Guide to Motor Disconnecting Means Questions on NEC Electrical Exams
NEC Exam Guides
Motor disconnecting means questions are crucial for electrical professionals as they directly impact safety and code compliance. These questions typically focus on selecting the appropriate disconnecting means based on motor specifications and operating conditions. This knowledge is essential for protecting both equipment and personnel, as improper disconnecting means can lead to hazardous situations during maintenance or emergencies.
Video Guide
Example Motor Disconnecting Means Questions on NEC Electrical Exams
A general-use switch rated at ________ amperes is the minimum permitted to serve as the disconnecting means for a 2 hp, 208-volt stationary motor.
An installation consists of one 5-hp, one 3-hp, and two ½-hp motors, all rated 240 V, 3-phase. All motors are Design B motors. Determine the size of the safety switch required for this combination load.
How to Identify a Motor Disconnecting Means Question on NEC Electrical Exams
Key phrases to look out for:
"Disconnecting means"
"General-use switch"
"Controller disconnect"
"Motor hp rating"
"Full-load current"
"Locked-rotor current"
"Combination load"
"Motor disconnect sizing"
When you spot these elements:
Confirm it's a motor disconnect question, not a motor circuit protector or overload protection question
Refer to NEC Article 430, Part IX (430.109-430.113)
Check if it involves single motor or combination loads
Verify if locked-rotor current considerations are needed
Motor Disconnecting Means Articles: NEC 430 Part IX
To correctly apply NEC Article 430 Part IX, focus on these main elements:
430.109: Required ratings and types of disconnecting means
Types of acceptable disconnecting means
Specific requirements for different motor applications
Conditions for using general-use switches
430.110: Ampere rating and interrupting capacity
Minimum ampere ratings for various applications
Requirements for combination loads
Special provisions for specific motor types
430.111: Switch or circuit breaker as disconnecting means
Requirements for indicating switch position
Accessibility requirements
Installation considerations
430.112: Motors served by single disconnecting means
Group motor installation requirements
Location requirements
Capacity considerations
430.113: Location requirements
Accessibility standards
Line-of-sight requirements
Emergency considerations
Key points to remember:
Must consider both full-load current and locked-rotor current
Both current rating and horsepower rating must be evaluated
Different types of motors and controllers have specific disconnect requirements
Location and accessibility of disconnecting means is crucial
Walkthrough for a NEC Electrical Exam Motor Disconnecting Means Question
Question:
An installation consists of one 5-hp, one 3-hp, and two ½-hp motors, all rated 240 V, 3-phase. All motors are Design B motors. Determine the size of the safety switch required for this combination load.
Step 1: Identify Full-Load and Locked-Rotor Currents
Using Table 430.250 for FLC and Table 430.251(B) for LRC:
5-hp motor:
FLC = 15.2A
LRC = 92A
3-hp motor:
FLC = 9.6A
LRC = 64A
Two ½-hp motors:
FLC = 2.2A each
LRC = 20A each
Step 2: Calculate Combined Load Currents
Calculate total full-load current (FLC):
Total FLC = 15.2A + 9.6A + (2 × 2.2A) = 29.2A
Calculate total locked-rotor current (LRC):
Total LRC = 92A + 64A + (2 × 20A) = 196A
Step 3: Determine Minimum Ampere Rating
Per 430.110:
Minimum ampere rating = FLC × 115%
Minimum ampere rating = 29.2A × 1.15 = 33.58A
Step 4: Determine Required Horsepower Rating
Using Table 430.251(B):
Combined LRC of 196A corresponds to a 15 hp motor
Table 430.250: A 15 hp, 230V motor has FLC of 42A
42A > 33.58A (calculated minimum ampere rating)
Therefore, 15 hp rating is sufficient
Step 5: Final Safety Switch Requirements
The safety switch must be:
Rated minimum 15 hp
Have ampere rating not less than 33.58A
Listed for use as motor disconnect
Rated for 240V, 3-phase service
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