PV combiner box protection [RE-wrenches]
Kurt Nelson
sunwise at cheqnet.net
Fri Jun 25 21:07:49 PDT 2004
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Hey again,
Okay, let me jump back in and offer my preferred method of doing the
combiner box. First, I don't do breakers out at the array, preferring
R-class fuses (FRN-5-30's). I buy a PVC NEMA3-R junction box of the
appropriate size/array, often in the 6, 8, 10 or 12 inch size by 4
inches deep. It gets bolted to the mast after I drill a 1.25 to 2 inch
hole in the bottom where the home-run conduit will come up to it along
with one to four, 1 inch holes in the sides for the SO connectors (pv
coming down). Inside the box I put a one to four pole R-class fuse
holder, depending on the number of PVs and or groups I'm bringing in on
the SO cables (movable array), usually two dasied and not to exceed
three (there's been four). I include a two pole distribution block (I
don't think split bolts make code either), one for the negative coming
directly from the array and the other for the positives from the fuse
blocks. Pop the FRN fuses in upon completion (size based on current
x1.56), and maybe hit em with a spray no-corrode and it's a go. I don't
usually have inspectors round here and when I do they have never looked
inside the combiner box.
BTW, in really small systems (perhaps 200 watts and less), I do not use
anything at the array, relying on the fuse that is installed just before
the CC and the one between the battery and the CC. In such systems where
the current the entire array can produce is under 20 amps, and all
wiring is 10 gauge or better, I actually consider the fuse between the
PVs and the CC to be unnecessary (though nice to be able to isolate the
CC and I include it).
Kurt Nelson
SOLutions
-----Original Message-----
From: Ray Walters [mailto:ray at solarray.com]
Sent: Friday, June 25, 2004 5:08 PM
To: RE-wrenches at topica.com
Subject: Re: PV combiner box protection [RE-wrenches]
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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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