What Is Motor Overload Protection?

Imagine your motor as a diligent worker tirelessly operating machinery. However, if it's pushed beyond its capacity—say, due to excessive load or mechanical issues—it can overheat, leading to potential damage.

Motor overload protection acts as a safeguard, monitoring the motor's current draw. If it detects that the motor is drawing more current than it's rated for over a sustained period, it intervenes by disconnecting the power supply, allowing the motor to cool down and preventing damage.

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Common Types of Overload Protection Devices

  • Thermal Overload Relays: Use a bimetallic strip that bends when heated by excessive current. Once it bends sufficiently, it trips a switch to cut off power to the motor.
  • Electronic Overload Relays: Monitor the motor's current electronically and can provide more precise protection, often with adjustable settings.
  • Embedded Thermal Protectors: Built-in thermal sensors directly monitor the temperature of the motor windings and disconnect power if overheating is detected.

⚠️ If you’re using fuses as Overload Protection, they have to be Time Delay!
Rule Reference: 28-302 (2)—14-200.

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Calculating Overload Protection

Overload protection is based on the Service Factor (SF) of the motor. There are only two multipliers to remember.

💡 Note: Whether the motor is “hot” or “cold” matters. “Cold” motors have a different way of calculating overloads.

Rule 28-306:

  • If SF is 1.15 or above: FLA × 125%
  • If SF is below 1.15 or unknown: FLA × 115%

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Example 1

Motor Specs: 3ø 575V 30HP Wound Rotor Motor
Service Factor: 1.15
FLA (Table 44): 32A
Rule Applied: 28-306 (1)(a) – 125%

Calculation:
32 × 1.25 = 40A
Leave it!

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Example 2

Motor Specs: 3ø 575V 30HP Wound Rotor Motor
Service Factor: 1.1
FLA (Table 44): 32A
Rule Applied: 28-306 (1)(b) – 115%

Calculation:
32 × 1.15 = 36.8A
Leave it!

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Example 3

Motor Specs: 3ø 575V 20HP Wound Rotor Motor
Service Factor: Unknown
FLA (Table 44): 22A
Rule Applied: 28-306 (1)(b) – 115%

Calculation:
22 × 1.15 = 25.3A
Leave it!

Summary:
Learn how to identify and apply proper motor overload protection using device types, code rules, and service factor-based calculations.

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