M-C branch plugs: Legal? Safe? [RE-wrenches]

Mon Feb 4 20:33:07 PST 2008

I am starting to be a bit interested in the discussion regarding
battery-less vs. battery.

What is the difference between having a hugh AC grid on the other side of a
transistor in an inverter and having a big battery on the other side of a
transistor in a charge controller.  Are the charge controllers so different
from the inverters that they can be expected to put current into the source
circuits? In a fault?

I don't really see how the batteries matter here.

Isn't this really about, generally, depending on the math, that when you
have three or more strings you should have individual string fusing per the
SMA white paper.

Perhaps someone could help clarify this for me.

Thanks,

MPF

-----Original Message-----
From: Phil Undercuffler [mailto:p.undercuffler at conergy.us] 
Sent: Monday, February 04, 2008 7:57 PM
To: RE-wrenches at topica.com
Subject: RE: M-C branch plugs: Legal? Safe? [RE-wrenches]

Mick,

If batteries are involved, then you really need one series fuse per string
-- branch plugs cannot be used.  Oh, in theory maybe they could, but
remember you need to protect the module from ALL sources of current:
the other string plus the batteries.  For instance, say you had a module
with a short circuit rating of 3.8 amps.  3.8 x 1.56 = 5.9 amps, and let's
round up to 6 to make the math easy.  In a batteryless application, two
strings could be connected with branch connectors because neither module
will see fault currents higher than it is rated to handle.  

Add a battery to the mix, however, and things get harder.  If you protect
the combined strings with a 15 amp fuse, a fault in one module will see a
potential for 21 amps:  15 flowing through the fuse from the batteries, and
another 6 from the second string.  If you wanted to choke down the series
fuse rating to stay below the sum of 15 amps, then the series fuse would
have to be 9 amps.  However, the 9 amp fuse would not be large enough to
handle the combined array current in normal operating conditions.....

So, in theory it may be possible to use branch connectors in battery based
applications, it would require modules with LOW short circuit ratings (1-2
amps, maybe) plus HIGH series fuse ratings.  

Finally, it shouldn't be forgotten that once you solve for the series fuse
rating, the ampacity of the wire in the conditions of use need to be
addressed.  Temperatures on the roof can be pretty high, and 2008 NEC
applies an even more stringent derate factor on conductors & conduit exposed
to sunlight.  It doesn't take long before the ampacity of that #10 and #12
wire runs out of headroom for paralleled modules.  

------------------------------------------------------------
Please note new phone number!

Phil Undercuffler
Director, Battery-based and Off-grid
Conergy, Inc.
Our World is Full of Energy

1730 Camino Carlos Rey Suite 103
Santa Fe NM 87507 USA
Office | 505.216.3841
Fax | 505.473.3830
www.conergy.us

------------------------------------------------------------ 

-----Original Message-----
From: Mick Abraham [mailto:mick at abrahamsolar.com]
Sent: Monday, February 04, 2008 6:20 PM
To: RE-wrenches at topica.com
Subject: RE: M-C branch plugs: Legal? Safe? [RE-wrenches]

Thanks to all who have replied...very helpful...

John Berdner: Thanks for reminding me about grid-tie situations where PV

string fuses are not needed. My present application is a battery based 
system where I must consider possible backfeed from the battery.

"Ray Walters wrote: "...you can't use those connectors to create more 
than 2 parallel strings, so I'm not sure how you would parallel 3 
strings."

Mick says: On page 13 of the M-C catalogs for SolarLine 1 and SolarLine 
2, they show trios plugged together in parallel. The M-C drawing looks 
nice but one must wonder if plugged trios are really workable in the 
real world, because that would need a PV module where the series fuse 
rating is almost four times greater than the short circuit amperage.

Ray's mention that the Solar World 165s have a series fuse rating of 15 
amps is most interesting, since their short circuit amperage is a much 
lower 5.1. With the fuse rating so high relative to its amperage, this 
module seems especially well suited to plug-parallel...in pairs at 
least. 

Thanks again,

Mick Abraham, Proprietor
www.abrahamsolar.com

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