[RE-wrenches] Square D QO breaker DC question

[jay peltz]

Hi Dan
I'm guessing the wire can handle the 100 amp if they are already running 60a. 
I've had QO breakers go bad, even ones not tortured like that one. 

The derate is interesting as it has to do with less atoms of air per volume. Less air, equals less cooling

Jay
Peltz power

Sent from my iPad

On Jul 8, 2013, at 10:07 PM, Dan Fink <danbob88 at gmail.com> wrote:

> 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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