Determining the ampacity of the conductors in a PV system requires
following the NEC guidelines below:
PV source circuit conductor’s ampacity will be 156 percent of the modules’
short-circuit current rating. [690.8(B)(2)]
The PV system output circuit conductor’s ampacity, that is, the A/C output
from the inverter, will be 125 percent of the inverter continuous output
rating. [690.8(A)]
The PV designer and installer needs to pay close attention to the ampacity of
conductors. Also, be sure to examine the size of overcurrent devices. As an
installer, make sure to address wiring methods and the site’s temperature when
choosing conductors.
Accurate PV system design will result in properly sized conductors and
appropriately rated overcurrent devices. The PV source circuit and the output cir-
cuit will be less likely to short-circuit if they are chosen properly.
It should be noted that overcurrent device ratings need to be less than, or
equal to, the cable’s ampacity. In order to calculate a conductor’s ampacity, start
by multiplying the Isc value by 125 percent. This gives you the continuous current
rating. After that, treat it as you would any continuous cur-
rent, that is, multiply by 125 percent, giving you the amperage
needed to determine the conductors and OCPD required.
When you multiply 125 percent by 125 percent, you get
156 percent.
Lastly, adjust the cable ampacity for temperature. Use
conductors having 90 degree Celsius insulation. Most PV
modules are rated for 90 degree Celsius conductors.
Other adjustments to the ampacity may be made in response to the following
questions:
Will the conductors be located in conduit or exposed to the elements,
especially sunlight?
What are the ambient temperature extremes for the area?
How many current-carrying conductors will be assembled together?
Standalone Systems—Inverters
Inverters are usually used in standalone systems. They are sometimes called
battery-based inverters or island inverters. Standalone inverters handle a wide
spectrum of loads, including:
Equipment on construction sites
Home appliances
Remote cabin electrical needs
Standalone inverters should have the following qualities:
TECH TIPS
Apply these adjustment factors to the
conductor ampacity calculated above.
196 ADVANCED PHOTOVOLTAIC INSTALLATIONS
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