PV combiner box protection [RE-wrenches]

[Ray Walters]

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Hi Bill;

I agree 48 v is good, but reality often dictates we use 24 and 12 v. Bergey 
for instance only makes the XL.1 in 24v. The venerable DR series inverters 
also are only available in12 or 24 v. DC pumps and lighting are alas 
limited to lower voltages.
This came up on a previous string, where I recommended paralleling 2 
strings and still fusing at the PV manufacturer's rating. (I too have seen 
overloaded module Jboxes burned from too many parallel circuits) At that 
time you or someone mentioned the possibility of simultaneously backfeeding 
from the batteries combined with the current from  the second string could 
exceed the module rating without blowing the fuse. Definitely a possibility 
but highly unlikely.
Then, I questioned whether PV was being made to jump through more hoops 
than standard house wiring. As you all know, standard house wiring allows a 
16 Ga lamp cord to be plugged into a circuit protected only by a 20 amp 
breaker. This poses a very real danger of overload at a much more dangerous 
voltage, yet IS allowed by code in millions of homes including yours. I 
always have taken the NEC relative to solar design with a healthy dose of 
salt.
Think about it : a 15 amp rated device(solar module) isn't allowed a 
possible 19.5 amps at 24 volt DC, but a 10 amp rated device (16Ga lamp 
cord) is allowed 20 amps at 120 VAC? One is outside on a pole, the other is 
behind your bed.....Hmmmm

Just wanting to level the playing field,

Ray







>48-Volts should be considered a minimum voltage in my book (it
>is also a maximum voltage if it is battery-based). This reduces current,
>fusing, copper, and overall losses.
>
>Bill.
>
>-----Original Message-----
>From: Kurt Nelson [mailto:sunwise at cheqnet.net]
>Sent: Tuesday, June 22, 2004 11:48 PM
>To: RE-wrenches at topica.com
>Subject: PV combiner box protection [RE-wrenches]
>
>Greetings all,
>
>I think this will be an easy one and not a can of worms, but then....
>
>A friendly competitor and I were speaking at the energy fair a while
>back and we disagreed on some issues regarding array wiring and over
>current protection.
>
>My position would be that the use of over-current protection
>(fuses/breakers) within the combiner box at the array is to protect the
>series and parallel interconnect wiring of the PV array which is
>typically a much smaller gauge wire than the home run (this primarily
>concerning low voltage 12, 24 & 48V systems).
>
>If a properly sized breaker or fuse is used at the DC box before the
>charge controller (protecting the array wiring from any high currents
>generated in the home/battery), and if the home run AND interconnecting
>wiring is capable of handling the total short circuit current of the
>entire array, is it necessary to have fusing in the combiner box?
>
>For instance if you had two 75 watt PVs wired into either a 12 or 24
>volt array and the smallest wire in the PV circuit was 10 gauge, would a
>fuse be required at the array if a properly sized fuse was used within
>the home prior to the charge controller?  BTW, I always use a second
>fuse/breaker between the CC and the battery.
>
>Also, as in the 12 volt configuration mentioned above but expanded a
>little.... a 600 watt, 24 volt array consisting of eight 75 watt
>modules.  I would think it acceptable to bring four, two series sets
>into one module's J-box (using liquid tight w/ten gauge/another post)
>and then use 10/4 SO(w) to bring the whole array (as two paralleled
>series sets) down to the pole mounted combiner box from the
>tracker/movable/seasonally adjustable top-of-pole mount.  I'd then use
>two FRN-15s in the combiner box before the distribution block/home run
>(necessary?).
>
>In any case, I've been told that you have to bring each series string
>down individually, resulting in eight conductors between the rack and
>the mast and nothing less than spaghetti in a similar 12 volt system.
>I'm hoping this is wrong and wondering what the logic is otherwise.
>Thanks for your brains.
>
>Kurt Nelson
>SOLutions
>
>
>
>.
>
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>
>
>
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