Page 101 - Solar Power in Building Design The Engineer's Complete Design Resource

ENTRANCE SERVICE CONSIDERATIONS FOR GRID-CONNECTED SOLAR POWER SYSTEMS 71

electrode conductor is connected. As per the NEC all PV systems—whether grid-
connected or stand-alone, in order to reduce the effects of lightning and provide a
measure of personnel safety—are required to be equipped with an adequate ground-
ing system. Incidentally, grounding of PV systems substantially reduces radio-
frequency noise generated by inverter equipment.
In general, grounding conductors that connect the PV module and enclosure frames
to the ground electrode are required to carry full short-circuited current to the ground;
as such, they should be sized adequately for this purpose. As a rule, grounding con-
ductors larger than AWG #4 are permitted to be installed or attached without special
protection measures against physical damage. However, smaller conductors are
required to be installed within a protective conduit or raceway. As mentioned earlier,
all ground electrode 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, which could be accidentally energized 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 ground-
ing 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 must also be oversized for voltage drops as
defined in NEC Article 250.122(B).
In some installations bare copper grounding conductors are attached along the rail-
ings that support the PV modules. In installations where PV current-carrying conduc-
tors are routed through metallic conduits, separate grounding conductors could be
eliminated since the metallic conduits are considered to provide proper grounding when
adequately coupled. It is, however, important to test conduit conductivity to ensure that
there are no conduction path abnormalities or unacceptable resistance values.

Entrance Service Considerations for
Grid-Connected Solar Power Systems

When integrating a solar power cogeneration within an existing or new switchgear, it is
of the utmost importance to review NEC 690 articles 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