Understanding how to calculate range demand is crucial for electrical professionals. 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 vital for passing NEC electrical exams and ensuring safe, code-compliant installations in the field.
Example Range Demand Calculation Questions on NEC Electrical Exams
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 commercial 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.
How to Identify a Range Demand Calculation Question on NEC Electrical Exams
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
Range Demand Calculation Articles: NEC 220.55
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
- Notes 1 through 5 under Table 220.55
- Column A for loads less than 3½ kW rating
- Column B for loads 3½ kW through 8¾ kW rating
- Column C for all ranges above 8¾ kW rating
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.
Walkthrough for a NEC Electrical Exam Range Demand Calculation Question
Example 1: Mixed Range Ratings
Question: 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.
Step 1: Identify all ranges and their ratings
- Two ranges: 4.5 kW each (Table 220.55, Column B)
- One range: 7 kW (Table 220.55, Column B)
- Column B for all ranges as they fall between 3½ kW and 8¾ kW
Step 2: Calculate the total connected load
- Total connected load = (4.5 kW + 4.5 kW + 7 kW) = 16 kW
Step 3: Find the demand factor in Column B for 3 appliances
- For 16 kW with 3 appliances, the demand factor is 55%
Step 4: 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.
Example 2: Single Range Above 12 kW
Question: In a commercial kitchen, there is one range rated at 15 kW. What is the calculated demand load for this range?
Step 1: Identify the range and its rating & determine which column of Table 220.55 to use
- Range: 15 kW (Column C, as it's above 8¾ kW)
Step 2: 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
Step 3: Apply Note 1 under Table 220.55
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."
Step 4: Calculate the increase based on rating above 12 kW
Additional kW over 12 kW = 15 kW - 12 kW = 3 kW
Increase = 5% × 3 = 15%
Step 5: Apply the increase to the base demand from Column C
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.
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.
Step 1: Identify the ranges and their ratings to determine which column of Table 220.55 to use
- Range 1: 14 kW (Column C)
- Range 2: 18 kW (Column C)
- Column C for both ranges as they're above 8¾ kW
Step 2: Calculate the total connected load
Total connected load = 14 kW + 18 kW = 32 kW
Step 3: 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
Step 4: Calculate the increase based on the average rating
Increase = (16 kW - 12 kW) × 5% = 4 × 5% = 20%
Step 5: 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.