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MILHDBK1038
5.5.5.17 Conductors. The selection of conductors, both for size and insulation
type, is required to comply with the National Electrical Code (NEC). Aluminum
conductors and connectors are not permitted to be used. All wiring is required to
be numbered or tagged at connection points.
NEC Table 61014(a) lists the short time ampacities for the different
sizes with various types of insulations. The allowable 30 and 60minute ratings
are listed and the method for calculating the allowable 15 minute rating is
prescribed. NEC Tables 31016 and 31018 list the allowable ampacities for
continuous applications. The short time ampacities listed are required to be used
for the selection of feeder, loop, or branch circuit conductors used with
similarly short time rated motors and their brakes; control circuit conductors and
conductors supplying ancillary equipment are required to be selected using the
continuous ampacities.
Under certain circumstances the published rating of the conductors are
required to be adjusted to be in compliance with the Code. Correction factors for
the conductor's ambient temperature are incorporated into NEC Tables 31016, 310
18, and 61014(a). Application of one of these factors either increases or
decreases the ampacity listed in the main body of the Table. The number of
simultaneously energized or current carrying conductors in a conduit also affects
the ampacity of the conductors in that conduit. Note 8 to NEC Tables 31016 and
31018 addresses the reduction of ampacity for conductors used in continuous
applications. Notes printed below NEC Table 61014(a) address the reduction of
ampacity for conductors used in short time applications. Equipment grounding
conductors and certain grounded conductors are not included in the calculations.
The recommendation of AISE, in their Standard No. 8, that the minimum
conductor size for motor and brake circuits should be No. 8 AWG is to be followed
for motors rated at 5 horsepower or higher. Conductors for motor branch circuits
and feeders are required to be sized to have an ampacity not less than 150 percent
of the motor's nameplate full load amperes. Additionally, voltage drop is
required to be calculated and to not exceed 5 percent. A voltage drop of less
than 5 percent from the supply to the drive motor and of less than 3 percent in
either the feeders and mains or in the branch circuit will provide reasonably
efficient operation. Voltage drop is required to be calculated using the
following "rule of thumb" formula:
EQUATION
VD = 10.37
x
I
x
F
x
L
x
CM
(16)
Where
VD = Voltage drop, in volts
I = Current, in Amperes
F = 2.0 (for DC circuits), 2.0 (for single phase or linetoneutral AC
(circuits), or 1.732 (for three phase AC circuits)
L = Conductor length in feet
CM = Conductor crosssectional area, in circular mils.
This formula does not account for reactance in AC circuits; however, for
conductors smaller than 3/0 AWG, reactance does not effectively contribute to
conductance impedance. The conductivity factor, 10.37, is required to be
corrected for an ambient temperature (t) other than 20 degrees (Celsius) by
multiplying it by [1+0.00393(t20)].
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