Hi Pat,
Thanks for your response. It appears that
we agree that it is now "code" to daisy chain 24 volt series sets in an
array. To your knowledge, was this allowed
in the old code?
There are quite a few systems wired with every
series set going to a combiner box. Some distributors have been selling
combiner boxes for installation on small systems.
The diagram in the '96 code, Figure
690-1, shows sets of three modules in series, with each series set feeding one
photovoltaic source circuit.
Since the '96 Code states that the removal of a
module shall not interrupt the grounded conductor from another source circuit,
many electricians and inspectors have interpreted that a pair of modules,
wired in series, could not have its grounded conductor run through another
module's junction box.
The '99 Code adds details on what a "source circuit
is."
Does the added wording, stating that a "single-source circuit" can be composed of "Sets
of modules interconnected as systems rated at 50 volts or less," constitute a
change in intent of the Code?
It certainly will affect the way in which the
wording and diagrams are interpreted. As so many of us have found
out, what is important is not what the Code says, but how it is interpreted!
It will lower system costs if word can be
gotten out to installers, inspectors and individuals who are building their own
systems, that it is not necessary to run a separate conduit to a combiner
box for each series pair of modules.
Good luck with your attempt to get the code language
clarified concerning the voltage designation of single-source circuits. It
seems very unclear what "rated voltage" means. One can interpret
nominal voltage, operating voltage and open circuit voltage, but what is rated
voltage? Is "rated voltage" even defined in the code?
The NEC uses the term "rated voltage" quite a
bit. Some sections seem to equate rated voltage with nominal
voltage.
The issue certainly needs clarification. Is a
system rated 48 volts if it powers a 48 volt inverter, or is its rating the open
circuit voltage of the series set of modules powering it? I hope you
succeed in getting an answer.
Drake
----- Original Message -----
Sent: Monday, April 10, 2000 11:45
AM
Subject: Re: Module Wiring Made Easy?
[RE-wrenches]
Drake and Don,
My understanding of 690-4(c) is that it allows
you to daisy chain the conduit runs of modules within a single source circuit.
A rack of 8 modules wired in a 24 volt series-parallel configuration could
qualify as a single source circuit. In theory, this allows you to run one
conduit whip for each panel assembly (rack). In practice, since most modules
have 1/2 inch knock outs, you may be limited by the wire volume
allowed in 1/2 inch conduit. The module j-box will also limit the number of
wires you can run through it. In my installations I commonly run two series
strings through one 1/2 inch conduit.
Each series string is still required to have its
own supplementary overcurrent device (supposedly to protect the modules per
UL). So if you want to run one whip for this rack you would still have to have
a fuse/combiner box on the rack.
The "rated at 50 volts" language is ambiguous.
The history of this article suggests the intent to limit it to 24 volt nominal
systems. The inspectors I have dealt with have always allowed 24 volt nominal
systems to be wired this way. Some have allowed 48 volt nominal systems to be
wired this way. I have submitted a proposal to amend the 2002 NEC to
clear up the language and allow 48 volt nominal systems to be wired this
way.
----- Original Message -----
Sent: Monday, April 10, 2000 10:02
AM
Subject: Module Wiring Made Easy?
[RE-wrenches]
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
----- Original Message -----
Sent: Monday, April 10, 2000 6:35
PM
Subject: Re: Wiring Made Easy?
[RE-wrenches]
Hi Pat,
Thanks for your response. We are trying
to figure out if the 1996 code allowed conductors from series sets to be wired
in the J-boxes of other modules. There are quite a few systems wired
with every series set going to a combiner box. Distributors have been
selling a lot of combiner boxes for installation on small systems.
The diagram in the '96 code, Figure
690-1, shows sets of three modules in series, with each series set feeding one
photovoltaic source circuit.
Since '96 Code states that the removal of a module
shall not interrupt the grounded conductor from another source circuit, many
electricians and inspectors have interpreted that a pair of modules,
wired in series, could not have its grounded conductor run through another
module's junction box.
The '99 Code adds details on what a "source
circuit is."
Does the added wording, stating that a
"single-source circuit" can be composed of "Sets of modules
interconnected as systems rated at 50 volts or less" constitute a change in
intent of the Code? It certainly will affect the way in which the
wording and diagrams are interpreted.
As we all know, what is important is not what the Code says, but how it
is interpreted!
Either way, hopefully word can be gotten out to
installers, inspectors and individuals who are building their own systems that
it is not necessary to run a separate conduit to a combiner box for each
series pair of modules.
Good luck with your attempt to get the code
language clarified concerning the voltage designation of single-source
circuits. It seems very unclear what "rated voltage"
means. One can interpret nominal voltage, operating voltage and
open circuit voltage, but what is rated voltage? Is "rated voltage" even
defined in the code?
The Code uses the term "rated voltage" quite a
bit. Some sections seem to equate rated voltage with nominal
voltage.
The issue does seem quite unclear. Is a
system rated 48 volts if it powers a 48 volt inverter, or is its rating the
open circuit voltage of the series set of modules powering it? I
hope you succeed in getting an answer.
The
RE LAB,
Drake
Don
----- Original Message -----
Sent: Monday, April 10, 2000 11:45
AM
Subject: Re: Module Wiring Made Easy?
[RE-wrenches]
Drake and Don,
My understanding of 690-4(c) is that it allows
you to daisy chain the conduit runs of modules within a single source
circuit. A rack of 8 modules wired in a 24 volt series-parallel
configuration could qualify as a single source circuit. In theory, this
allows you to run one conduit whip for each panel assembly (rack). In
practice, since most modules have 1/2 inch knock outs, you may be
limited by the wire volume allowed in 1/2 inch conduit. The module
j-box will also limit the number of wires you can run through it. In my
installations I commonly run two series strings through one 1/2 inch
conduit.
Each series string is still required to have
its own supplementary overcurrent device (supposedly to protect the modules
per UL). So if you want to run one whip for this rack you would still have
to have a fuse/combiner box on the rack.
The "rated at 50 volts" language is ambiguous.
The history of this article suggests the intent to limit it to 24 volt
nominal systems. The inspectors I have dealt with have always allowed 24
volt nominal systems to be wired this way. Some have allowed 48 volt nominal
systems to be wired this way. I have submitted a proposal to amend the
2002 NEC to clear up the language and allow 48 volt nominal systems to be
wired this way.
----- Original Message -----
Sent: Monday, April 10, 2000 10:02
AM
Subject: Module Wiring Made Easy?
[RE-wrenches]
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