Dakota Prep - Guide to Range Demand Calculation Questions on NEC Electrical Exams

Understanding how to calculate range demand is useful for electrical professionals, especially when preparing for a licensing exam. These calculations are essential for properly sizing service entrance conductors and other components of electrical systems in residential and commercial kitchens. Mastering this topic is a helpful skill for passing NEC electrical exams and ensuring safe, code-compliant installations in the field.

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Example Range Demand Calculation Questions on NEC Electrical Exams

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A single-family dwelling has two ranges rated at 4.5 kW each and another range rated at 7 kW. Calculate the demand load for these kitchen appliances on the ungrounded service entrance conductors using the standard method of calculation for dwellings.

In a household kitchen, there is one range rated at 15 kW. What is the calculated demand load for this range?

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.

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How to Identify a Range Demand Calculation Question on NEC Electrical Exams

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Key phrases to look out for in range demand calculation questions:

"Household cooking appliances"
"Range(s)", "Oven(s)", "Cooktop(s)"
"Demand factor"
"Service entrance conductors"
"kW ratings" of appliances

When you spot these elements:

Confirm it's a range demand calculation question, not a general load calculation question
Refer to NEC section 220.55

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Range Demand Calculation Articles: NEC 220.55

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To correctly apply NEC Article 220.55, focus on these main elements:

Table 220.55: Demand Factors and Loads for Household Electric Ranges, Wall-Mounted Ovens, Counter-Mounted Cooking Units, and Other Household Cooking Appliances
Column C for all ranges above 18¾ kW rating except as otherwise permitted in Note 3.
Column A for loads less than 3½ kW rating when using Note 3
Column B for loads 3½ kW through 8¾ kW rating when using Note 3
Notes 1 through 7 under Table 220.55

To summarize, use Table 220.55 to determine the appropriate demand factor based on the ratings of individual appliances. Pay special attention to the notes, which provide guidance for specific scenarios and adjustments to the demand factors.

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Walkthrough for a NEC Electrical Exam Range Demand Calculation Question

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Example 1: Mixed Range Ratings

A single-family dwelling has two ranges rated at 4.5 kW each and another range rated at 7 kW. Calculate the maximum demand load for these kitchen appliances on the ungrounded service entrance conductors using the standard method of calculation for dwellings.

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Step 1: Identify all ranges and their ratings

Two ranges: 4.5 kW each One range: 7 kW

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Step 2: Determine which column of Table 220.55 to use

Column C shows a maximum demand of 14 kW for three appliances
Now, because each range is rated between 1¾ and 8¾ kW, we can use Note 3 which may be less than the Column C maximum demand
Calculate the Note 3 demand using Column B for all ranges as they fall between 3½ kW and 8¾ kW

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Step 3: Calculate the total connected load

Total connected load = (4.5 kW + 4.5 kW + 7 kW) = 16 kW

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Step 4: Find the demand factor in Column B for 3 appliances

For 16 kW with 3 appliances, the demand factor is 55%

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Step 5: Calculate the demand load

Demand load = Total connected load × Demand factor

Demand load = 16 kW × 55% = 8.8 kW

Therefore, the calculated demand load for these kitchen appliances on the ungrounded service entrance conductors is 8.8 kW because it is in fact lower than the 14 kW demand that would be allowed by Column C.

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Example 2: Single Range Above 12 kW

Question: In a dwelling unit kitchen, there is one range rated at 15 kW. What is the calculated demand load for this range?

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Step 1: Identify the range and its rating

Range: 15 kW

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Step 2: Determine which column of Table 220.55 to use

The rating of the range is above 8¾ kW so we cannot use Note 3 and must use Column C
Per the heading, Column C demand factors are for appliances rated less than 12 kW so we must look at the table notes.
Note 1 states: "Over 12 kW through 27 kW ranges all of same rating. For ranges individually rated more than 12 kW but not more than 27 kW, the maximum demand in Column C shall be increased 5% for each additional kilowatt of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW."

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Step 3: Find the maximum demand in Column C

For a 15 kW range, we start with base maximum demand of 8 kW value in Column C

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Step 4: Apply Note 1 under Table 220.55

Again, Note 1 states we have to increase the Column C demand (8 kW in this case), 5% for every kW the rating of the appliance is over 12 kW

Additional kW over 12 kW = 15 kW - 12 kW = 3 kW

Increase = 5% × 3 = 15%

Final demand load = 8 kW + (8 kW × 15%) = 8 kW + 1.2 kW = 9.2 kW

Therefore, the calculated demand load for the 15 kW range is 9.2 kW.

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Example 3: Two Ranges Above 12 kW

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.

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Step 1: Identify the ranges and their ratings

Range 1: 14 kW
Range 2: 18 kW

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Step 2: Determine which column of Table 220.55 to use

The rating of the range is above 8¾ kW so we cannot use Note 3 and must use Column C

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Step 3: Calculate the total connected load

14 kW + 18 kW = 32 kW

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Step 4: Apply Note 2 under Table 220.55

Note 2 states: "Over 12 kW through 27 kW ranges of unequal ratings. For ranges individually rated more than 12 kW but not more than 27 kW, an average value of rating shall be calculated by adding together the ratings of all ranges to obtain the total connected load (32 kW) and dividing by the number of ranges (2). Then the maximum demand in Column C shall be increased 5% for each kW or major fraction thereof by which this average value exceeds 12 kW."

Average rating = 32 kW ÷ 2 = 16 kW

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Step 5: Calculate the increase based on the average rating

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

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Step 6: Apply the increase to the base demand from Column C

Base demand for 2 appliances from Column C = 11 kW

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

Therefore, the calculated demand load for the two ranges (14 kW and 18 kW) on the service entrance conductors is 13.2 kW.

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