Understanding when and how to size primary protection without requiring separate secondary protection is crucial for electrical licensing exams. This specific scenario is governed by NEC Article 240.21(C)(1) and requires careful attention to voltage ratios and current calculations.

Example Primary-Only Protection Question on NEC Electrical Exams

The maximum primary overcurrent rating allowed for a 300 kVA, dry-type, 2400 V-240/416 V, 3-phase, 4-wire transformer without requiring separate secondary overcurrent protection is __________ A.

A 225 kVA, dry-type, 4160 V-277/480 V, 3-phase, 4-wire transformer can have a maximum primary overcurrent rating of _____ A without requiring separate secondary overcurrent protection.

For a 75 kVA, dry-type, 480 V-120/208 V, 3-phase, 4-wire transformer, the maximum primary overcurrent rating allowed without requiring separate secondary overcurrent protection is _____ A.

How to Identify Primary-Only Protection Questions on NEC Electrical Exams

Key phrases to look out for:

  • "Without secondary protection"
  • "Without requiring separate secondary overcurrent protection"
  • "Primary-only protection"
  • "Maximum primary overcurrent rating allowed"

When you spot these elements:

  1. Check if question specifically mentions absence of secondary protection
  2. Note both primary and secondary voltages
  3. Calculate voltage ratio
  4. Refer to NEC Article 240.21(C)(1)

Primary-Only Protection Articles: NEC 240.21(C)(1)

To correctly apply NEC Article 240.21(C)(1), focus on these main elements:

  1. Requirements for Omitting Secondary Protection:some text
    • Primary protection must be properly sized
    • Voltage ratio must be considered
    • Secondary conductors must be properly sized
  2. Key Calculations:some text
    • Secondary full-load current
    • Secondary-to-primary voltage ratio
    • Equivalent primary current
  3. Important Rules:some text
    • Must round down to next standard size
    • Protection must not exceed secondary conductor ampacity
    • Both primary and secondary conductors must be properly sized

Steps for Primary-Only Protection Calculations

  1. Calculate Secondary Full-Load Current:some text
    • Three-phase: I = VA ÷ (V × √3)
    • Single-phase: I = VA ÷ V
  2. Calculate Voltage Ratio:some text
    • Ratio = Secondary Voltage ÷ Primary Voltage
    • Use line-to-line voltages for three-phase
  3. Calculate Equivalent Primary Current:some text
    • Multiply secondary current by voltage ratio
    • This gives maximum primary protection allowed
  4. Select Standard Size:some text
    • Round DOWN to next standard size
    • Use Table 240.6(A) for standard sizes

Walkthrough for Primary-Only Protection Question

Question: The maximum primary overcurrent rating allowed for a 300 kVA, dry-type, 2400 V-240/416 V, 3-phase, 4-wire transformer without requiring separate secondary overcurrent protection is __________ A.

Step 1: Calculate Secondary Full-Load Current

  • kVA = 300
  • Secondary voltage = 416V
  • Using three-phase formula:
  • I = 300,000 VA ÷ (416V × 1.732)
  • I = 300,000 ÷ 720.512
  • I = 416.85 amperes

Step 2: Calculate Secondary-to-Primary Voltage Ratio

  • Secondary voltage = 416V
  • Primary voltage = 2400V
  • Ratio = 416V ÷ 2400V
  • Ratio = 0.173

Step 3: Calculate Equivalent Primary Current

  • Multiply secondary current by voltage ratio
  • 416.85A × 0.173
  • = 72.12 amperes

Step 4: Select Standard Size

  • Must round DOWN to next standard size
  • 72.12A rounds down to 70A per Table 240.6(A)

The maximum primary overcurrent protection allowed is 70A.

Common Mistakes to Avoid

  1. Rounding Up Instead of Down:some text
    • Always round DOWN to next standard size
    • This is different from most other NEC calculations
  2. Using Wrong Voltage Values:some text
    • Must use line-to-line voltages for ratio
    • Don't mix line-to-line and line-to-neutral voltages
  3. Forgetting Standard Sizes:some text
    • Must use standard sizes from Table 240.6(A)
    • Can't use calculated value directly
  4. Incorrect Secondary Current:some text
    • Must use full-load current
    • Don't apply multipliers to secondary current

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