MIXED PANEL ARRAY'S [RE-wrenches]
allan
allan at positivenergy.com
Tue Aug 15 20:55:49 EDT 2000
Drake and all,
We have been consistently using 10 AWG USE-2 to our combiner boxes,
without any conduit, and have been happy with it. It sidesteps the above
issue, so I will hold off and listen to others' responses on the NEC
interpretation of your issue re PV input wiring.
The related point I want to make is that the code doesn't require a
disconnect at the array. Disconnects should be grouped together, and in
fact a disconnect at the array can be a safety hazard, as terminals on both
sides could be energized in an open position. I went through this with the
NM Head State Inspector on a job, and got John Wiles' support, and he went
with our argument. Wiles says the 2002 NEC will probably have a revision to
this effect.
Allan at Positive Energy
-----Original Message-----
From: Steve Willey, Backwoods Solar <steve at backwoodssolar.com>
To: RE-wrenches at topica.com <RE-wrenches at topica.com>
Date: Tuesday, August 15, 2000 11:12 AM
Subject: Re: MIXED PANEL ARRAY'S [RE-wrenches]
>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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