PV combiner box protection [RE-wrenches]

[Ray Walters]

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Hey Phil;

I'm aware and proud to share Nuevo Mexico with you, I was referring to the 
rest of the list and didn't want anyone to think I was suggesting these 
methods for Higher voltage grid tie.
  I love your example and with your numbers, I would drop the cost even 
lower by doing a modest amount of Daisy chaining (3 modules would be 
pushing it, but would still be around 10 amps actual operating current per 
circuit. ) Assuming they're Siemens M50 modules, the manufacturer's 
literature clearly shows daisy chaining up to 4 modules. Then I would bring 
the 2 circuits into a 2 pole QO box indoors with 2 15 A breakers.

QO 2P box  $14.00
2 breakers    $15.88
60' 12-2 TC     $14.40

Total   $44.28

Three times cheaper.

Perfect: no.
NEC no, but Taos inspectors allow this all the time since its in the  PV 
manufacturer's instructions. (and I quote from the latest Siemens manual: " 
A small number of modules may be paralleled using the Junction box on the 
back of the module. Contact your local reseller for more information.")
Is it safer than daisy chaining the whole thing with no fuses:  Yes.
Am I going to lose sleep over this arrangement going up in smoke , 
absolutely not, it won't.
Are the power losses worth going to other methods:  Not without MPPT. The 
almost 1 volt drop while high, will not be noticed by a regular charge 
controller. The modules even hot will  produce close to full amps at 15 
volts, leaving full current up to 14 volts at the batteries. As the 
batteries near full charge (over 14), the volt drop will only help taper 
the charging current slightly. (3% volt drop is too conservative for 12 v 
systems and not conservative enough at high voltages, 48 and up. Instead I 
use power drop as stated in previous emails)
As noted, jumping into an MPPT controller would solve many problems and 
would be the next recommended upgrade for this customer. The same lines at 
24 v and 5 amps would only cause a total power loss of 5 watts or $25 worth 
of  solar power. So the wiring holds for the MPPT upgrade too. Maybe I 
would run a third line and put  each series string on its own circuit then 
too and everyone would be happy.

I'm sure a bunch of wrenches are rolling around thinking I'm cracked, but 
that's great. That's why I love this list and respect everyones' views; its 
getting us ALL to think and rethink about what we're doing.

Ray




At 01:39 PM 6/24/2004, you wrote:

>California Grid tie guy?  Man, Ray, that's hitting below the belt! ;-)
>
>Actually, I live in Madrid, New Mexico--the closest thing this country has
>to the third world next to Taos.  I've lost count of the systems I've done
>for neighbors with next to nothing to their names.
>
>But let's look at nuts and bolts.
>
>Hypothetical situation:  Extremely poor couple, living in a bus.  300 watts
>of array, 50 watt modules, pole mounted, 30'from batteries.  12v system
>required, in order to run the stereo in the bus.
>
>Using your proposal, 12-2 TC would run from each trio of daisy chained
>modules to one of two QO breakers in a 2 pole NEMA 3 disconnect.  From
>there,  #4 USE (no conduit in this neighborhood!) runs to a second
>disconnect protecting the controller.
>
>1 QO 3R box     60.01
>1 QO indoor box 31.52
>3 QO breakers   23.82
>60' #4 wire     32.40
>Total:          $147.75
>
>My proposal:  12/2 tray cable from each module back to one of 6 input
>breakers in a QO 6-12 disconnect mounted near the batteries, serving as
>array disconnect and series fusing.
>
>180' 12/2 TC    43.20
>1 QO 6-12       40.01
>6 QO breakers   47.64
>Total:          $130.85
>
>
>
>PS--beyond 300 watts, their array is larger than the one my wife and I lived
>very nicely with, thank you very much, and their cries of poverty become
>much dimmer in my ears.  And they should be 24 volts anyway.
>
>Phil Undercuffler ext.238

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