Application Note

3 Fluke Corporation Determining load horsepower, wiring, and breaker size for safe and efficient installations
generally conductors and circuit
breakers are sized at 125 % of
the continuous load plus 100
% of the non-continuous load.
A “continuous load” is one
where the maximum current is
expected to continue for three
hours or more. One important
point: when sizing conductors
and breakers for motors use the
appropriate table in the NEC for
motor full-load amperage—not
the value previously measured
or motor nameplate information.
The value previously measured
is for helping to determine the
size of the load. Sizing wires
and breakers to supply a motor
is based on code tables provid-
ing the full-load current values
for specific phase, voltage, and
horsepower motors. Manufac-
turer’s ratings and measured
values are used for other than
motor loads.
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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In Europe/M-East/Africa [phone] or
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From other countries [phone] or
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Web access: http://www.fluke.com
©2011 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 6/2011 4067476 A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
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up and running.
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For example, a three-phase, 25
horsepower chilled water pump
motor is expected to operate at
full load for three hours or more.
The NEC tables indicate that
the full load current of a three-
phase, 460volt, 25 horsepower
motor is 34 amps. Therefore, the
conductors supplying the motor
must be sized at 34 x 1.25 =
43 amps (125% of 34 amps).
Ampacity tables in the NEC are
used to determine the actual
conductor size based on insula-
tion type, ambient temperatures,
and other conditions. The maxi-
mum circuit breaker or fuse size
for the motor is based on another
NEC table, Table 430.52. The
maximum value of this overcur-
rent protective device may range
anywhere from 175 % to 250%
of the full load current. Always
consult the National Electrical
Code or a qualified electrician
for exact sizes for motor wiring,
fuses, and circuit breakers, as
well as the overload protection
requirements for motors. The
same holds true for hermetic
refrigerant motor-compressors
and other types of electrical
equipment.
The goal: A properly
sized and safe
installation operating at
maximum efficiency
You must determine motor horse-
power in the field to verify that
the right-size motor has been
applied. If the motor is oversized,
consider replacing the motor
or installing a variable-speed
drive. Routinely measuring and
recording current and voltage
values is also an important part
of a quality preventive mainte-
nance program. Size the wiring
and breakers for any type of
load using the National Electrical
Code. Remember, the goal is a
properly sized and safe instal-
lation operating at maximum
efficiency.