Q1. When sizing Motor circuits, when does the code require the use of
the Code tables as opposed to the nameplate current?
A1. According to 430.6(A)(1),
the motor full-load current ratings listed in Tables 430.247, 430.248, and
430.250 are used to determine the conductor ampacity [430.22], the
branch-circuit short-circuit and ground-fault overcurrent device size [430.52
and 430.62], and the ampere rating of disconnecting switches [430.110].
Author’s Comment:
The actual current
rating on the motor nameplate full-load amperes (FLA) [430.6(A)(2)] isn’t
permitted to be used to determine the conductor ampacity, the branch-circuit
short-circuit and ground-fault overcurrent device size, nor the ampere rating
of disconnecting switches.
Motors built to operate at less
than 1,200 RPM or that have high torques may have higher full-load currents,
and multispeed motors have full-load current varying with speed, in which case
the nameplate current ratings must be used.
430.6(A)(1) Ex 3:
For a listed motor-operated appliance, the actual current marked on the
nameplate of the appliance must be used instead of the horsepower rating on the
appliance nameplate to determine the ampacity or rating of the disconnecting
means, the branch-circuit conductors, the controller, and the branch-circuit
short-circuit and ground-fault protection.
430.6(A)(2) Motor Nameplate Current Rating (FLA). Overload devices must be sized based
on the motor nameplate current rating in accordance with 430.31.
Author’s Comment:
The motor nameplate
full-load ampere rating is identified as full-load amperes (FLA).
The FLA rating is the current in amperes the
motor draws while producing its rated horsepower load at its rated voltage,
based on its rated efficiency and power factor.
The actual current drawn by the
motor depends upon the load on the motor and on the actual operating voltage at
the motor terminals. That is, if the load increases, the current also
increases, or if the motor operates at a voltage below its nameplate rating, the
operating current will increase.
CAUTION: To prevent
damage to motor windings from excessive heat (caused by excessive current),
never load a motor above its horsepower rating, and be sure the voltage source
matches the motor’s voltage rating.
Q2. What is the Code rule for sizing conductors to a single continuous
duty motor?
A2. Conductors to a single motor must
be sized not less than 125 percent of the motor FLC rating as listed in Table
430.247 Direct-Current Motors, Table 430.248 Single-Phase Motors, or Table
430.250 Three-Phase Motors [430.22].
Question: What size branch-circuit conductor is required
for a 7½ hp, 230V, three-phase motor with 75ºC
terminals?
(a) 14 AWG (b) 12 AWG (c) 10 AWG (d) 8 AWG
Answer: (c) 10 AWG
Motor FLC = 22A
[Table 430.250]
Conductor’s
Size = 22A x 1.25
Conductor’s Size = 27.50A,
10 AWG, rated 35A at 75°C
[Table 310.15(B)(16)]
Note: The branch-circuit short-circuit and ground-fault protection device
using an inverse time breaker is sized at 60A according to 430.52(C)(1) Ex 1:
Circuit Protection =
22A x 2.50
Circuit Protection =
55A, next size up 60A [240.6(A)]
Q3. When sizing conductors for more than one motor, what does the Code
consider to be the highest rated motor?
A3. When sizing motor circuit conductors, the highest rated
motor is the motor with the highest rated full-load current rating (FLC) [430.17].
Question: Which of the following motors has the highest
FLC rating?
(a) 10 hp, three-phase, 208V (b) 5 hp, single-phase, 208V
(c) 3 hp, single-phase, 120V (d)
none of these
Answer: (c) 3 hp, single-phase, 120V
10 hp = 30.80A
[Table 430.250]
5 hp = 30.80A [Table 430.248]
3 hp = 34.00A [Table 430.248]
Q4. What is the Code requirement for sizing conductors that supply
several motors?
A4. Circuit conductors that supply several motors must not
be sized smaller than the sum of the following [430.24]:
(1) 125 percent of the full-load current of the
highest rated motor
(2) The full-load current ratings of other motors
Question: What size feeder conductor is required for two
7½ hp, 230V, three-phase motors, if the terminals are rated for
75°C?
(a) 14 AWG (b) 12 AWG (c) 10 AWG (d) 8 AWG
Answer: (d) 8 AWG
Motor FLC = 22A [Table 430.250]
Motor Feeder Conductor = (22A x 1.25) + 22A
Motor Feeder
Conductor = 49.50A, 8 AWG rated 50A at 75°C
[Table 310.15(B)(16)]
Author’s
Comment:
The feeder
overcurrent device (inverse time circuit breaker) must comply with 430.62 as
follows:
Step 1: Determine the largest branch-circuit
overcurrent device rating [240.6(A) and 430.52(C)(1) Ex 1]:
22A x 2.50 = 55A, next size up 60A
Step 2: Size the feeder overcurrent device in
accordance with 240.6(A) and 430.62:
Feeder Inverse Time
Breaker: 60A + 22A = 82A,
next size down, 80A
Author’s
Comment:
The “next size up protection” rule for
branch circuits [430.52(C)(1) Ex 1] doesn’t
apply to motor feeder short-circuit and ground-fault protection device sizing.
Q5. What is the Code requirement for protecting a motor from short
circuit, ground fault, and overload with a single device?
A5. A motor can be
protected against overload, as well as short circuit, and ground fault by a
single overcurrent device sized to the overload requirements contained in
430.32 [430.55].
Question: What size dual-element fuse is permitted to
provide overload as well as short circuit and ground fault protection for a 5
hp, 230V, single-phase motor with a service factor of 1.15 and a nameplate
current rating of 28A?
(a) 20A (b)
25A (c)
30A (d)
35A
Answer: (d) 35A
Overload Protection
[430.32(A)(1)]
28A x 1.25 = 35A
Q6. What is the Code rule for sizing short circuit and ground fault
protection for a single motor branch circuit?
A6. The motor branch-circuit short-circuit and ground-fault
protective device must comply with 430.52(B) and 430.52(C).
(B) All Motors. A motor
branch-circuit short-circuit and ground-fault protective device must be capable
of carrying the motor’s starting current.
(C) Rating or Setting.
(1) Table 430.52. Each motor
branch circuit must be protected against short circuit and ground faults by a
protective device sized no greater than the following percentages listed in
Table 430.52.
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Table 430.52
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Motor Type
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Nontime Delay
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Dual-Element Fuse
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Inverse Time Breaker
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Wound Rotor
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150%
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150%
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150%
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Direct Current
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150%
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150%
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150%
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All Other Motors
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300%
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175%
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250%
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Question: What size conductor and inverse time circuit
breaker are required for a 2 hp, 230V, single-phase motor
with 75ºC terminals?
(a) 14 AWG, 30A breaker (b)
14 AWG, 35A breaker
(c) 14 AWG, 40A breaker (d) 14 AWG, 45A breaker
Answer: (a) 14 AWG, 30A breaker
Step 1: Determine the branch-circuit conductor [Table
310.15(B)(16), 430.22, and Table 430.248]:
12A x 1.25 = 15A, 14 AWG, rated 20A at 75°C
[Table 310.15(B)(16)]
Step 2: Determine the branch-circuit protection
[240.6(A), 430.52(C)(1), and Table 430.248]:
12A x 2.50 = 30A
Author’s
Comment:
I know it bothers many in the electrical industry to see
a 14 AWG conductor protected by a 30A circuit breaker, but branch-circuit
conductors are protected against overloads by the overload device, which is
sized between 115 and 125 percent of the motor nameplate current rating
[430.32]. The small conductor rule contained in 240.4(D) which limits 15A protection
for 14 AWG doesn’t apply to motor circuit protection. See 240.4(D) and
240.4(G).
Ex 1: If the motor short-circuit and ground-fault protective device
values derived from Table 430.52 don’t correspond with the standard
overcurrent device ratings listed in 240.6(A), the next higher overcurrent
device rating can be used.
Question: What size conductor and inverse time circuit
breaker are required for a 7½ hp, 230V, three-phase
motor with 75ºC terminals?
(a) 10 AWG, 50A breaker (b)
10 AWG, 60A breaker
(c) a or b (d)
none of these
Answer: (b) 10 AWG, 60A breaker
Step 1: Determine the branch-circuit conductor
[Table 310.15(B)(16), 430.22, and Table 430.250]:
22A x 1.25 = 27.50A, 10 AWG, rated
35A at 75°C
[Table 310.15(B)(16)]
Step 2: Determine the branch-circuit protection
[240.6(A), 430.52(C)(1) Ex 1, and Table 430.250]:
22A x 2.50 = 55A, next size up = 60A
Q7. What is the Code rule for sizing short circuit and ground fault
protection for a motor feeder?
A7. Feeder conductors must be protected against short
circuits and ground faults by a protective device sized not more 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 [430.62(A)].
Question: What size feeder protection (inverse time
breakers with 75°C terminals) and conductors are required for the following
two motors?
Motor 1—20 hp,
460V, three-phase = 27A FLC [Table 430.250]
Motor 2—10 hp,
460V, three-phase = 14A FLC
(a) 8 AWG, 70A breaker (b)
8 AWG, 80A breaker
(c) 8 AWG, 90A breaker (d)
10 AWG, 90A breaker
Answer: (b) 8 AWG, 80A breaker
Step 1: Determine the feeder conductor size [430.24]:
(27A x 1.25) + 14A = 48A
8 AWG rated 50A at 75°C
[110.14(C)(1) and Table 310.15(B)(16)]
Step 2: Feeder protection [430.62(A)] isn’t
greater than the largest branch-circuit ground-fault and short-circuit
protective device plus the other motor FLC.
Step 3: Determine the largest branch-circuit
ground-fault and short-circuit protective device [430.52(C)(1) Ex]:
20 hp Motor = 27A x 2.50 = 68, next size up = 70A
10 hp Motor = 14A x 2.50 = 35A
Step 4: Determine the size feeder protection:
Not more than 70A + 14A, = 84A,
next size down = 80A [240.6(A)]
Author’s
Comment:
The “next size up protection” rule for
branch circuits [430.52(C)(1) Ex 1] doesn’t
apply to a motor feeder protection device rating.
Q8. How do I size the equipment grounding conductor to a motor branch
circuit?
A8. Equipment grounding conductors of the wire type must be
sized not smaller than shown in Table 250.122, based on the rating of the
circuit overcurrent device; however, the circuit equipment grounding conductor
isn’t required to be larger than the circuit conductors [250.122(A)].
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Table 250.122
Sizing Equipment Grounding Conductor
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Overcurrent Device Rating
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Copper Conductor
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15A
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14 AWG
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20A
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12 AWG
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25A—60A
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10 AWG
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70A—100A
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8 AWG
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110A—200A
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6 AWG
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225A—300A
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4 AWG
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350A—400A
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3 AWG
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450A—500A
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2 AWG
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600A
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1 AWG
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700A—800A
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1/0 AWG
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1,000A
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2/0 AWG
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1,200A
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3/0 AWG
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250.122 (D)(1) specifically addresses sizing the equipment grounding
conductor for motor branch circuits.
Question: What size equipment grounding conductor is
required for a 2 hp, 230V, single-phase motor?
(a) 14 AWG (b) 12 AWG (c) 10 AWG (d) 8 AWG
Answer: (a) 14 AWG
Step 1: Determine the branch-circuit conductor size
[430.22(A) and Table 310.15(B)(16)]
2 hp, 230V Motor FLC = 12A [Table
430.248]
12A x 1.25 = 15A, 14 AWG, rated
20A at 75°C [Table 310.15(B)(16)]
Step 2: Determine the branch-circuit protection
[240.6(A), 430.52(C)(1), and Table 430.248]
12A x 2.50 = 30A
Step 3: The
circuit equipment grounding conductor must be sized to the 30A overcurrent
device—10 AWG [Table 250.122], but it’s not required to be sized
larger than the circuit conductors—14 AWG [250.122(A)].
Q9. What is the Code requirement for the rating of a motor controller?
A9. The controller must have one of the following ratings [430.83(A)]:
(1) Horsepower
Rating. Controllers, other than circuit breakers and molded case switches, must
have a horsepower rating not less than that of the motor.
(2) Circuit
Breakers. A circuit breaker can serve as a motor controller [430.111].
Author’s Comment:
Circuit breakers
aren’t required to be horsepower rated.
(3) Molded Case
Switch. A molded case switch, rated in amperes, can serve as a motor
controller.
Author’s
Comment:
A molded case switch
isn’t required to be horsepower rated.
430.83(C) Stationary
Motors of Two Horsepower or Less. For stationary motors rated at 2 hp or less,
the controller can be:
(2) General-Use Snap
Switch. A general-use alternating-current snap switch, where the motor
full-load current rating isn’t more than 80 percent of the ampere rating
of the switch.
Author’s
Comment:
A general-use snap switch is a general-use switch constructed for
installation in device boxes or on box covers [Article 100].
Q10. What are the Code requirements for motor disconnects?
A10. A disconnecting means is required for each motor
controller, and it must be located within sight from the controller [430.102(A)].
Author’s
Comment:
According to Article 100, within sight means that
it’s visible and not more than 50 ft from one to the other.
A motor disconnect must be provided in accordance with 430.102(B)(1)
or (B)(2).
(1) Separate Motor Disconnect. A disconnecting means is
required for each motor, and it must be located in sight from the motor
location and the driven machinery location.
(2) Controller Disconnect. The controller disconnecting
means [430.102(A)] can serve as the disconnecting means for the motor, if the
disconnect is located in sight from the motor location.
Ex to (1) and (2): A
motor disconnecting means isn’t required under either condition (a) or
(b), if the controller disconnecting means [430.102(A)] is lockable, as described in 110.25.
(a) If locating the disconnecting means is
impracticable or introduces additional or increased hazards to persons or
property.
(b) In industrial installations, with written
safety procedures, where conditions of maintenance and supervision ensure only
qualified persons will service the equipment.
Informational Note 2: For information on lockout/tagout
procedures, see NFPA 70E, Standard for
Electrical Safety in the Workplace.
