Unit 7: AC Motor Calculations Flashcards
Applying NEC® provisions to determine motor full-load current, conductor sizing, overcurrent protection, and voltage drop considerations for safe and efficient AC motor operation.
The branch-circuit conductors supplying a continuous-duty ac motor shall have an ampacity of NOT less than what percent of the motors full-load current rating?
125 percent
Fill in the blank:
Luminaires installed under metal-corrugated sheet roofing decking shall be installed and supported so there is NOT less than ________ clearance from the roof decking to the top of the luminaires.
1 1⁄2 inches
Fill in the blank:
For the purpose of calculating the general lighting load for a dwelling unit, the floor area should include all the following EXCEPT ________.
an open porch not adaptable for future use
Fill in the blank:
Where flexible cord is approved for use and used with a specific listed appliance, it shall be considered to be protected ________.
when applied within the appliance listing requirements
Fill in the blank:
Electrical equipment, such as distribution panelboards, motor controllers, and motor control centers that are located in other than dwelling units, and are likely to require examination, adjustment, servicing, or maintenance while energized shall be field or factory marked to warn qualified persons of ________.
potential electric arc flash hazards
Determine the MINIMUM required ampacity of the branch-circuit conductors to supply a continuous-duty, Design B, 3-phase, 208-volt, 10 hp, ac motor.
38.5 amperes
FLC of motor = 30.8 amperes amperes x 125% = 38.5 amperes
What is the MAXIMUM ampere rating permitted for a 125-volt, single-phase, receptacle having a cord-and-plug connected motor-load that does NOT have overload protection?
15 amperes
Fill in the blank:
A ________, rated at 125 volts, single-phase, 15 or 20-ampere, does NOT require GFCI protection for personnel in a dwelling unit.
receptacle behind the refrigerator in the kitchen, where more than 6 ft. from the sink
Fill in the blank:
Fastened in place utilization equipment, other than luminaires, that is connected to a branch circuit with other loads shall NOT exceed ________ of the branch circuit rating.
50 percent
Fill in the blank:
In compliance with the NEC®, a 3 in. x 2 in. x 11⁄2 in. metal device box may contain NO more than ________ size 12 AWG conductors.
three
Fill in the blank:
The workspace in front of electrical equipment such as panelboards, switchboards and motor control centers rated 1000 volts or less, shall be at LEAST ________ wide or the width of the equipment, whichever is greater.
30 inches
Fill in the blank:
Each receptacle of dc plugging boxes in motion picture studios shall be rated at NOT less than ________.
30 amperes
Where a retail clothing store has an actual connected fluorescent general lighting load of 14,000 VA how many 120-volt, single phase branch-circuits are required for the general lighting? Given: The lighting loads will be used continuously throughout the business day.
Eight
14,000 VA (continuous load) x 125%= 17,500 VA
120 volts x 20 amperes = 2,400 VA of 1 circuit
17,500 VA (load)/2,400 VA (circuit) = 7.29 = 8 – 120 volt, 20-ampere lighting circuits
Fill in the blank:
The NEC® mandates specific branch-circuits, receptacle outlets and utilization equipment to be provided with a ground-fault circuit interrupter (GFCI); this device is intended ________.
for the protection of personnel
Fill in the blank:
For a grounded electrical system, the equipment grounding conductor shall be sized in accordance with the ________.
overcurrent protective device
Determine the MINIMUM size 75°C copper conductors permitted to supply a 30 hp, 230- volt, 3-phase, induction-type, continuous-duty motor.
3 AWG THW
FLC of motor – 80 amperes x 125% = 100 amperes
Size 3 AWG 75°C rated CU conductors with an ampacity of 100 amperes should be selected from Table 310.16.
Given: A rigid metal conduit (RMC) originates from a panelboard in an area that is unclassified, then passes through a Class I, Division 1 area, and then terminates in equipment that is also in an unclassified area. The RMC is without any connections or fittings except at the terminations. How many seal-off fittings are required in the RMC?
none
What is the MAXIMUM distance between conduit supports for trade size 1 in. Schedule 40 rigid PVC conduit?
3 feet
Fill in the blank:
Receptacles supplying power to freestanding-type office furnishings shall be located NOT more than ________ from the furnishing that is connected to it.
1 foot
For the purpose of determining conductor fill in a device box, the NEC® mandates a switch to be counted as equal to two (2) conductors. The volume allowance for the two (2) conductors shall be based on the:
largest size wire connect to the switch
Fill in the blank:
A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected is a ________.
controller
Fill in the blank:
Motors of ________ are NOT approved for use in a Class I, Division 1 location.
totally enclosed fan cooled motors
Fill in the blank:
Unless listed and identified otherwise, terminations and conductors of circuits rated 100 amperes or larger, shall be rated for ________.
75°C
Determine the conductor ampacity where given the following related information:
- conductors are size 500 kcmil THWN copper
- ambient temperature is 125°F
- eight (8) current-carrying conductors are in the raceway
178.2 amperes
500 kcmil THWN copper ampacity before derating = 380 amperes
380 amps x .67 (temp. correction) x .7 adj. factor = 178.22 amperes
