What’s an Overcurrent Device Anyway?

It’s your motor’s personal superhero—the circuit breaker or fuse that steps in when things get too heated (literally).

Too much current? KA-POW! The overcurrent device shuts it down before damage happens.

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Which Device Can I Use?

You’ve got three main options (and one very sneaky one):

1. Time Delay Fuse (TD)

  • Allows short startup surges without blowing.
  • Melts and opens circuit if high current persists.
  • Great for motors with initial surge.

2. Non-Time Delay Fuse (NTD)

  • Blows immediately with overcurrent.
  • Doesn’t tolerate startup surges.
  • Better for sensitive equipment—not motors.

3. Inverse Time Circuit Breaker (CB)

  • Slower for minor overloads, faster for big faults.
  • Handles both overloads and short circuits.

4. Instantaneous Circuit Breaker

  • Trips immediately at high current.
  • Only protects against short circuits.
  • No delay, no mercy.

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The Golden Rule: YOU. SHALL. NOT. EXCEED.

With motor overcurrent protection, you must round down to the nearest standard value in Table 13.

Example:

  • FLA = 54 A
  • SCIM motor with Full-Voltage Starting (FVS), Circuit Breaker:
    54 × 250% = 135 A
  • Table 13 shows 125 A and 150 A → Pick 125 A.

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First Step: Know Your FLA

Everything starts with the Full Load Amperage (FLA).

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Is the Motor HOT or COLD?

  • “Cold” Motors (Hermetic Refrigerant Compressors) follow different rules.
    ❌ Do not use Table 29
  • For “Hot” motors → Use Table 29

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AC or DC Motor?

DC Motors

  • Use a 150% multiplier for any overcurrent device.

AC Motors

  • Identify phase (1ø or 3ø), motor type (SCIM or Synchronous), and starting type.
  • Use Table 29 to find the multiplier, then go to Table 13 and round down.

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Important Notes to Check

  • Note 1: Super low torque & speed synchronous motors → use 200% multiplier
  • Note 2: Instantaneous CBs → Refer to Rule 28-210 → 1300% multiplier

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Examples

Easy

Q: What size Time Delay Fuse for a 3ø, 575V, 50HP SCIM motor with FVS?

  • Table 44 FLA: 52 A
  • Table 29 Multiplier: 175% → 52 × 1.75 = 91 A
  • Table 13: Go down → 90 A fuse

Medium

Q: What size TD Fuse for a 3ø, 575V, 50HP Synchronous Motor @ 90% PF?

  • FLA: 44 A
  • PF adjustment: 44 × 1.1 = 48.4 A
  • Multiplier: 175% → 48.4 × 1.75 = 84.7 A
  • Table 13: Go down → 80 A fuse

Hard

Q: What size Instantaneous CB for a 3ø, 575V, 50HP Auto-Transformer Start Synchronous Motor @ 80% PF?

  • Table 44 FLA: 44 A
  • PF adjustment: 44 × 1.25 = 55 A
  • Note 2 → Rule 28-210 = 1300%
  • 55 × 13 = 715 A → Table 13: 700 A CB

💡 Note: Even if the motor is Auto-Transformer Start, you're asked for Instantaneous CB—so ignore inverse time.

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“Judge for Yourself” Example

Q: A 300A NTD fuse is not allowing a 3ø, 230V, 60HP Auto-Transformer Start Synchronous Motor (80% PF) to start. What size is required?

  • FLA (Table 44): 128 A
  • PF adjustment: 128 × 1.25 = 160 A
  • Rule 28-200(4)(a)(i): Max 400% → 160 × 4 = 640 A
    → Table 13: Closest = 600 A

🤔 But that’s high! What if we try 300%?

  • 160 × 3 = 480 A → Table 13: 400 A

➡️ Both could be acceptable depending on the question wording.