Aspiring electricians, get ready to boost your knowledge with a free NEC electrical exam question! Whether you're prepping for your journeyman or master electrician exam, mastering the art of transformer calculations is key. Today, we’ll tackle a practice question on transformer primary overcurrent protection, breaking it down step by step.

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Question Breakdown

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

This type of question is common on journeyman and masters practice tests because it reinforces your ability to apply NEC code knowledge to real-world scenarios.

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Step-by-Step Solution

Step 1: Calculate the Secondary Full-Load Current

The secondary full-load current is the starting point for solving this problem. Using the three-phase formula:

I = kVA × 1000 ÷ (Voltage × 1.732)

Plugging in the values:

I = 300,000 ÷ (416 × 1.732)

I = 300,000 ÷ 720.512

I = 416.85 A

The full-load secondary current is 416.85 A.

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Step 2: Calculate the Secondary-to-Primary Voltage Ratio

The next step involves finding the voltage ratio between the transformer’s secondary and primary sides.

Voltage Ratio = Secondary Voltage ÷ Primary Voltage

Voltage Ratio = 416 ÷ 2400 = 0.173

The voltage ratio is 0.173.

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Step 3: Calculate the Equivalent Primary Current

Using the voltage ratio, determine the equivalent current on the primary side:

Primary Current = Secondary Current × Voltage Ratio

Primary Current = 416.85 × 0.173 = 72.12 A

The equivalent primary current is 72.12 A.

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Step 4: Select the Standard Overcurrent Protection Size

NEC Table 240.6(A) specifies standard overcurrent protection sizes. For this transformer, the overcurrent protection must round down to the next standard size.

72.12 A rounds down to 70 A.

Thus, the maximum primary overcurrent protection allowed is 70 A.

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Code Reference: Why 70 Amps?

Per NEC 450.3(B), overcurrent protection for a transformer must be carefully selected to prevent excessive damage while accommodating inrush currents during startup. For transformers of this size and configuration, secondary overcurrent protection isn’t required, provided the primary protection does not exceed the limits set by the code.

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Key Takeaways for Your Exam

  1. Understand Three-Phase Calculations: Knowing how to apply the three-phase current formula is essential for solving transformer-related questions.
  2. Voltage Ratios Matter: Always calculate the voltage ratio to transition between the primary and secondary circuits.
  3. Rounding Rules Apply: For overcurrent protection, rounding down to the next standard size ensures compliance with NEC standards.
  4. Memorize Key NEC Tables: Familiarize yourself with NEC Table 240.6(A) for overcurrent protection sizes and NEC 450.3(B) for transformer protection requirements.

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Why This Question Matters

This problem is a classic example of what you’ll face in NEC practice tests for journeymen and masters electricians. Understanding transformer calculations ensures you’re equipped to handle questions that test both theoretical knowledge and practical application.

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