For electricians preparing for their exams, tackling range demand load calculations is a must. In this free NEC electrical exam question, we’ll guide you step by step to calculate the demand load for two residential ranges, each rated above 12 kW. Let’s get started!

Question Breakdown

Question: A residential kitchen is equipped with two ranges: one rated at 14 kW and another at 18 kW. Determine the demand load for these appliances on the service entrance conductors.

This question leverages NEC Table 220.55 and its notes, commonly tested in journeyman and masters practice tests.

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Step-by-Step Solution

Step 1: Identify the Ranges and Determine the Relevant Table Column

Both ranges exceed 8¾ kW, so they fall under Column C of NEC Table 220.55.

  • Range 1: 14 kW (Column C)
  • Range 2: 18 kW (Column C)

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Step 2: Calculate the Total Connected Load

Add the ratings of both ranges:

Total connected load = 14 kW + 18 kW = 32 kW

The total connected load is 32 kW.

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Step 3: Apply Note 2 Under Table 220.55

Note 2: For ranges individually rated more than 12 kW but not more than 27 kW, calculate an average value by dividing the total connected load by the number of ranges. Then increase the demand in Column C by 5% for each kW (or major fraction thereof) exceeding 12 kW.

Average rating = Total connected load ÷ Number of ranges
Average rating = 32 kW ÷ 2 = 16 kW

The average rating for the two ranges is 16 kW.

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Step 4: Calculate the Percentage Increase Based on the Average Rating

Increase = (Average rating - 12 kW) × 5%
Increase = (16 kW - 12 kW) × 5%
Increase = 4 × 5% = 20%

The demand increase is 20%.

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Step 5: Apply the Increase to the Base Demand from Column C

From Table 220.55, the base demand for two appliances in Column C is 11 kW. Now, apply the 20% increase:

Final demand load = Base demand + (Base demand × Increase)
Final demand load = 11 kW + (11 kW × 20%)
Final demand load = 11 kW + 2.2 kW = 13.2 kW

The calculated demand load for the two ranges is 13.2 kW.

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Code Reference: When to Use Note 2

  • NEC Table 220.55 Note 2 applies when dealing with ranges rated over 12 kW and up to 27 kW.
  • It’s crucial to use this note to properly adjust the demand based on the average range rating, ensuring compliance with NEC standards.

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Key Takeaways for Your NEC Practice Test

  1. Column Selection Matters: Always identify the correct column in Table 220.55 for range calculations.
  2. Apply Notes Carefully: Use Note 2 when dealing with ranges rated above 12 kW.
  3. Demand Increase: Calculate and apply the appropriate percentage increase based on the average rating.
  4. Practice is Key: These calculations frequently appear in journeyman and masters practice tests, so repetition is vital.

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Why This Question Matters

This problem highlights a real-world application of NEC Table 220.55, essential for electricians working on residential installations. Mastering range demand load calculations ensures safe and compliant design of service entrance conductors.

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