Motor Disconnect Guides for Red Seal Exam (2024 CEC Updated!)
Red Seal Exam Question Guides
What is a Disconnect?
It’s a nearby switch that completely cuts off electricity to the motor, making it safe to inspect, maintain, or repair. It’s like giving the motor a clear signal:
“Take a break—we need to work safely.”
It’s usually installed within sight of the motor so workers can easily confirm it’s off before starting any hands-on work. Once everything’s good to go, the switch gets turned back on—and the motor is ready to work again.
What Kinds Are There?
1. Fused Disconnect Switch
Includes fuses that provide overcurrent protection.
Shuts off the motor and protects against short circuits and overloads.
Often used when separate fuse protection is required.
2. Non-Fused Disconnect Switch
Disconnects the motor from power, but has no built-in fuse protection.
Typically used when overcurrent protection is already provided elsewhere (like in a panelboard).
3. Circuit Breaker with Disconnect Function
Acts as both a breaker and disconnect.
Can trip automatically during a fault or be switched manually to isolate the motor.
Handy when space is limited.
4. Safety Switch (Knife Switch or Toggle)
Often mounted in a weatherproof or dustproof enclosure.
Clearly shows ON/OFF positions.
Common in industrial settings for lockout/tagout procedures.
5. Enclosed Disconnects (NEMA-rated or IP-rated)
Housed in special enclosures to protect against:
Dust (NEMA 12)
Water or washdowns (NEMA 4X or IP66)
Explosive atmospheres (hazardous locations)
Essential for outdoor or harsh environments.
What Sizes Are There?
Disconnects come in standard sizes: 30A, 60A, 100A, 200A, 400A, etc.
How Do We Know Which Size to Pick?
That handy old codebook. Whether or not it is a hot motor or cold motor matters. So make sure you go to the right Code Rule: 28-602!
Code Rules and Examples
⚡ 28-602 (1)(a) Manually Operated Fused or Unfused
Must not be less than the HP of the motor it serves.
Example:
Motor: 3ø 208V 15HP SCIM
FLA (Table 44): 46.2A
Result: Go UP to a 60A Fused Disconnect Switch
⚡ 28-602 (1)(b) Moulded Case Switch/Circuit Breaker
Calculation: FLA × 115%
Example:
Motor: 3ø 208V 15HP SCIM
FLA (Table 44): 46.2A
46.2A × 1.15 = 53.13A
Result: Go UP to a 60A Moulded Case Switch
⚡ 28-602 (1)(c) Instantaneous Circuit Breaker
Follow 28-210: Not less than 1300% of FLA
Example:
Motor: 3ø 208V 15HP SCIM
FLA (Table 44): 46.2A
46.2A × 13 = 600.6A
Result: Go DOWN to a 600A Instantaneous Circuit Breaker
Additional Note
Sometimes the disconnect doesn’t cut power directly to the motor but to its controller (like a VFD or starter). In that case, the controller must be marked with a “motor disconnecting means” label. Refer to Rule 28-602 (3)(b) - Appendix
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