Refrigerant motor-compressor protection questions are common on electrical licensing exams, particularly focusing on Article 440. These questions specifically deal with air conditioning and refrigeration equipment - NOT standard motors. This is a crucial distinction as the protection requirements for refrigerant motor-compressors differ significantly from those of standard motors covered in Article 430.

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Important Note

A common mistake is treating refrigerant motor-compressor questions like standard motor questions. Remember: if the question mentions air conditioning units, refrigeration equipment, or motor-compressors, you must apply Article 440 rules, not standard motor rules from Article 430.

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Refrigerant Motor-Compressor Protection Categories

There are two main types of protection covered in Article 440:

  1. Overcurrent Protection (Article 440.22)
    • Branch circuit protection
    • Maximum fuse/breaker sizing
    • Starting current considerations
  2. Overload Protection (Article 440.52)
    • Running overload protection
    • Protection against extended overload conditions
    • Temperature considerations

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How to Identify Refrigerant Motor-Compressor Protection Questions

Look for these key terms to identify refrigeration equipment questions:

  • "Air conditioning unit"
  • "Refrigeration equipment"
  • "Motor-compressor"
  • "Condensing unit"

Key phrases for Overcurrent Questions:

  • "Branch-circuit protective device"
  • "Maximum fuse/breaker size"
  • "Cannot handle starting current"
  • "Branch circuit short-circuit"

Key phrases for Overload Questions:

  • "Running overload protection"
  • "Overload relay"
  • "Temperature rating"
  • "Thermal protector"
  • "Trip time"

When you spot these elements:

  • First, verify it's a refrigerant motor-compressor question, not a standard motor
  • Identify if it's overcurrent or overload protection
  • Check for rated-load current values
  • Look for starting current issues
  • Note any temperature considerations

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Motor-Compressor Protection: NEC Article 440

Key Requirements for Overcurrent Protection (440.22):

  1. Standard Rating: 175% of rated-load current
  2. If starting issues: Up to 225% allowed
  3. Use standard fuse/breaker sizes

Key Requirements for Overload Protection (440.52):

  1. Maximum Setting for Separate Overload Relay: 140% of rated-load current
  2. Maximum Setting for Fuse/Inverse Time Circuit Breaker: 125% of rated-load current

Important Multipliers:

  • 175% - Standard overcurrent protection
  • 225% - Maximum with starting issues
  • 140% - Maximum for separate overload relay
  • 125% - Maximum for fuse/inverse time circuit breaker overload protection

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Example Problems and Solutions

Overcurrent Protection Example

Question: A motor-compressor has a rated-load current of 65A. The initial 100A fuse cannot handle the starting current. The ________ amp fuse is the maximum allowed for this motor-compressor.

Step 1: Identify the Scenario

  • Starting current issues exist
  • Use 225% maximum multiplier per 440.22

Step 2: Calculate Maximum Protection

  • 65A × 2.25 = 146.25A

Step 3: Round to Standard Size

  • Next standard size down = 125A (since 146.25 A is the maximum possible)
  • Answer: 125 amperes

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Overload Protection Example

Question: What is the maximum setting for a separate overload relay protecting a motor-compressor with a rated-load current of 28A?

Step 1: Identify Protection Type

  • Separate overload relay
  • Use 140% per 440.52(A)(1)

Step 2: Calculate Maximum Rating

  • 28A × 1.40 = 39.2A
  • Answer: 39.2 amperes

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Common Pitfalls to Avoid

  1. Most Critical Mistakes
    • Confusing refrigerant motor-compressor rules with standard motor rules
    • Using Article 430 values instead of Article 440 requirements
    • Not recognizing air conditioning/refrigeration equipment questions
  2. Overcurrent Protection Mistakes:
    • Using wrong multiplier (175% vs 225%)
    • Not rounding to standard sizes
    • Confusing with overload requirements
    • Not considering starting current issues
  3. Overload Protection Mistakes:
    • Using wrong multiplier for device type (140% vs 125%)
    • Not considering device type (relay vs fuse/breaker)

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