Beatrice Lang
Calculating the electrical load for multifamily dwellings can be confusing due to the availability of multiple methods, including the standard method (Art. 220, Part II) and the optional method (Art. 220, Part III or IV). The standard method involves six steps, including determining the general load, which consists of lighting and receptacle loads, small appliance loads, and laundry circuit loads. The optional method, on the other hand, is a simpler three-step process that results in a smaller service and is generally preferred in the field. It is important to note that the optional method has specific conditions, such as having no more than one feeder per dwelling unit and requiring electric cooking equipment and space heating or air conditioning in each unit. When calculating the electrical load, it is crucial to follow the Code rules for the specific calculation and not mix the standard and optional methods. Additionally, demand factors, such as the number of units and the presence of electric cooking and heating, should be considered, as outlined in resources like the National Electrical Code (NEC) and UpCodes.
| Characteristics | Values |
|---|---|
| Definition of a multifamily dwelling | A dwelling becomes "multifamily" when it contains three or more dwelling units |
| Calculating service | Calculate the load and apply the appropriate demand factors to the sum of the individual dwelling units of that multifamily dwelling |
| Applicable methods | Standard method (Art. 220, Part II) and optional method (Art. 220, Part III) |
| Standard method | A six-step process |
| Step 1: Determine the general load | Consists of the general lighting and receptacle loads (3VA per square foot), a minimum of two 1,500VA small appliance loads (3,000VA), and the laundry circuit loads (1,500VA) |
| Step 2: Calculate the load | Apply the demand factor from Table 220.84 to the total connected load |
| Step 3: Size feeder and service conductors | Size the ungrounded conductors per Table 310.16, based on the calculated load |
| Optional method | A three-step process |
| Conditions for using the optional method | There must be no more than one feeder per dwelling unit. Each dwelling unit is equipped with electric cooking equipment and/or electric space heating, air conditioning, or both. |
| Step 1: Determine the total connected load | Add up the loads, including corridor lighting, outside lighting, parking lot lighting, sign lighting, and sprinkler systems |
| Feeder for a two-family dwelling unit | Calculated using the standard method in Part III of Art. 220 |
| Sizing of branch circuits, feeders, and service conductors | Similar to the sizing for single-family dwellings |
| Demand factors | Demand percentages that decrease as the number of units increases, starting at 45% for 3-5 units and dropping to 23% for 62 or more units |
| Calculating appliance demand load | Use a demand factor of 75% for four or more appliances fastened in place, such as dishwashers, waste disposals, trash compactors, or water heaters |
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What You'll Learn
Calculating feeder or service loads
Calculating the feeder or service load for a multi-family dwelling is a complex process that requires a good understanding of electrical engineering and the National Electrical Code (NEC). The NEC defines a dwelling unit as a single unit that provides complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation. A dwelling becomes "multi-family" when it contains three or more of these dwelling units.
When calculating the service load for a multi-family dwelling, you must first calculate the load for each individual dwelling unit and then apply the appropriate demand factors to the sum of these units. This can be done using either the standard method from Part III of Art. 220 or the optional method from Part IV of Art. 220.
The standard method involves six steps, including determining the general load, which consists of general lighting and receptacle loads, small appliance loads, and laundry circuit loads. It is important to note that a laundry circuit is not required for an individual dwelling unit if the multi-family unit has common laundry facilities.
The optional method, on the other hand, is a three-step process and can be used if the dwelling meets certain conditions, such as having no more than one feeder per dwelling unit and each dwelling unit being equipped with electric cooking equipment and space heating, air conditioning, or both. This method takes a broader approach, considering the individual nameplate ratings for the loads and providing a generalized allowance for overall Volt-Amps expected to be consumed.
It is important to follow the Code rules for the specific calculation and not to intermix the standard and optional methods. Additionally, demand factors for multi-family dwellings with three or more units should be considered, as outlined in relevant tables and articles in the NEC. These demand factors decrease as the number of units increases, with percentages starting at 45% for 3-5 units and dropping to 23% for 62 or more units.
By carefully applying these methods and considering the relevant demand factors, one can successfully calculate the feeder or service loads for a multi-family dwelling.
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Applying demand factors
When calculating the electrical service for a multi-family dwelling, it is essential to apply the appropriate demand factors. This process involves considering the diversity of usage and loads across the separate units. Unlike a single-family dwelling, where load diversity exists among different types of loads, a multi-family dwelling presents diversity among the individual units. This means that the electrical calculations must account for variations in usage patterns and load demands between different families.
To apply demand factors effectively, follow these steps:
Step 1: Determine the Total Connected Load
Start by adding up all the relevant loads. This includes loads from various appliances, such as dishwashers, waste disposal units, water heaters, dryers, and cooking equipment. For example, consider a 25-unit apartment building, where each apartment has specific loads for different appliances. Calculate the total connected load by summing up the individual loads from each unit.
Step 2: Apply Demand Factors
Once you have the total connected load, refer to the appropriate demand factor tables, such as Tables 220.84, 220.18, 220.19, and 220.32. These tables provide demand factors based on the number of dwelling units and the specific type of load. For instance, if there are four or more appliance loads on the service, apply a 75% demand factor to the connected load of those appliances.
Additionally, consider specific adjustments, such as using 5,000W or the nameplate rating (whichever is greater) for the clothes dryer load and then adjusting it according to the demand factors in Table 220.18. Similarly, for household cooking appliances rated greater than 1.75W, apply the demand factors from Table 220.19.
Step 3: Calculate the Net Computed Demand Load
After applying the relevant demand factors, calculate the net computed demand load. This involves using the demand factor from Table 220.32 and applying it to the total connected load. This step helps determine the overall electrical demand required by the multi-family dwelling.
Step 4: Size the Ungrounded Conductors
Refer to Table 310.16 to size the ungrounded conductors based on the calculated demand load. This table provides guidelines for selecting the appropriate conductor sizes to ensure they meet or exceed National Electrical Code (NEC) requirements.
It is important to note that the calculation process may vary slightly depending on local electrical codes and regulations. Therefore, always refer to the specific Code rules and guidelines applicable to your region when performing electrical calculations for multi-family dwellings.
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Sizing branch circuits
Branch circuits for lighting and appliances, including motor-operated appliances, must be provided to supply the loads computed in accordance with 220.3. In addition, branch circuits must be provided for specific loads not covered by 220.3 (e.g. a hot tub) and for dwelling unit loads as specified in 210.11(C).
At least one 20-ampere branch circuit must be provided to serve the laundry circuit. Additionally, at least two 20-ampere small-appliance branch circuits are required. The bathroom branch circuit supplying receptacle outlets must be rated at 20 amperes, and no additional load calculation is required.
Load calculations for small-appliance and laundry branch circuits are covered in 220.52. Caution is advised when installing receptacles on branch circuits in dwellings. Local jurisdictions may limit the number of receptacle outlets permitted on 15- and 20-ampere, general-purpose branch circuits.
To determine conductor sizes for branch circuits, refer to 210.19. Specifications for branch-circuit overcurrent protection are in 210.20.
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Determining appliance demand load
When determining the appliance demand load, it is important to follow the specific Code rules for the calculation. The National Electrical Code (NEC) defines a dwelling unit as a single unit that provides complete and independent facilities for living, sleeping, cooking, and sanitation. A dwelling becomes "multifamily" when it contains three or more of these dwelling units.
To calculate the appliance demand load, you must first calculate the total load from all the appliances. This involves looking at the individual nameplate ratings for the loads, which will give you a generalized allowance for overall Volt-Amps expected to be consumed by the appliances.
Once you have the total load, you can then apply the appropriate demand factors. For a single-family dwelling, you calculate the load and apply the demand factors. For a multifamily dwelling, you do the same but apply the demand factors to the sum of the individual dwelling units.
It is important to note that the optional method for calculating appliance demand load can only be used if each dwelling unit is equipped with electric cooking equipment, electric heating or air-conditioning, and is supplied by no more than one feeder.
There are specific considerations for certain appliances. For example, a 75% demand factor can be applied when there are four or more fastened-in-place appliances such as water heaters, trash compactors, and dishwashers. For electric clothes dryers, you must use the nameplate rating or 3000 VA, whichever is larger.
By following these steps and applying the appropriate demand factors, you can successfully determine the appliance demand load for a multi-family dwelling.
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Calculating total load
Calculating the total load for a multi-family dwelling can be done using either the standard method (Art. 220, Part II) or the optional method (Art. 220, Part III). The optional method is generally preferred in the field as it has fewer steps and results in a smaller service, but for electrical exams, the standard method should be used unless otherwise specified.
The standard method involves six steps:
- Determine the general load, which includes general lighting and receptacle loads (3VA per square foot), a minimum of two 1500VA small appliance loads, and the laundry circuit loads (1500VA).
- Add in loads that have their own demand factor tables, such as dryers and ranges.
- Calculate the total connected load by adding up the individual loads from each dwelling unit.
- Apply the demand factor from Table 220.84 to the total connected load.
- Size the feeder and service conductors based on the calculated load.
- Size the ungrounded conductors per Table 310.16.
The optional method, on the other hand, is a three-step process and can only be used if the dwelling meets certain conditions: there must be no more than one feeder per dwelling unit, each unit must be equipped with electric cooking equipment, and each unit must have electric space heating, air conditioning, or both. The steps for this method are as follows:
- Determine the total connected load, which includes the loads from each dwelling unit as well as any additional loads from the house panel, such as lighting in common areas or a sprinkler system.
- Apply the demand load from Table 220.84.
- Compare the heating load to the AC load and take the larger of the two loads.
It is important to note that the sizing of branch circuits, feeders, and service conductors for multi-family dwellings is similar to that of single-family dwellings. Additionally, when calculating the service load, it is crucial to follow the specific Code rules for the chosen calculation method and not intermix the two methods.
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