Page 112 - Alternative Energy Systems in Building Design

88 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

WIRING DESIGN

In essence, solar power installations include a hybrid of technologies consisting of
basic ac and dc electric power and electronics—a mix of technologies, each requiring
specific technical expertise. Systems engineering of a solar power system requires
an intimate knowledge of all hardware and equipment performance and application
requirements. In general, major system components such as inverters, batteries, and
emergency power generators, which are available from a wide number of manufactur-
ers, each have a unique performance specification specially designed for specific
applications.
The location of a project, installation space considerations, environmental settings,
choice of specific solar power module and application requirements, and numerous other
parameters usually dictate specific system design criteria that eventually form the basis
for the system design and materials and equipment selection.
Issues specific to solar power include the fact that all installations are outdoors, and
as a result, all system components, including the PV panel, support structures, wiring,
raceways, junction boxes, collector boxes, and inverters, must be selected and designed
to withstand harsh atmospheric conditions and must operate under extreme tempera-
tures, humidity, and wind turbulence and gust conditions. Specifically, the electrical
wiring must withstand, in addition to the preceding environmental adversities, degrada-
tion under constant exposure to UV radiation and heat. Factors to be taken into consid-
eration when designing solar power wiring include the PV module’s short-circuit current
(I ) value, which represents the maximum module output when output leads are shorted.
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The short-circuit current is significantly higher than the normal or nominal operating
current. Because of the reflection of solar rays from snow, a nearby body of water or
sandy terrain can produce unpredicted currents much in excess of the specified nominal
or I current. To compensate for this factor, interconnecting PV module wires are assigned
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a multiplier of 1.25 (25 percent) above the rated I .
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PV module wires, based on NEC requirements, are allowed to carry a maximum load
or an ampacity of no more than 80 percent; therefore, the value of current-carrying
capacity resulting from the previous calculation is multiplied by 1.25, which results in
a combined multiplier of 1.56. The resulting current-carrying capacity of the wires, if
placed in a raceway, must be further derated for specific temperature conditions, as
specified in NEC wiring tables (Article 310, Tables 310.16 to 310.18).
All overcurrent devices also must be derated by 80 percent and have an appropriate
temperature rating. It should be noted that the feeder-cable temperature rating must be
the same as that for overcurrent devices. In other words, the current rating of the devices
should be 25 percent larger than the total amount of current generated from a solar array.
For overcurrent device sizing, NEC Table 240.6 outlines the standard ampere ratings. If
the calculated value of a PV array somewhat exceeds one of the standard ratings of this
table, the next higher rating should be chosen.
All feeder cables rated for a specific temperature should be derated by 80 percent
or the ampacity multiplied by 1.25. Cable ratings for 60, 75, and 90°C are listed in
NEC Tables 310.16 and 310.17. For derating purposes, it is recommended that cables
rated for 75°C ampacity should use 90°C column values.