SQ D HU 362 A Ratings? [RE-wrenches]

[Mark Frye]

William,

Thanks for getting me that link.

I think the essence of the SMA white paper is in agreement with 
Exception B. Generally with 2 strings or less the short circuit current 
from all sources including other PV source circuits will be less than 
the ampacity of the conductor. With 3 strings or more it may or may not, 
depending.  I always use #10 so I don't worry about it too much. I use 
the fuses because the "string fuse rating" of the modules I use is only 
10A.

With an Isc of 5.4A I could theoretically be in troblem with a short in 
a 3 string system and in more with a 4 string system. Will the fuse help 
protect the modules? Probably not. Do I still do it? Yes.

But, I am building a job right now and I'm thinking, "You know, there 
will never be a short in my string wiring. I don't really need to 
install the fused combiner. It's just overkill."  Still I have the 
materials budgeted, so in it goes.  But I do remember a saleman in India 
back in '82 telling me, "Cheap Is Best!", so I wonder.

For service, the integrated DC disconnect is sufficient to interupt the 
current flow so the PV source circuits can be opened either at the 
fusued combiner box or in my cse usually at an MC connector.

I guess in my case for everything except small two string systems, I 
have double disconnects, but I don't use one under load.


William Miller wrote:
> 
> 
> 
> 
> 
> Mark:
> 
> The SMA white paper is on my web site:
> 
> http://mpandc.com/resources/resources.html
> 
> You can hook as many strings as you want to a single pole of a 
> disconnect 
> as long as the result complies with the code section 690-9 (A) Exception 
> b.
> 
> On a related subject, I attended a day-long training session hosted by 
> Fronius.  I like and use their inverters.  I do disagree with the notion 
> 
> that the built-in DC and AC disconnect switches obviate the need for 
> external switches.  Fronius spent an inordinate amount of time 
> justifying 
> the notion that all you need to do to remove an inverter is to cover the 
> 
> modules feeding the inverter.
> 
> The last time I used Fronius inverters we installed 6 4500 watt units.  
> To 
> render one of the units safe to remove, one would have to set up a 24 
> foot 
> ladder, figure out which 30 of 180 modules needed to be covered and then 
> 
> produce an opaque tarp measuring 30 feet by 30 feet to cover those 
> modules.  I say this is ridiculous.  The inverters we install all have 
> external AC and DC disconnects.
> 
> William Miller


Berkeley Solar Electric Systems


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