PV combiner box protection [RE-wrenches]

[Bill Brooks]

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Kurt,

All the SMA inverter systems have made the series fusing issue something
many installers have gotten rusty on. Fusing is required by the UL listing
of the module. The one exception is when the inverter has been listed to
provide no backfeed current. In this case, all modules can handle two
strings of modules in parallel before the inverter, and in a few cases,
three strings.

I am still unaware of any crystalline silicon module that can parallel
strings before going to a fuse for inverters not listed to provide no
backfeed. 12-Volt inverters are only battery-based at this time so they
cannot be listed for no backfeed since the battery is in the circuit. This
means that every single module in a 12-Volt system needs a fuse. How many
12-Volt systems have fuses on every module--my guess is not many but it is a
code requirement. Fuses in module junction boxes are problematic because of
the extreme temperatures in the junction box and the difficulty to access
the box in the case of a blown fuse.

Fusing protects the module, but it does have the secondary effect of being
able to protect the array wiring since we are typically stepping down the
wire size to connect to the module. Most modules with junction boxes can
only handle 10AWG and only a few can handle 8AWG (with great pain). For any
system providing a reasonable amount of power (residential on or off grid 1
kW or more) 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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