Lesson 1 - NEC Requirements For Motors And Motor Circuits In Residential Applications Flashcards
Other than for motors built for low speeds (less than 1,200 rpm) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine all of the items listed except ______ instead of the actual current rating marked on the motor nameplate.
a. ampacity of conductors
b. ampere ratings of switches
c. branch-circuit short-circuit and ground-fault protection
d. separate motor overload protection
NEC REF:
d. separate motor overload protection
430.6(A)(1)
A listed overhead door opener is installed in a residential garage. The nameplate on the appliance is marked with both the horsepower rating and the full-load current rating of the opener. To determine the size of the branch-circuit conductors required, the horsepower rating of the cord- and-plug-connected appliance should be referenced.
a. True
b. False
NEC REF:
False
430.6(A)(1) Exception No. 3
Note: The motor in this example is a listed appliance, making the nameplate the preferred source of information required to size the branch-circuit conductors.
The full-load current rating of a continuous duty, general purpose, single-phase motor rated 0.5 horsepower at 120 volts is ______. The motor is automatically started.
a. 4.4 A
b. 7.2 A
c. 9.8 A
d. 13.8 A
c. 9.8 A
Note: See Table 430.248. The voltages in the Table are based on the NEMA voltage ratings for motors. The heading of the Table explains that the 115-volt column is acceptable for motors utilized on system voltages of 110 to 120 volts.
The full-load current rating of a continuous duty, general purpose, single-phase motor rated 7.5 horsepower at 240 volts is ______. The motor is automatically started and has a service factor of 1.15 and a marked temperature rise of 40°C.
a. 28 A
b. 40 A
c. 44 A
d. 80 A
b. 40 A
Note: See Table 430.248
The size of conductors that supply equipment covered by Article 430 of the NEC shall be ______.
a. calculated in accordance with 310.14(B)
b. selected from the allowable ampacity tables in accordance with Section 310.15
c. selected in accordance with Section 400.5 where flexible cord is used
d. all of the above
NEC REF:
d. all of the above
In general, conductors that supply a single motor used in a continuous-duty application shall have an ampacity of not less than 125% of the motor full-load current rating, as determined by 430.6(A)(1).
True or False
True
Note: See Section 430.22.
A single-phase, one-horsepower motor is installed for a 120-volt residential gate opener. The minimum size THWN branch-circuit conductors required to supply the motor is ______.
______ AWG Copper
14 AWG Copper
Note: NEC 430.6(A)(1) states the motor FLC is to be determined from Table 430.248. Table 430.248 lists the FLC for a single-phase, 120-volt, one-horsepower motor as 16 amperes. Section 430.22 requires the minimum ampacity for branch-circuit conductors to be 125% of the full load current.
16 × 1.25 = 20 amperes minimum conductor ampacity
Table 310.16 lists the smallest copper conductor with THWN insulation with an ampacity of at least 20 amperes as a 14 AWG, with an ampacity of 20 amperes. NEC 240.4(G) and Table 240.4(G) state the OCPD limitations for small conductors located in 240.4(D) do not apply to a motor installation.
A large wood shaper is installed in a residential workshop. The equipment is driven by a single-phase, five-horsepower, 230-volt motor. The minimum size THWN branch-circuit conductors required to supply the motor is ______.
______ AWG Copper
10 AWG Copper
Note: NEC 430.6(A)(1) states the motor FLC is to be determined from Table 430.248. Table 430.248 lists the FLC for a single-phase, 240-volt, five-horsepower motor as 28 amperes. Section 430.22 requires the minimum ampacity for branch-circuit conductors to be 125% of the full load current.
28 × 1.25 = 35 amperes minimum conductor ampacity
Table 310.16 lists the smallest copper conductor with THWN insulation with an ampacity of at least 35 amperes as a 10 AWG, with an ampacity of 35 amperes. NEC 240.4(G) and Table 240.4(G) state the OCPD limitations for small conductors located in 240.4(D) do not apply to a motor installation.
The maximum standard rating of an inverse-time circuit breaker permitted to provide the motor branch-circuit short-circuit and ground-fault protection for a single-phase, 1/3-horsepower, 120-volt motor is ______.
20 A
Note: NEC 430.6(A)(1) requires the motor full-load current to be determined by the values in Table 430.248. The Table gives a value of 7.2 amperes for a single-phase, 1/3-horsepower, 120-volt motor. 430.52(C)(1) requires the protective device to have a rating not exceeding the value calculated in accordance with Table 430.52. Table 430.52 gives a percentage of 250% for an inverse-time circuit breaker.
7.2 × 2.5 = 18 amperes
Table 240.6 lists the standard ratings of overcurrent protective devices. The maximum OCPD rating that does not exceed the calculated value is 15 amperes. As stated in 430.52(C)(1) Exception No. 1, when the rating determined from Table 430.52(C)(1) does not correlate with a standard rating, a higher rating that does not exceed the next higher rating shall be permitted. Table 240.6 lists a 20-ampere rating as the next higher standard rating.
The maximum standard rating of a dual-element, time-delay fuse permitted to provide the motor branch-circuit short-circuit and ground-fault protection for a single-phase, 7.5-horsepower, 240-volt motor is ______.
70 A
Note: NEC 430.6(A)(1) requires the motor full-load current to be determined by the values in Table 430.248. The Table gives a value of 40 amperes for a single-phase, 7.5-horsepower, 240-volt motor. NEC 430.52(C)(1) requires the protective device to have a rating not exceeding the value calculated in accordance with Table 430.52(C)(1). Table 430.52(C)(1) gives a percentage of 175% for a dual-element, time-delay fuse.
40 × 1.75 = 70 amperes
Table 240.6 lists the standard ratings of overcurrent protective devices. The maximum OCPD rating that does not exceed the calculated value is 70 amperes. Since this is a standard rating, the next higher rating is not permitted to be used.
The maximum standard rating of a non–time-delay fuse permitted to provide the motor branch-circuit short-circuit and ground-fault protection for a single-phase, 1.5-horsepower, 208-volt motor is ______.
35 A
Note: NEC 430.6(A)(1) requires the motor full-load current to be determined by the values in Table 430.248. The Table gives a value of 11 amperes for a single-phase, 1.5-horsepower, 208-volt motor. 430.52(C)(1) requires the protective device to have a rating not exceeding the value calculated in accordance with Table 430.52(C)(1). Table 430.52(C)(1) gives a percentage of 300% for a non-time-delay fuse.
11 × 3.0 = 33 amperes
Table 240.6 lists the standard ratings of overcurrent protective devices. The maximum OCPD rating that does not exceed the calculated value is 30 amperes. 430.52(C)(1) Exception No. 1 states that when the rating determined from Table 430.52(C)(1) does not correlate with a standard rating, a higher rating that does not exceed the next higher rating shall be permitted. Table 240.6 lists a 35-ampere rating as the next higher standard rating.
The maximum standard rating of a non–time-delay fuse permitted to provide the motor branch-circuit short-circuit and ground-fault protection for a single-phase, 1.5-horsepower, 208-volt motor is ______. The motor has difficulty starting due to high inrush current.
40 A
Note: NEC 430.6(A)(1) requires the motor full-load current to be determined by the values in Table 430.248. The Table gives a value of 11 amperes for a single-phase, 1.5-horsepower, 208-volt motor. For installations where the standard value determined in 430.52 is not sufficient for the starting current of the motor, 430.52(C)(1) Exception 2(a) requires the non–time-delay fuses to have a rating not exceeding 400% of the motor FLC.
11 × 4.0 = 44 amperes
Table 240.6 lists the standard ratings of overcurrent protective devices. 430.52(C)(1) Exception No. 2(a) states the rating of the selected fuse shall in no case exceed the calculated value of 44 amperes; therefore, it is necessary to select the next lower standard ampacity. Table 240.6 lists a 40-ampere rating as the next lower standard rating.
A three-horsepower single-phase air-compressor motor is to be installed in a residential garage where the motor will be both automatically and manually started. The motor nameplate is shown.
The motor is provided with integral overload protection that meets the requirements of NEC 430.32(A)(3). An inverse-time circuit breaker installed in a subpanel located in the garage will provide the motor branch-circuit short-circuit and ground-fault protection in addition to serving as the disconnecting means for the motor. The motor will be supplied with copper conductors with THWN insulation. Calculate the following:
1) Maximum rating or setting of the short-circuit ground-fault protection if motor starting is an issue.
a. 40 A
b. 45 A
c. 50 A
d. 60 A
2) Maximum rating or setting of the short-circuit ground-fault protection if motor starting is not an issue.
a. 40 A
b. 45 A
c. 50 A
d. 60 A
3) Minimum branch-circuit conductor size.
a. 14 AWG
b. 12 AWG
c. 10 AWG
d. 8 AWG
4) Minimum copper equipment grounding conductor if motor starting is not an issue.
a. 14 AWG
b. 12 AWG
c. 10 AWG
d. 8 AWG
1) d
2) b
3) b
4) b
The following protection methods are the only acceptable means recognized by the Code to provide overload protection for motors rated at one horsepower or less that are automatically started:
-Impedance protection of the windings
-Protective devices integral with the motor
-Separate overload devices
a. True
b. False
NEC REF:
Separate overload relays are used for the overload protection of a three-horsepower, 208-volt, single-phase, continuous duty motor with a motor nameplate full-load current rating of 16.2 amperes, service factor of 1.10, and temperature rise of 50°C. The maximum overload protective device for this installation is ______.
Separate overload relays are used for the overload protection of a three-horsepower, 208-volt, single-phase, continuous duty motor with a motor nameplate full-load current rating of 16.2 amperes, service factor of 1.10, and temperature rise of 50°C. The overload device selected in accordance with 430.32(A)(1) is not capable of carrying the motor starting current and constantly trips. The maximum overload protective device for this installation is ______.
Motor branch-circuit short-circuit and ground-fault protection and motor overload protection ______ be permitted to be combined in a single protective device.
a. shall
b. shall not
NEC REF:
In general, a disconnecting means shall be located ______ the motor location and the driven machinery.
a. adjacent to the panelboard serving
b. as near as practicable to
c. in sight from
d. on the controller for
NEC REF:
The Code specifies that equipment shall be considered to be “in sight from,” “within sight,” “within sight from,” and so forth of other equipment if the specified equipment is visible and not more than ______ from the other.
a. 6’
b. 25’
c. 50’
d. 75’
NEC REF:
In general, the disconnecting means for motor circuits shall have a current rating not less than ______ of the full-load current rating of the motor.
a. 50%
b. 75%
c. 100%
d. 115%
NEC REF:
A 15-ampere general-use snap switch that is rated for use only on AC circuits is installed to serve as the disconnecting means for a single-phase motor. The motor nameplate is shown.
The switch is installed at a readily accessible location within sight of the motor. The disconnecting means selected for the motor installation is ______.
a. a violation
b. acceptable
The minimum required current rating of the motor disconnecting means for a five-horsepower, 208-volt, single-phase motor is ______.
