABCEP / IREC accredited Continuing Education Providers
970.672.4342
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