Well, not exactly. Yes, all of those things
will usually keep a photovoltaic (PV) system from generating
power, but that is to be expected when your power system uses
sunlight for fuel. However, these and other weather conditions
also affect how a PV system is designed and installed to
comply with the requirements of the National Electrical
Code. With a PV power system lifetime exceeding 40 years,
Mother Nature is going to use every trick in the book to make
that system fail before its time. PV designers, installers,
and inspectors need to devote significant attention to the
weather-related safety requirements for PV systems to help
ensure long-lived, hazard-free electrical installations.
Intense Solar
Radiation and High Temperatures
Before we look at the dark side of PV, let’s examine hot and
sunny. PV modules are designed to operate in sunlight and the
more sunlight they get, the more power and energy they will
produce. The "Perspectives on PV" article in the
July-August 2004 IAEI News (available on the SWTDI web
site) addressed how the PV module’s electrical output is
affected by the intensity of the solar radiation. But what
about the related issues of the effects of temperature and
ultra-violet radiation on the other equipment? Any equipment
exposed to the sunlit environment should be rated for the
exposure. Exposed conductors and cables must be marked
sunlight resistant (UF and TC cables) or be tested during the
listing process for UV exposure (USE and SE, Table 310.13)
cables. Using the wrong conductors can lead to failures (photo
1). Because of the normal operating environment, cables
attached to PV modules must be listed for wet (the W
designator) and hot environments (the HH designator)
and a -2 after the cable type gets them both. We have tested
PV installations that have been in hot, sunny, dry weather for
two weeks or more, opened module j-boxes and conduit bodies
and had hot water run out. -2 cables are a must.
Underwriters Laboratory is releasing a
specification for a new "PV" cable. Cables meeting
this specification will have to pass a 720-hour accelerated UV
exposure test, be rated for wet locations, have at least a
90°C rated insulation, have a flame retardant compound, and
have a physically tougher insulation than type USE cable.
Although the intent of the specification was that compliant
cables would now meet the requirements for use in ungrounded
PV systems as permitted by Section 690.35 of the NEC,
it has yet to be determined how the "PV" cable will
be used, given the existing code language in 690.35(D). The
issue will hopefully be clarified in the 2008 NEC and
the PV industry is looking at ways to use this cable long
before the 2008 Code is enacted. Enlightened electrical
inspectors who may see the new cable as an acceptable
alternative to USE-2 may be the key to early its adoption.
Cord grips and cable clamps used on outdoor
junction boxes should be UV rated. In some cases, metal cord
grips have been used, and while metal is resistant to UV,
these generally have not been listed for outdoor use because
they can corrode rapidly. Nylon cable ties are frequently used
to tie conduits and exposed cables to module racks. The white
cable ties have no UV resistance, and even some of those that
are black fail in a few months. The use of listed cable ties
specifically marked (at least on the package) for outdoor use
and sunlight resistance should be encouraged. Even better is
the use of stainless steel pipe clamps with neoprene rubber
inserts to firmly secure exposed single conductor cables to
racks and frames (photo
2).
The Fine Print Note No. 2 in 310.10 of the 2005
NEC points out that conduits on buildings in sunlight
operate at temperatures of 17°C above the ambient
temperatures. Because conduits in PV systems are exposed to
sunlight for decades, the raceways many times become
discolored or darken with age (photo
3). Therefore, I suggest to the PV installers that 20°C
be added to the highest ambient temperature when doing
ampacity calculations, to account for the higher solar energy
absorption of the aged materials. With PV module junction
boxes operating in the 65–75°C (and hotter) ranges and
conduits in sun in ambient temperatures of 40–50°C plus the
added 20°C for solar heating, it becomes evident that 310.15
temperature corrections are critical in calculating ampacities
of wiring for PV systems.
PV combiner boxes that combine the outputs
of strings of PV modules are also mounted in the sun. These
devices (photo 4)
usually contain overcurrent devices, and most overcurrent
devices are rated for operation in ambient temperatures up to
40°C. With ambient temperatures in many locations of 40°C
(45–50°C in the Southwest), solar heating of these
enclosures pushes the internal temperature well above 40°C.
The overcurrent device manufacturer must be consulted for
appropriate temperature corrections. After applying the
corrections by increasing the rating of the overcurrent
device, the installer must then go back and verify that
conductors and modules are properly protected from fault
currents.
We also have to deal with those often
overlooked terminal temperature limitations in NEC 110.14(C) because the high PV module temperatures require the
PV installer to use 90°C rated conductors. While the modules
all have terminals rated for use with 90°C conductors, the
combiner boxes and most of the other fused disconnects and
overcurrent devices have terminals that are restricted to use
with 60°C or 75°C rated conductors. Some of the combiner
boxes do not have temperature markings, and since the
overcurrent devices are usually below 100 amps, a conductor
temperature limitation of 60°C must be assumed. Things get
pretty complex when we deal with a fused combiner box, for
example, with unmarked terminal temperature limitations.
Consider one operating in the sun in Phoenix, Arizona, where
the ambient temperature may be 45°C for weeks at a time. The
box temperature could be 55–60°C (requiring 90°C insulated
conductors), the fuse rating must be temperature corrected for
the 55–60°C operating temperature, and then the operating
temperature of the conductor/terminal at these elevated
temperatures must be estimated to be less than 60°C.
Obviously, if the internal temperature of the enclosure is
near 60°C, it is going to be difficult to have fuse terminals
operating below 60°C with any appreciable current in the
terminals. In this case, the prudent path would be to replace
this PV combiner with one that is marked for use with 75°C
insulated conductors. At the very least, any enclosure
containing overcurrent devices installed in the hot Southwest
(and other hot locations) should be mounted in the shade where
it will be subjected to no more than the high ambient
temperatures (photo
5).
Wind, Sleet, Snow
and Rain
Moving from the hot summer Southwest (and other parts of the
country) to winter and the colder locations, we see other
weather related issues. Equipment has to be able to withstand
wind-driven rains. The use of appropriate types of NEMA
enclosures will generally ensure that the internal equipment
will not be subject to direct water spray. The use of listed
devices will ensure that the internal connections are also
generally immune to the effects of wind-driven rain. However,
some custom, field-assembled enclosures may have been made
with materials that are not well designed for even a little
moisture. Rust may form when the internal components have not
been properly specified for outdoor/damp areas (see photo
6).
High winds are an issue in coastal areas
where hurricanes are common as well as in many other areas of
the country. Building codes in these areas generally specify
how items on the roof and on the ground are to be fastened
down to resist the lifting forces of the wind. The Study
Guide for the North American Board of Certified Energy
Practitioners (NABCEP) has some guidance for PV installers in
this area that is based on information in the National
Design Specification for wood construction and on roofing
manuals. The Study Guide may be downloaded from the
Resources section of the NABCEP web site (www.nabecp.org).
In areas of the country where there is snow
buildup on roofs, attention must be directed to securely
fastening all conductors and cables to the module racks or
mounts and to the roof. Otherwise, sliding snow can rip wires
loose and pull conduits loose. Similar attention to these
workmanship details should be applied to windy areas and in
all installations, a neat, workman-like installation will
usually be safer that a messy installation (photo
2).
The PV designer/installer will usually be
required to make a tradeoff between the best tilt angle for PV
array performance and the angle that will best shed snow.
Fortunately, as the installation location moves farther north
(into snow country), the tilt angle for best performance gets
greater and even assists in shedding snow. However, these
higher tilt angles usually result in the PV modules being
subjected to higher wind loading, so secure mounting is a
must.
At very low temperatures, snow, sleet and
freezing rain may adhere to the PV modules and must be removed
if full output from the PV system is desired (photo
7). Obviously rooftop installations may make this more
difficult. On the other hand, ground-mounted arrays must be
high enough to avoid deep snow and drifts.
Hail? Usually, hail doesn’t pose too much
of a problem. The PV modules are made with tempered glass and
the modules are tested with impacts simulating hailstones.
Summary
Yes, sleet, snow, rain, and the dark of night will prevent a
PV system from producing energy. But when the snow melts and
the sun comes up, that PV system will again be generating
power for a very, very long time. The wide range of
environmental conditions in which PV systems are installed
impose significant design and installation requirements. The NEC has been addressing such requirements for many years. The long
life of these systems points to the need for durable hardware
and high levels of workmanship. The equipment is required to
be up to the task. Installers and inspectors must also be up
to the job.
For Additional
Information
If this article has raised questions, do not hesitate to
contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu
Phone: 505-646-6105
A PV Systems Inspector/Installer Checklist
will be sent via e-mail to those requesting it. A color copy
of the 143-page, 2005 edition of the Photovoltaic Power
Systems and the National Electrical Code: Suggested Practices,
published by Sandia National Laboratories and written by the
author, may be downloaded from this web site: (http://www.nmsu.edu/~tdi/roswell-8opt.pdf.
) A black and white printed copy will be mailed to those
requesting a copy via e-mail if a shipping address is
included. The Southwest Technology Development web site (http://www.nmsu.edu/~tdi)
maintains all copies of the previous "Perspectives on
PV" articles. Copies of "Code Corner " written
by the author and published in Home Power Magazine over
the last 10 years are also available on this web site.
Draft proposals for the 2008 NEC being developed by the PV Industry Forum may be downloaded
from this web site: http://www.nmsu.edu/~tdi/pdf-resources/2008NECproposals2.pdf
The author makes 6–8 hour presentations
on "PV Systems and the NEC" to groups of 40
or more inspectors, electricians, electrical contractors, and
PV professionals for a very nominal cost on an as-requested
basis. A schedule of future presentations can be found on the
SWTDI web site.
John Wiles works at the Southwest
Technology Development Institute (SWTDI) at New Mexico
State University. SWTDI has a contract with the US
Department of Energy to provide engineering support to
the PV industry and to provide that industry, electrical
contractors, electricians, and electrical inspectors
with a focal point for code issues related to PV
systems. He serves as the secretary of the PV Industry
Forum that will be submitting 30+ proposals for Article
690 in the 2008 NEC. He provides draft comments to NFPA
for Article 690 in the NEC Handbook. As an old solar
pioneer, he lives in a stand-alone PV-power home in
suburbia with his wife, two dogs, and a cat—permitted
and inspected, of course.
This work was supported by the United
States Department of Energy under Contract
DE-FC04-00AL66794 |
