|NEC Article 430 - Motors, Motor Circuits, and Controllers
|Charting a course
through the largest chapter of the NEC
Mike Holt for EC&M
Article 430, with its tough subject and thirteen parts, presents
a challenge. At first glance, correctly applying Article 430 may seem
impossible. But, a closer look shows reveals usability features-such as its
tables-that make Article 430 more user-friendly than many people realize. One
especially notable feature is Figure 430.1, which provides a graphical
representation of how to apply Article 430. Figure 430.1 allows you to proceed
through Article 430 methodically and not miss a key requirement. Let's see what
some of those requirements are.
Part I. General Requirements
load currents. Use the motor full load current rating (see Tables 430.147,
430.148, or 430.150) when determining conductor ampacity [430.22], the
branch-circuit short-circuit and ground-fault protection device [430.52 and 62],
and the ampere rating of switches [430.110]. Do not use the current rating on
the motor nameplate for this purpose (Figure 430-2).
Note: Graphics not part
of this newsletter, they are contained in the textbook, Understanding the
National Electrical Code.
Motor nameplate current rating. Use the motor
nameplate current rating when selecting devices intended to protect motors,
motor control apparatus, and motor branch-circuit conductors against excessive
heating due to motor overloads and failure to start [430.31].
controller terminals. Connect motor controllers and terminals of control circuit
devices with copper conductors, unless identified otherwise. Torque motor
control conductors 14 AWG and smaller at a minimum of 7 lb-in. for screw-type
pressure terminals, unless identified otherwise. See [430.9], [110.3(B)], and
Motor locations. Locate motors to facilitate maintenance and
provide adequate ventilation [430.14].
Part II. Conductor Size
people misapply Chapter 3 ampacity tables when sizing motor conductors, and
therefore undersize the motor conductors. Motors have inrush current, which
conductor sizing must allow for.
Single motors. Per [430.22], size motor
branch-circuit conductors no smaller than 125 percent of the motor FLC rating
listed in Table 430.147 or 430.148 (Figure 430-4). Size the branch-circuit
short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1)
Multiple motors. Per [430.24], size multiple motor conductors as
follows. First, multiply the full-load current rating of the highest-load motor
by 1.25. Then, add up the full-load current ratings of all the other motors in
the group. Add these two numbers. That's your motor load for calculating
ampacity. Add any other loads on that conductor, to calculate total conductor
Test your knowledge, with the following question. What size
branch-circuit conductor does a 71/2-hp, 230V, 3Ø motor need if the conductor
terminals are rated 60°C (Figure 430-5)?
(a) 14 AWG
(b) 12 AWG
(d) none of these
Answer: (c) 10 AWG
22A x 1.25 = 27.5A, 10
AWG, rated 30A at 60°C.
Size the branch-circuit short-circuit and
ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1. For an
inverse-time breaker: 22A x 2.5 = 55A, next size up = 60A.
Let's try sizing a
motor feeder conductor. Here's your question. What size 75°C feeder conductor is
required for two 71/2-hp, 230V, 3Ø motors (Figure 430-7)?
(b) 12 AWG
(c) 10 AWG
(d) 8 AWG
Answer: (d) 8 AWG
1.25) + 22A = 49.5A, 8 AWG rated 50A at 75°C
Size the feeder protection
device [430.62] as follows:
Step 1. Determine the branch-circuit
protection rating. Inverse-Time Breaker: 22A x 2.5 = 55A, next size up
Step 2. Size the feeder protection device. Inverse-Time Breaker: 60A +
22A = 82A, next size down, 80A.
Part III. Overload Protection
overload is not a short-circuit or ground fault. It's an operating current that
is just too high. Overload protection devices will interrupt a current that is
too high, when it persists for too long (typically on the order of seconds). The
time factor allows for the starting current of the motor, which is higher than
the operating current but only momentary. A branch-circuit short-circuit and
ground-fault protection device protects the motor, the motor control apparatus,
and the conductors against short circuits or ground faults, but not against
overload (Figure 430-11).
You must protect each motor branch circuit
against short circuit and ground fault by a protection device sized no greater
than the percentages listed in Table 430.52. Motor branch-circuit conductors are
protected against overcurrent by overloads sized at 115 to 125 percent of motor
nameplate current rating [430.32].
Part VI. Motor Control
You must provide motor control circuit conductors with a
disconnecting means that opens all conductors of the motor control circuit
[430.74]. The controller disconnect can serve as the disconnecting means for
control circuit conductors, if the control circuit conductors are tapped from
the controller disconnect [430.102(A)]. If the control circuit conductors are
not tapped from the controller disconnect, provide a separate disconnect for the
control circuit conductors and locate it adjacent to the controller disconnect
(Figure 430-17). The control circuit disconnect cannot not be higher than 6 ft 7
in. above the floor or working platform, unless located adjacent to the
equipment it supplies [404.8(A)].
Part V. Feeder Short-Circuit and
Per [430.62], protect feeder conductors against
short circuits and ground faults by a protection device sized:
greater than the largest rating of the branch-circuit short-circuit and
ground-fault protective device for any motor
Plus the sum of the full-load
currents of the other motors in the group.
What size feeder protection
(inverse-time breakers with 75°C terminals) and conductor do you need for the
following two motors (Figure 430-15)?
Motor 1: 20-hp, 460V, 3Ø =
Motor 2: 10-hp, 460V, 3Ø = 14A
(a) 8 AWG/70A
(c) 8 AWG/90A
(d) none of these
Answer: (b) 8
(27A x 1.25) + 14A = 48A
8 AWG rated 50A at 75°C [110.14(C) and
Feeder Protection [430.63(A)] not greater than largest
branch-circuit protection device plus other motor FLC.
Step 1. Determine
largest branch-circuit protection device [430-52(C)(1)]
20-hp Motor = 27A
x 2.5 = 68, next size up = 70A [430.52(C)(1) Ex.]
10-hp Motor = 14A x 2.5 =
Step 2. Size Feeder Protection = 70A + 14A, = 84A, next size down =
Part VII. Motor Controllers
Each motor requires its own
controller [430.87]. Select an enclosure suitable for the environment that
controller occupies, per Table 430.91.
Controllers other than circuit
breakers and molded case switches must have a horsepower rating no less than
that of the motor. A circuit breaker can serve as a motor controller [430.111].
A molded case switch, rated in amperes, can serve as a motor controller.
stationary motors rated at 2-hp or less and 300V or less, the controller can be
either of the following:
A general-use switch having an ampere rating
not less than twice the full-load current rating of the motor.
snap switch where the motor full-load current rating is not more than 80 percent
of the ampere rating of the switch.
The motor controller is required to open
only as many conductors of the circuit as necessary to start and stop the motor
[430.84]. For example, one conductor must open to control a 2-wire, 1Ø motor;
two conductors must open to control a 3-wire, 3Ø motor (see Figure 430-19). The
controller starts and stops the motor; it is not a disconnecting means
Part IX. Disconnecting Means
You need a disconnect for
each motor controller. You must locate it within sight of the controller (see
Figure 430-20). "Within sight" means visible and not more than 50 ft from each
other [Article 100]. Under certain circumstances, [430.102(B)] allows exceptions
to this requirement.
The controller disconnect must open all circuit
conductors simultaneously [430.103] (see Figure 430-21). The controller
disconnect can serve as the disconnect for motor control circuit conductors
[430.74] and the motor [430.102(B) Ex.].
The disconnecting means for the
motor controller and the motor must open all ungrounded supply conductors
simultaneously [430.103] (see Figure 430-24).
The disconnecting means
must be legibly marked to identify its intended purpose [110.22 and 408.4]. When
operated vertically, the "up" position corresponds to the "on" state [240.81 and
404.6(C)]. The controller disconnect or motor disconnect required by [430.102]
must be readily accessible (Figure 430-25).
Table 430.148 lists the full-load current for single-phase
alternating-current motors. Use these values to determine motor conductor
sizing, ampere ratings of disconnects, controller rating, and branch-circuit and
feeder protection-but not overload protection [430.6(A)(1)].
430.150 lists the full-load current for 3-phase alternating-current motors. Use
these values to determine motor conductor sizing, ampere ratings of disconnects,
controller rating, and branch-circuit and feeder protection-but not overload
Table 151(A) contains locked-rotor current for
single-phase motors, and Table 151(B) contains the locked-rotor current for
3-phase motors. Use these values in the selection of controllers and
disconnecting means when the horsepower rating is not marked on the motor
Article 430 is an amazing document, given the complexity of
the subject it covers and the ease with which you can apply it. That ease of
application is possible because of usability features, such as the various
tables and Figure 430.1. If you proceed methodically, you will correct apply
Article 430 every time.
Copyright © 2002 Mike Holt