UFC 3 -520-01
June 10, 2002
conductor ratings. For larger systems, a computer program is necessary to evaluate
properly all of the possible facility modes of operation.
VOLTAGE DROP ANALYSIS .
Voltage Drop Calculations.
2-6.1.1 A power flow analysis as described in pa ragraph 2-5 provides equivalent
voltage drop results also by displaying the voltages throughout the electrical system.
Manual voltage drop calculations are necessary for smaller systems in which a power
flow analysis was not performed or to check smaller load panels in larger systems.
2-6.1.2 Verify adequate voltage drop throughout the electrical distribution system.
Paragraph B-2 provides information regarding how to calculate voltage drop. The
allowed voltage drop is typically no more than 5 percent for continuous loads and
depends of the voltage range required by the load. Refer to paragraph 6 -2 for specific
2-6.1.3 As part of a voltage drop study, evaluate the voltage regulation throughout the
system while considering the effect of overvoltage and undervoltage on the various load
types. Distribute voltage drop throughout the system in the most economical way.
Specify transformer tap settings as needed to satisfy system voltage drop limitations.
Verify that selected transformer tap settings are acceptable for both full-load and no-
Motor Starting Studies.
2-6.2.1 Confirm system loading, conductor sizing, and transformer sizing is adequate
to ensure that voltage drops do not reduce motor torque values below mi nimum values.
Ensure that starting current of any motor will not actuate undervoltage protection or
cause stalling of other motors.
2-6.2.2 If a computer program was used to perform the power flow analysis, the same
electrical system model will often be used to evaluate the voltage drop during motor
starting. An impact motor starting analysis is acceptable to determine the minimum
voltage during motor starting.
2-6.2.3 If large motors are to be started by an emergency generator, a more detailed
transient analysis will be necessary; contact the generator manufacturer for assistance.
2-6.2.4 The voltage drop caused by motor starting can be calculated by hand if a
computer program is not available. Calculate the maximum voltage drop under locked
rotor conditions to determine the worst -case effect of motor starting. Follow the
procedure described in Appendix B. If actual motor data is not available, obtain motor
full-load current from NEC Table 430.150 (2002 Edition) and obtain maximum motor
locked roto r current from NEC Table 430.151B (2002 Edition). Energy efficient Design
B motors can have a locked rotor current near that of Design E motors. Use the