Guide to Transformer Protection Questions on NEC Electrical Exams

Transformer protection questions are critical on electrical licensing exams as they test understanding of NEC Article 450.3 requirements. These questions involve calculating proper overcurrent protection for both primary and secondary sides, with different rules applying based on voltage levels and transformer characteristics.

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Example Transformer Protection Questions on NEC Electrical Exams

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In a data center, a three-phase, 75 kVA transformer steps down 480V to 208Y/120V. What is the maximum standard ampere rating for the secondary overcurrent protection allowed by NEC when both primary and secondary protection are required?

In an office building, a 15 kVA, 240V-120V, 1Ø transformer needs primary and secondary protection. What are the maximum overcurrent protection ratings permitted by NEC for the primary and secondary, respectively?

The NEC allows a primary fuse no larger than _____ A for a 1000 kVA, 13.8kV-480V dry-type transformer in a supervised location.

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How to Identify Transformer Protection Questions on NEC Electrical Exams

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Key phrases to look out for:

• "Maximum overcurrent protection"
• "Primary/secondary protection"
• "Fuse size"
• "Circuit breaker rating"
• References to Table 450.3(A) or 450.3(B)
• "Supervised location"
• "Without secondary protection"

When you spot these elements:

1. Check if voltage is above or below 1000V
2. Check for the rated impedance
3. Check for the type of overcurrent device
4. Determine if both primary and secondary protection are required
5. Note if it's a supervised location
6. Refer to appropriate section of NEC 450.3

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Understanding Different Protection Requirements

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Voltage Level Differences (450.3)

1. Over 1000V Primary (Use Table 450.3(A))
2. 1000V or Less (Table 450.3(B))
3. Apply rules based on supervised or unsupervised, device type & rated impedance

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Primary vs Secondary Protection Requirements

1. Primary Protection:
   
   • Always required
   • Size depends on presence of secondary protection
   • Different percentages for supervised locations
2. Secondary Protection:
   
   • May not be required if primary is properly sized
   • In general, secondary protection is required where the system has a neutral conductor 
   • Note* permissions found in NEC 240.21(C) allow for connection to secondary without overcurrent protection under certain conditions
   • Must be 125% of secondary current when over 9A
   • Special rules for specific transformer types

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Walkthrough for NEC Electrical Exam Protection Questions

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Over 1000V Protection Question

The NEC allows a primary fuse no larger than _____ A for a 1000 kVA, 13.8kV-480V dry-type transformer in a supervised location.

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Step 1: Calculate Primary Full Load Current

• kVA = 1000
• Primary voltage = 13.8kV
• I = 1000 kVA ÷ (√3 × 13.8kV)
• I = 41.89 amperes

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Step 2: Apply Protection Factor (Table 450.3(A))

• Supervised location uses 250%
• Maximum protection = 41.89 × 2.50
• Maximum protection = 104.72A

Step 3: Select Standard Size

• Use Table 240.6(A)
• Next standard size above 104.72A
• Standard size = 125A

The maximum primary fuse size allowed is 125A.

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Under 1000V Protection Question (Both Primary and Secondary)

In an office building, a 15 kVA, 240V-120V, 1Ø transformer needs primary and secondary protection. What are the maximum overcurrent protection ratings permitted by NEC for the primary and secondary based on the rating of the transformer, respectively?

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Step 1: Calculate Primary Current

• Single-phase formula: I = VA ÷ V
• I = 15000 ÷ 240
• Primary current = 62.5A

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Step 2: Calculate Secondary Current

• I = 15000 ÷ 120
• Secondary current = 125A

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Step 3: Apply Protection Factors (Table 450.3(B))

• Primary with secondary protection: 250%
• Primary maximum = 62.5 × 2.50 = 156.3A
• Secondary > 9A: 125%
• Secondary maximum = 125 × 1.25 = 156.3A

Per Table 240.6(A), the maximum primary overcurrent device rating is 150 amps. The maximum for the secondary would be 175 amps per note 1 of Table 450.3(B).

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Secondary Protection Question

In a data center, a three-phase, 75 kVA transformer steps down 480V to 208Y/120V. What is the maximum standard ampere rating for the secondary overcurrent protection?

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Step 1: Calculate Secondary Full Load Current

• Three-phase formula: I = kVA × 1000 ÷ (√3 × V)
• I = 75000 ÷ (√3 × 208)
• I = 208.4 amperes

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Step 2: Apply 125% Factor (Table 450.3(B))

• Secondary current > 9A
• Maximum protection = 208.4 × 1.25
• Maximum protection = 260.5A

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Step 3: Select Standard Size

• Use Table 240.6(A)
• Next standard size above 260.5A
• Standard size = 300A

The maximum secondary overcurrent protection allowed is 300A.

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