UFC 3 -520-01
June 10, 2002
maximum locked rotor motor current in the calculation of voltage drop. The impedance
is the total impedance from the service entrance to the motor. If other significant loads,
such as other large motors, are also contributing to the total feeder current, add this
current to the motor starting current in the feeder. If appropriate for the situation,
demand factors can be applied to the other load currents.
2-6.2.5 Refer to paragraph 7 -2.3 for design alternatives to consider if motor starting
current causes more than a 20 percent transient voltage dip during starting.
2-6.2.6 Evaluate the effect of moto r starting transients with respect to flicker of
incandescent lamps. Refer to IEEE 241, IEEE Recommended Practice for Electric
Power Systems in Commercial Buildings, for information regarding the calculation and
effect of flicker. This is particularly important for any electrical system with a large
proportion of motor loads or a system with cycling motor loads.
ELECTRICAL COORDINATION .
Determine if electrical coordination is a design requirement for the electrical
system or some portion of the electrical system. If coordination is required, p erform a
coordination study to ensure that protective device settings are appropriate for the
expected range of conditions. The coordination study of an electric power system
consists of an organized time-current study of all protective devices in series from the
electrical source (commercial power or a backup power source) down to the utilization
device. This study compares the time it takes the individual devices to operate when
certain levels of norma l or abnormal current are sensed. The objective of a coordination
study is to generate a comprehensive one -line-diagram representation of the distribution
system performance to abnormal currents. This is intended to ensure that protective
devices will isolate a fault or overload anywhere in the system with the least possible
effect on unfaulted sections of the system. At the same time, the devices and settings
selected must provide satisfactory protection against overloads on the equipment and
must inter rupt short circuits as rapidly as possible. Depending of the design and
construction schedule, it might be necessary to design adequate protective devices with
adjustable features, followed by a coordination study during construction to specify the
Ensure electrical systems are selectively coordinated to the maximum degree
practical. Critical systems require selective coordination to the first panelboard level, as
a minimum. Refer to C hapter 9 for specific electrical coordination study criteria.
Refer to C hapter 12 for specific analysis criteria for power quality design
requirements. Chapter 12 provides a technical basis for power system quality as a
design consideration and explains different methods of solving power quality problems.
Unlike other system electrical design criteria, power quality design solutions are very
dependent on the types of transients and disturbances that can and will occur in power