Page 115 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM CONFIGURATIONS 91

installed within a protective conduit or raceway. As mentioned earlier, all ground elec-
trode conductors are required to be connected to a single grounding electrode or a
grounding bus.

Equipment grounding Metallic enclosures, junction boxes, disconnect switches,
and equipment used in the entire solar power system that could be accidentally ener-
gized are required to be grounded. NEC Articles 690, 250, and 720 describe specific
grounding requirements. NEC Table 25.11 provides equipment-grounding conductor
sizes. Equipment-grounding conductors similar to regular wires are required to provide
25 percent extra ground current-carrying capacity and are sized by multiplying the
calculated ground-current value by 125 percent. The conductors also must be oversized
for voltage drops, as defined in NEC Article 250.122(B).
In some installations, bare copper grounding conductors are attached along the railings
that support the PV modules. In installations where PV current-carrying conductors are
routed through metallic conduits, separate grounding conductors could be eliminated
because the metallic conduits are considered to provide proper grounding when coupled
adequately. Nevertheless, it is important to test conduit conductivity to ensure that there
are no conduction-path abnormalities or unacceptable resistance values.

ENTRANCE SERVICE POWER CONSIDERATIONS FOR
GRID-CONNECTED SOLAR POWER SYSTEMS
When integrating a solar power cogeneration within existing or new switchgear, it is of
the utmost importance to review NEC Article 690 related to switchgear bus capacity.
As a rule, when calculating switchgear or any other power distribution system bus
ampacity, the total current-bearing capacity of the bus bars is not allowed to be loaded
more than 80 percent of the manufacturer’s equipment nameplate rating. In other words,
a bus rated at 600 A cannot be allowed to carry a current burden of more than 480 A.
When integrating a solar power system with the main service distribution switchgear,
the total bus current-bearing capacity must be augmented by the same amount as the cur-
rent output capacity of the solar system. For example, if we were to add a 200-A solar
power cogeneration to the switchgear, the bus rating of the switchgear must, in fact, be
augmented by an extra 250 A. The additional 50 A represents an 80 percent safety mar-
gin for the solar power output current. Therefore, the service entrance switchgear bus
must be changed from 600 to 1000 A or, at a minimum, to 800 A.
As suggested earlier, the design engineer must be fully familiar with the NEC Article
690 related to solar power design and ensure that solar power cogeneration system elec-
trical design documents become an integral part of the electrical plan-check submittal
documents. The integrated solar power cogeneration electrical documents must incor-
porate the solar power system components, such as the PV array systems, solar collec-
tor distribution panels, overcurrent protection devices, inverters, isolation transformers,
fused service-disconnect switches, and net metering, within the plans and must be
considered as part of the basic electrical system design.
Electrical plans should incorporate the solar power system configuration in the elec-
trical single-line diagrams, panel schedule, and demand-load calculations. All exposed,