MIXED PANEL ARRAY'S [RE-wrenches]

[Steve Willey, Backwoods Solar]

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.
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