3.2 Fixed Appliance, Dryer, & Cooking Unit Loads

3.2 Fixed Appliance, Dryer, & Cooking Unit Loads

Once general lighting and receptacle loads are determined, feeder and service calculations require sizing major residential loads: fixed appliances, electric clothes dryers, and household cooking equipment. Because these heavy electrical loads do not operate at full power simultaneously, NEC Article 220 provides specific demand factors and rules to reduce calculated service demand while ensuring adequate ampacity.

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1. Fastened-in-Place Appliance Demand Factor (NEC 220.53)

In a dwelling unit, NEC 220.53 permits a demand factor of 75% (0.75) to be applied to the total nameplate rating of four (4) or more appliances fastened in place that are served by the same feeder or service.

Qualifying vs. Excluded Appliances

To apply the 75% demand factor correctly, you must know which appliances qualify and which are strictly excluded:

+-----------------------------------------------------------------------+
|                   NEC 220.53 APPLIANCE COUNT RULES                    |
|                                                                       |
|   QUALIFYING APPLIANCES (Count towards 4+):                           |
|   - Electric Water Heater                                             |
|   - Garbage Disposal                                                  |
|   - Dishwasher                                                        |
|   - Trash Compactor                                                   |
|   - Built-in Microwave                                                |
|   - Attic Exhaust Fan / Sump Pump                                     |
|                                                                       |
|   EXCLUDED APPLIANCES (Do NOT count towards 4+):                      |
|   - Electric Household Cooking Equipment (Ranges, Ovens, Cooktops)    |
|   - Electric Clothes Dryers                                           |
|   - Space Heating Equipment (Baseboard or Central Heat)               |
|   - Air-Conditioning Equipment                                        |
+-----------------------------------------------------------------------+

Exam Trap: If a dwelling has 3 fastened appliances (Water Heater 4,500W, Dishwasher 1,200W, Disposal 900W) plus an electric range and dryer, you CANNOT apply the 75% factor! You must have at least 4 qualifying fastened-in-place appliances before applying 75%.

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2. Electric Clothes Dryer Load Sizing (NEC 220.54)

Under NEC 220.54, the load for household electric clothes dryers in a dwelling unit is calculated based on the following minimum rules:

1. Minimum Load Rule: The calculated load for each dryer must be 5,000 VA or the nameplate rating, whichever is larger.
   • Example: A dryer rated 4,200 VA on its nameplate must be calculated at 5,000 VA.
   • Example: A dryer rated 6,500 VA on its nameplate must be calculated at 6,500 VA.
2. Demand Factors for Multiple Dryers (NEC Table 220.54):

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3. Household Electric Ranges & Cooking Appliances (NEC 220.55)

Calculating electric range and cooking appliance loads is one of the most tested subjects on electrical licensing examinations. NEC Table 220.55 allows demand factors for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances rated over $1.75\text{ kW}$.

Table 220.55 Demand Factor Columns Explained

• Column A (Ratings less than $3.5\text{ kW}$): Applied as a percentage of total nameplate ratings.
• Column B (Ratings $3.5\text{ kW}$ to $8.75\text{ kW}$): Applied as a percentage of total nameplate ratings.
• Column C (Ratings $8.75\text{ kW}$ to $12\text{ kW}$): Specifies a maximum demand load in kW directly, rather than a percentage!

Key Values from Table 220.55 (Column C)

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4. Range Rating Adjustments Over 12 kW (Table 220.55 Note 1)

When individual ranges exceed $12\text{ kW}$, Note 1 of Table 220.55 requires an increase to the Column C maximum demand value:

1. Determine the amount by which the range rating exceeds $12\text{ kW}$.
2. For each additional kW or major fraction thereof ($0.5\text{ kW}$ or greater), increase the Column C maximum demand value by 5% (0.05).
3. Multiply the Column C demand by $(1 + \text{Percentage Increase})$.

Formula for Ranges Over 12 kW:

$\text{Excess kW} = \text{Nameplate Rating (kW)} - 12\text{ kW}$ $\text{Multiplier} = 1 + (\text{Rounded Excess kW} \times 0.05)$ $\text{Final Feeder Demand Load} = \text{Column C Base Demand} \times \text{Multiplier}$

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5. Step-by-Step Worked Calculation Examples

Example 1: Fastened Appliance Load Calculation

A single-family dwelling contains the following fixed appliances connected to the service:

• Electric Water Heater: $4,500\text{ VA}$
• Dishwasher: $1,400\text{ VA}$
• Garbage Disposal: $900\text{ VA}$
• Trash Compactor: $1,000\text{ VA}$
• Attic Fan: $600\text{ VA}$

Solution:

1. Count qualifying appliances: There are 5 qualifying fastened appliances (exceeds the minimum of 4).
2. Total Nameplate Sum: $4,500 + 1,400 + 900 + 1,000 + 600 = 8,400\text{ VA}$
3. Apply NEC 220.53 Demand Factor ($75%$): $\text{Feeder Demand Load} = 8,400\text{ VA} \times 0.75 = \mathbf{6,300\text{ VA}}$

Example 2: Range Load Calculation ($15.4\text{ kW}$ Range)

Calculate the feeder demand load for a single household range with a nameplate rating of $15.4\text{ kW}$.

Solution:

1. Find Base Column C Demand for 1 range: $8\text{ kW}$.
2. Determine Excess over $12\text{ kW}$: $15.4\text{ kW} - 12\text{ kW} = 3.4\text{ kW}$
3. Apply Major Fraction Rule: $3.4\text{ kW}$ rounds down to $3\text{ kW}$ (since $0.4$ is less than $0.5$).
4. Calculate Percentage Increase: $3\text{ kW} \times 5\% = 15\%\text{ increase}$
5. Compute Final Demanded Load: $\text{Feeder Demand} = 8\text{ kW} \times (1 + 0.15) = 8\text{ kW} \times 1.15 = \mathbf{9.2\text{ kW} (9,200\text{ VA})}$

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6. Branch Circuit Sizing for Cooking Equipment (NEC 210.19(A)(3))

Under NEC 210.19(A)(3), the branch-circuit rating for a single household range rated $8.75\text{ kW}$ or more must not be less than 40 amperes.

$\text{Calculated Current} = \frac{8,000\text{ VA}}{240\text{ V}} = 33.33\text{ Amperes}$ Since $33.33\text{ A}$ exceeds $30\text{ A}$, the circuit must be protected by a minimum 40-ampere overcurrent device using minimum #8 AWG copper conductors.