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PAGE 74 — INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07)
AC ELECTRICAL CONNECTIONS
Emergency Standby Generator Systems (600 Volts and below)
The National Electric Code (NEC) requires the engine-generator
be provided with phase overcurrent protection such as fuses or
circuit breakers. In some applications, ground fault protection
may be also be required.
Generator Main Line Circuit Breaker
a) Generator-Mounted Main Line Circuit Breaker (MCB) -
Industry practice is to provide a molded-case circuit breaker,
sized to protect the generator feeder conductors against
overcurrent, and provide provisions for a disconnecting
means, to meet National Electric Code (NEC) requirements.
b) Neutral Conductors – The ampacity of the neutral
conductor is generally permitted to be equal to or greater
than the calculated maximum single-phase unbalance of
the load. Where a significant portion of the load is non-
linear, the neutral conductor should be sized in accordance
with anticipated neutral current but never less than 100
percent rated.
Sizing A Generator Main Line Circuit Breaker
Sizing a generator main line circuit breaker is typically the result
of electrical engineering review of generator load schedules
and design calculations for a feeder and its overcurrent device,
keeping in mind that the primary purpose of the generator main
line circuit breaker is to protect the feeder conductors as per the
National Electric Code (NEC).
MQ Power offers several factory-mounted circuit breaker options
per model, based on generator output voltage and current
ampacity. Unless specified otherwise, these circuit breakers, both
thermal-magnetic and electronic trip types, are factory sized for
the maximum output current of each engine-generator, with
regards to their respective voltage connection. The circuit
breakers are mounted on the engine-generator so as to meet
code requirements which stipulate the overcurrent protective
device be located within 25-feet of the generator output terminals.
It should be noted too, when sizing a main line circuit breaker,
that feeder ampacity and overcurrent device ratings should be
calculated by summing the total of load currents of all branch
circuits being supplied by the engine-generator, multiplied by
any applicable demand factors allowed by National Electric
Code (NEC). In any event, the minimum size of the generator
main line circuit breaker should be at least equal to the ampacity
rating of the feeder conductors (or the next largest standard
rating).
Field Installing A Generator Main Line Circuit Breaker
All work should be completed by qualified persons familiar
with the installation, construction and operation of generator
sets. All work should be completed in accordance with the
National Electric Code (NEC), Uniform Building Code (UBC)
and other state or local codes.
When installing a main line circuit breaker NOT factory
supplied by MQ Power, it is code required that the circuit
breaker be UL listed. The overcurrent protective device
should be installed with the correct voltage, current and
short-circuit interruption ratings that are appropriate for the
generator output. The interrupting capacity of the circuit
breaker must be equal to or greater than the amount of fault
current that can be delivered at the point in the system
where the circuit breaker is applied.
Once the circuit breaker has been properly sized and the
appropriate cable and lugs have been determined, the circuit
breaker should be mounted on the engine-generator in a
suitable location. The circuit breaker should be mounted on
the engine-generator so as to minimize vibrations produced
by the engine while running.
Connection Of Generator Leads For Correct Voltage
Output
It is required of the installer to connect the generator main
stator leads (12-lead generator) in a configuration required
to meet the system voltage output requirement. Refer to
the MQ Power reconnection diagram to review the various
voltage connection configurations. Once the voltage
selection and correct wiring configuration has been
completed, the wiring is terminated at the circuit breaker
input lugs and/or bus bar.
DO NOT attempt to field install a main line circuit breaker
while the engine-generator is capable of starting and
running. Serious injury or
death
could result. Make sure
the generator control is in the OFF position, then
disconnect the engine starting battery by lifting the cables
(ground cable first). It is advisable to use "Lock-Out" tags
accordingly.