Page 98 - Solar Power in Building Design The Engineer's Complete Design Resource

68 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS

WIRING DESIGN

Essentially 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 manufacturers, 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 material and equipment selection.
Issues specific to solar power relate to the fact that all installations are of the outdoor
type, 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
temperatures, humidity, and wind turbulence and gust conditions. Specifically, the elec-
trical wiring must withstand, in addition to the preceding environmental adversities,
degradation under constant exposure to ultraviolet radiation and heat. Factors to be
taken into consideration when designing solar power wiring include the PV module’s
short-circuit current (Isc) value, which represents the maximum module output when
output leads are shorted. The short-circuit current is significantly higher than the nor-
mal 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 Isc current. To compensate for this factor, interconnecting
PV module wires are assigned a multiplier of 1.25 (25 percent) above the rated Isc.
PV module wires as per the 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 must also be derated by 80 percent and have an appropriate
temperature rating. Note 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 sum of the 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 highest 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. Various device terminals,