MIXED PANEL ARRAY'S [RE-wrenches]

jay peltz jay at asis.com
Wed Aug 16 11:25:36 EDT 2000 

Hi All,

The other issue is that of the "series fuse" rating on  each panel.  From what I
have figured out that is the main  issue with how many panels you can parallel.

For example the BP- 75 series fuse is like 7.5 amps: well its a 4.5 amp panel,
no way to run 2 without exceding the amp rating.  Other manufacuters ( Solarex
is good this way) have up to 20 amp series fuse ratings, therefor you can string
them together again as long as the wire is protected etc.

Hope that helps somewhat.

jay

"Steve Willey, Backwoods Solar" wrote:

> Here is one of those.  I am no longer installing and code is not real strict
> though definitely exists here, so you may have a better view on this than I.
> Here is the start of that discussion, and it carries on a few letters if you
> have the archive access:
>
> Hey Wrenches,
>
> There has been a change in the wording of the 99 Code that might make array
> wiring easier.  The use of a combiner box might become a rare thing on
> domestic PV systems.  Let me know if you think this is correct.
>
> In the old code it appeared that every series set of modules needed to have
> its output run directly to an overcurrent device in a combiner box.
> Connections could not be made in another module's junction box, if removal
> of that module would disrupt the "grounded conductor" of another set.
>
> This is how it has often been interpreted in Colorado anyway.  With this
> method, a rack of 8 modules wired for 24 volts requires four flexible
> conduit "whips" coming down to a combiner box with four fuses.  There are
> often positive and negative busbars to which the system conductors are
> connected.  Below is the old wording.
>
> ****************************************************************************
> *********************
> 690-4(c) 1996 NEC
>
>  (c) Module Connection Arrangement. The connections to a module or panel
> shall be so arranged that removal of a module or panel from a photovoltaic
> source circuit does not interrupt a grounded conductor to another
> photovoltaic source circuit.
>
> ****************************************************************************
> ********************
> The new code goes into detail about the meaning of "source circuit."  It now
> appears that a source circuit is any configuration of modules rated at 50
> volts or less, terminating in a single over current device.  Here is the
> section.
>
> ****************************************************************************
> ******************
>
> 690-4(c) 1999 NEC
>
> (c) Module Connection Arrangement. The connections to a module or panel
> shall be arranged so that removal of a module or panel from a photovoltaic
> source circuit does not interrupt a grounded conductor to another
> photovoltaic source circuit. Sets of modules interconnected as systems rated
> at 50 volts or less, with or without blocking diodes, and having a single
> overcurrent device shall be considered as a single-source circuit.
> Supplementary overcurrent devices used for the exclusive protection of the
> photovoltaic modules are not considered as overcurrent devices for the
> purpose of this section.
>
> ****************************************************************************
> ***************
>
> The first sentence of the new code reads exactly like the old.  To simplify
> the changes, we can eliminate it.   The changed part has a lot of modifying
> wording.  Although necessary to consider, we can eliminate the modifiers for
> present purposes.  The reduced statement boils down to:
>
> Sets of modules interconnected as systems rated at 50 volts or less having a
> single overcurrent device shall be considered as a single-source circuit.
>
> This would seem to imply that a rack of 8 modules wired in a 24 volt
> series-parallel configuration is a single source circuit.  It would
> therefore need only one whip going to a disconnect, rather than 4 whips
> connecting to a combiner box.
>
> The series sets apparently may be paralleled in the module junction boxes
> now.  If true, this will not only save hundreds of dollars in materials and
> labor on large systems, it will also make arrays look a lot cleaner.
>
> Here is an example of how an array might be designed with the new code.
>
> Eight Siemens SP 75s wired in series-parallel for 24 volts, have a short
> circuit rating of 19.2 amps (4.8 X 4) .  Multiply that times 1.25 twice (to
> comply with code) and you get 30 amps.  A pair of number 10 THWN-2 wires
> should satisfy any inspector.  (* Note:  The rating of THWN-2 comes on many
> THHN conductors).
>
> So two #10 AWG wires can be used instead of eight.  One conduit is needed
> instead of four.  One overcurrent device is needed instead of four.  The
> wiring can be run directly to a fused disconnect, rather than to a combiner
> box first.  A 16 module, 24 volt array can have two whips run to a 2 pole
> fused disconnect.
>
> Due to the language, it looks hopeful that this rule will also apply to 48
> volt systems.  The code now seems to say that photovoltaic source circuits
> are "Sets of modules interconnected as systems rated at 50 volts or less."
> The word "rated" seems to imply nominal voltage rather than the actual
> measured, operating voltage.
>
> Wrenches, let us know if we are on track.
>
>                                                                   Drake
> Chamberlin
>
>                                                                   Don
> Wallingford
>
> Steve Willey   www.backwoodssolar.com
> Backwoods Solar Electric Systems
> A catalog business on a mountain
> 2 miles from utility lines,
> powered entirely by our catalog products.
> IF UTILITY POWER IS 1/2 MILE OR MORE FROM YOUR SITE,
> get our free 160 page solar-electric Planning Guide/Catalog.
> (Your postal address required - Our list is never given or sold).
>
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