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.

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.

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.

PAGE 74 — INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07)