They should produce smooth sinusoidal AC sine waves.
They should have a simple conversion process from
DC to AC.
They should be highly efficient.
They should be capable of handling surges.
They should be able to detect automatic loads.
They should have input- and output-side protection from short-circuiting.
They should have great compatibility with EMI suppression.
They should operate quietly.
Using a 24V PV system with a 2,500-watt inverter will
create an ampacity of 105A at full load. The ampacity require-
ment for conductors flanked by the battery and the inverter is
125 percent of the 105 amps, or 131 amps.
Overcurrent Protection
An overcurrent device must protect every ungrounded con-
ductor. The overcurrent device safeguards the conductor from
power surges. [690.9(a)] Sources of power surges within a PV
system include:
PV modules
Batteries and battery chargers
Generators
Ampere Rating
According to NEC Article 690.8(a), all overcurrent devices for
PV module string output circuits must be rated 156 percent
of the short-circuit current. This is the ampere, or unit of
electric current, rating. There are standard ratings for over-
current devices. You should step up to the next size overcur-
rent device if the ampacity is below one of the standard values.
All ampacity calculations have to be corrected for the
gear with the lowest temperature ratings. This is usually the terminals. Some ter-
minals have a 60 degrees Celsius rating and others a rating of 75 degrees Celsius.
The cable ampacity may need to be reduced if 75- or 90-degree Celsius conduc-
tors are used.
Amperes Interrupting Rating (AIR)—Short-Circuit Conditions
Short-circuit circumstances can be harsh. It is possible for batteries to create over
8,000A of short-circuit currents in less than a second. Current that high can
cause the overcurrent device to blow up. All overcurrent devices have interrupt
capabilities or interrupt ratings. They often are called the amperes interrupting
rating (AIR).
TECH TIPS
Minimize voltage drops by upgrading
to a 48-volt system. Voltage drops can
also decrease when you increase the
conductor size.
NOTE
Inverters shut down automatically if
battery power gets too low.
TECH TIPS
The following are not vercurrent
devices:
• Blocking diodes, which are used
to prevent current flow toward a
failed module or from the battery to
the PV array during shading or low
current productivity
• Charge controllers
• Inverters
CHAPTER 8
Connecting the Sun to the Home and the Utility 197
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