Motor Circuit Protection
Group Motor Protection
Group Fusing
Group Motor Installation (Group Fusing) NEC® 430.53
430.53 covers the requirements for group motor installations. Two or more
motors, or one or more motors and other loads may be protected by the same
branch circuit overcurrent protective device if:
(A)
Branch Circuit
Fuse
All motors are 1Hp or less, protected at not over 20A at 120V or at 15A at
600V or less, the full load amp rating of each motor does not exceed 6
amps, the device rating marked on the controller is not exceeded, and
individual overload protection conforms to 430.32.
or (B)
The circuit for the smallest motor is protected per 430.52; i.e. the branch
circuit overcurrent protective device protecting the group meets 430.52
for the circuit with the smallest motor.
or (C)
The complete assembly of properly sized branch circuit overcurrent
protective device, controller, and overload devices is tested, listed, and
marked for a group installation.
Branch Circuit
Conductor
Taps
M
and one of the following:
M
(D)(1) the ampacity of conductors to motors are no less than the ampacity
of the branch circuit conductors
or
or
(D)(2) the conductors to motors have at least 1⁄3 the ampacity of the branch
circuit conductors, are protected from physical damage and are not more than
25 feet long before being connected to the motor overload device.
(D)(3) The tap conductors from the branch circuit overcurrent protective device
(OCPD) to each manual motor controller* marked “Suitable for Tap Conductor
Protection in Group Installations” shall have an ampacity of at least 1⁄10** the
amp rating of the branch circuit OCPD. These tap conductors shall be 10 feet
or less, enclosed and protected from physical damage; if not, then these
conductors shall have an ampacity of at least the same as the branch circuit
conductors. The conductor ampacity from the controller to the motor shall be
per 430.22.
Another Approach
Typically, group motor installations protected by one branch circuit OCPD
and group switching are considered for cost savings. However, caution
should be taken where a conductor is expected to be protected by an
overcurrent protective device significantly greater than the conductor
ampacity. The NEC® implies this caution in 430.53(C) FPN, referring back
to 110.10. Under short circuit conditions, smaller conductors are difficult to
protect, especially by non current-limiting protective devices. Also, group
protection sacrifices selective coordination; a fault on one circuit shuts down
all the loads on the group circuit. As a better alternative, consider group
switching with fuses/fuse holders for each motor or other type load. See
page 144 on group switching. Use holders such a OPM-NG, OPM1038SW,
OPM1038, CH Series, JT Series or TCFH & TCF.
* If a manual motor controller is utilized for this application, it must:
1. Be marked “Suitable for Tap Conductor Protection in Group Installations”.
Group Motor Protection
Group Motor
Application
(Group Fusing)
Must Meet 430.53
NO
Single Motor
Branch Circuit
Must Meet 430.52
YES
[430.53 (A)]
Are all motors
1 HP or less?
YES
NO
NO
[430.53 (B)]
Is smallest motor
protected according
to 430.52?
NO
[430.53(D)(1)
Do conductors to motors have
same ampacity as branch
circuit conductors?
YES
[430.53 (D)(2)]
Do tapped conductors
to each motor have
an ampacity of at
least 1/3 of the
incoming branch
circuit conductor?
YES
YES
NO
NO
[430.53 (C)]
Is the entire assembly
of Branch Circuit Over YES
current Devices and
motor controllers
NO
tested, listed and
marked for a group
installation?
NO
Group Motor
installation not
possible. Each motor
branch circuit must be
individually protected NO
by a branch circuit
overcurrent device.
[430.53(D)(3)]
Do tap conductors from branch
circuit OCPD that supply
manual motor controller* which
is marked "Suitable for Tap
Conductor Protection in Group
Installations" have an ampacity
of at least 1/10** the rating of
the branch circuit OCPD?
[430.53 (D)(2)]
Are tapped
conductors to
each overload
device 25 feet
or less?
YES
Ok to use Group
Motor Protection
but must still meet
Group Switching
430.112. (Motors
served by a single
disconnecting means)
YES
NO
[430.53(D)(3)]
Are these tap conductors
(lineside of controller) 10 feet
or less, enclosed, and
protected from physical
damage?
YES
NO
[430.53(D)(3)]
Is the ampacity of each tap
conductor from controller to
motor per 430.22?
YES
2. Be applied within its voltage limitations (slash voltage rating), if applicable.
3. Be protected by a branch circuit protective device that meets all limitations of the
manual motor controller listing criteria. For instance, it may be required to be
protected by a fuse no greater than a specified amp rating.
** Even though permitted by this section, the branch circuit overcurrent
protective device may not be able to provide adequate short-circuit protection
for a conductor having an ampacity 1/10 the rating of the branch circuit
overcurrent protective device. This is especially the case with non currentlimiting branch circuit protective devices. It is suggested an engineering
conductor protection analysis be conducted for this application (110.10).
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©2005 Cooper Bussmann