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