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.
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.
Refrigerant Motor-Compressor Protection Categories
There are two main types of protection covered in Article 440:
- Branch-Circuit Short-Circuit and Ground-Fault Protection (Article 440.22)
- Short-circuit protection
- Maximum fuse/breaker sizing
- Starting current considerations
- Overload Protection (Article 440.52)
- Running overload protection
- Protection against extended overload conditions
- Temperature considerations
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"
- "Heat pump"
Key phrases for Overcurrent Questions:
- "Branch-circuit protective device"
- "Maximum fuse/breaker size"
- "Cannot handle starting current"
- "Branch circuit short-circuit"
- References to Article 440.22
Key phrases for Overload Questions:
- "Running overload protection"
- "Overload relay"
- "Temperature rating"
- "Thermal protector"
- "Trip time"
- References to Article 440.52
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
Motor-Compressor Protection: NEC Article 440
Key Requirements for Overcurrent Protection (440.22):
- Standard Rating: 175% of rated-load current or branch-circuit selection current (BSCS) (whichever is greater)
- If starting issues: Up to 225% allowed
- Must use standard fuse/breaker sizes
Key Requirements for Overload Protection (440.52):
- Maximum Setting for Separate Overload Relay: 140% of rated-load current or BCSC
- Maximum Setting for Fuse/Inverse Time Circuit Breaker: 125% of rated-load current
- Must protect against running overloads
- Separate from branch-circuit protection
Important Multipliers:
- 175% - Short-circuit and ground-fault protection
- 225% - Maximum with starting issues
- 140% - Maximum for separate overload relay
- 125% - Maximum for fuse/inverse time circuit breaker overload protection
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. A ________ amp fuse is the maximum allowed for short-circuit protection 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
Step 3: Round to Standard Size
- Next standard size down = 125A (since starting issues exist)
- Answer: 125 amperes
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
Common Pitfalls to Avoid
- Most Critical Mistake:
- 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
- Overcurrent Protection Mistakes:
- Using wrong multiplier (175% vs 225%)
- Not rounding to standard sizes
- Confusing with overload requirements
- Not considering starting current issues
- Overload Protection Mistakes:
- Using wrong multiplier for device type (140% vs 125%)
- Not considering device type (relay vs fuse/breaker)
- Confusing with branch circuit protection
- Rounding to standard sizes (don't round)
Quick Reference Guide
Remember - These rules apply ONLY to refrigeration equipment:
Short-Circuit and Ground-Fault Protection (440.22):
- Standard protection: 175%
- With starting issues: 225%
- Always round to standard sizes
Overload Protection (440.52):
- Separate overload relay: 140% maximum
- Fuse/inverse time circuit breaker: 125% maximum
- Separate from branch circuit
Tips for Success:
- Always verify if it's a refrigerant motor-compressor question
- Check for starting current issues
- Use correct multiplier for situation
- Remember rounding rules
- Verify against code requirements
