Many installers don't think about sizing generator protection nearly as much as they think about the protective arrangements for the conductors the generator supplies. In terms of electrical design, this is a classic example of ignoring the forest for the trees. Although we certainly don't want to damage the conductors from overloading, the Code also requires overload protection for generators. In addition, a large generator represents a major capital expenditure. While the NEC requires overload protection of generators, it isn't as straightforward as using a main output molded case thermomagnetic circuit breaker or set of fuses based on the wire size. This is very important when either local design constraints or other standards require selective coordination with downstream protective devices. Selective coordination means that, even under the worst-case available fault, a fault in a smaller feeder originating below the level of the main generator protection will do two things. First, the overcurrent device next upstream will open and clear that fault safely. Second, all other overcurrent devices further upstream will stay closed, limiting the extent of the outage as much as possible. The NEC requires this kind of selective coordination on most elevator feeders in Sec. 620-62. In addition, Sec. 4-5.1 in NFPA 110, Emergency and Standby Power Systems, requires designers to "optimize selective tripping of the circuit overcurrent protective devices when a short circuit occurs." The word "optimize" was chos
Many installers don't think about sizing generator protection
nearly as much as they think about the protective arrangements for
the conductors the generator supplies. In terms of electrical
design, this is a classic example of ignoring the forest for the
trees. Although we certainly don't want to damage the conductors
from overloading, the Code also requires overload protection for
generators. In addition, a large generator represents a major
capital expenditure.While the NEC requires overload protection of
generators, it isn't as straightforward as using a main output
molded case thermomagnetic circuit breaker or set of fuses based on
the wire size. This is very important when either local design
constraints or other standards require selective coordination with
downstream protective devices. Selective coordination means that,
even under the worst-case available fault, a fault in a smaller
feeder originating below the level of the main generator protection
will do two things. First, the overcurrent device next upstream
will open and clear that fault safely. Second, all other
overcurrent devices further upstream will stay closed, limiting the
extent of the outage as much as possible.The NEC requires this kind
of selective coordination on most elevator feeders in Sec. 620-62.
In addition, Sec. 4-5.1 in NFPA 110, Emergency and Standby Power
Systems, requires designers to "optimize selective tripping of the
circuit overcurrent protective devices when a short circuit
occurs." The word "optimize" was chos
