PV combiner box protection [RE-wrenches]

Wed Jun 23 19:05:32 PDT 2004

-----------------------------------------------------------
Give Your Team Access to Their PCs from Anywhere. Increase 
productivity with a secure remote-access solution from 
GoToMyPC Pro. Stay in touch with your office. FREE TRIAL:
http://click.topica.com/caaciqXbz8Qcsbz9JC9a/ExpertCity
-----------------------------------------------------------

Wrenches:

The UL guy in charge of module listings once gave me some information I
find valuable when thinking about arrays and fusing.  He said:

Think of the module as a piece of wire with one magical property "it
generates electricity when you put it in the sun".
Other than this magical property, think of it as just a piece of wire.

The UL series fuse rating is nothing more than the ampacity of the
magic wire.

If you mentally translate "series string" into "length of magic wire"
it becomes pretty easy to calculate fault currents and see where you
need protection.
Think about a fault in one of "N" magic wires all twisted together
(paralled).
The Code says we have to consider fault currents from all sources so:

Ifault = ((1.56*Isc)*(N-1))+Ibackfeed

You use N-1 because the faulted wire does not feed it's own fault but
the remaining N-1 series strings can deliver fault current to the
fault.
Ibackfeed typically comes from the inverter, the charge controller, or
the battery.
Ibackfeed is normally the rating of over current device in the PV
output circuit.
The one exception is where the inverter or charge controller has been
tested to show it will not deliver backfeed current into a fault in the
array wiring. 
If Ibackfeed = 0 the no OC device is required >>IF<< all of the wire
(including the magic wire) connected to that circuit has an ampacity
higher than the total available fault current.

Other than this limited case the Code requires minimum rating of the
output OC device to be >= N*1.56*Isc so the minimum fault current we
need to consider is:

Ifault = ((1.56*Isc)*(N-1))+((1.56 *Isc)*N) or;
Ifault = (1.56*Isc)*((2*N)-1)

As Bill Brooks pointed out there are no crystalline modules with series
fuse ratings higher than this
Effectively this means you have to have over current protection on each
string unless Ibackfeed = 0.
Combiner boxes give you a convenient, code compliant way to provide the
necessary OC protection.
They also protect the wire going to the magic wire but that is really
just a secondary benefit.

I hope this helps.

Best Regards,

John Berdner

>>> billb at endecon.com 6/23/2004 08:52:14 AM >>>

Your free subscription is supported by today's sponsor:
------------------------------------------------------------------
Get a FREE Mortgage Quote from Mortgage War. Simple 60-second 
application for mortgage information. Rates are the lowest 
theyve been in 40 years!
http://click.topica.com/caacneLbz8QcsbAs9Tla/411Web 
------------------------------------------------------------------

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.

--
[Non-text portions of this message have been removed]

-----------------------------------------------------------
Apply now for a No-Annual-Fee Discover® Platinum Card 
0% Intro APR*, No Annual Fee, Up to 2% Cashback Bonus® 
award* Start Saving Today – APPLY NOW! It's fast, easy and 
secure. 
http://click.topica.com/caaciqQbz8Qcsbz9JC9f/Discover Card
-----------------------------------------------------------

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://lists.topica.com/lists/RE-wrenches/read

List rules & etiquette: www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

For Topica's complete suite of email marketing solutions visit:
http://www.topica.com/?p=TEXFOOTER
--^----------------------------------------------------------------