Demand Factor for Multi Dwelling Unit: NEC Calculation & Examples
The demand factor for multi dwelling unit calculations helps electrical engineers size a service or feeder without simply adding every connected load at 100% of its rating. In a multifamily building, it is unlikely that all dwelling units will reach their maximum electrical demand at the same time. The National Electrical Code (NEC) therefore provides demand factors that can reduce the calculated load for certain residential applications.
For multifamily projects in the United States, the most useful reference is NEC Article 220, particularly the optional calculation methods in Sections 220.82 and 220.84. The correct method depends on the type of building, number of dwelling units, connected loads, and local code requirements.
Table of Contents
Table of Contents
Quick Reference: Multifamily Demand Factors
The following table summarizes the dwelling-unit demand percentages commonly used with the NEC optional method for a feeder or service supplying three or more dwelling units.
| Number of Dwelling Units | Demand Factor |
|---|---|
| 1 | 100% |
| 2 | 45% |
| 3 | 44% |
| 4 | 43% |
| 5 | 42% |
| 6 | 41% |
| 7 | 40% |
| 8 | 39% |
| 9 | 38% |
| 10 | 37% |
| 11 | 36% |
| 12 | 35% |
| 13 | 34% |
| 14 | 33% |
| 15 | 32% |
| 16 | 31% |
| 17 | 30% |
| 18 | 29% |
| 19 | 28% |
| 20 | 27% |
| 21–25 | 26% |
| 26–30 | 25% |
| 31–35 | 24% |
| 36–40 | 23% |
| 41–45 | 22% |
| 46–50 | 21% |
| 51–55 | 20% |
| 56–60 | 19% |
| 61–65 | 18% |
| 66–70 | 17% |
| 71–75 | 16% |
| 76–80 | 15% |
| 81–85 | 14% |
| 86–90 | 13% |
| 91–95 | 12% |
| 96–100 | 11% |
| Over 100 | 10% |
These percentages are associated with the NEC optional multifamily dwelling calculation and should not be applied blindly to every building load. Always verify the adopted NEC edition and local amendments before finalizing a service or feeder design.
Important: Demand factors are not the same as load diversity factors. A demand factor is the ratio of maximum demand to connected load, while diversity considers the relationship between individual maximum demands and the overall system maximum demand.
What Is the Demand Factor for Multi Dwelling Unit?
The demand factor for multi dwelling unit electrical design represents the percentage of the calculated residential load expected to occur simultaneously. As the number of dwelling units increases, the percentage generally decreases because simultaneous maximum usage becomes less likely.
For example, a building with 10 dwelling units does not normally require the same service capacity as 10 completely independent homes added together at 100% of their calculated loads. The NEC optional method recognizes this difference and provides a 37% factor for 10 dwelling units under the applicable conditions.
This approach can significantly reduce the calculated service load while still providing a practical electrical capacity for the building.
NEC Multifamily Demand Factor Table Explained
The NEC table is designed around the number of dwelling units served by the feeder or service. The percentage decreases progressively as more units are added.
For example:
| Dwelling Units | Applicable Factor | Example Connected Load | Calculated Load |
|---|---|---|---|
| 3 | 44% | 30,000 VA | 13,200 VA |
| 5 | 42% | 50,000 VA | 21,000 VA |
| 10 | 37% | 100,000 VA | 37,000 VA |
| 20 | 27% | 200,000 VA | 54,000 VA |
| 50 | 21% | 500,000 VA | 105,000 VA |
| 100 | 11% | 1,000,000 VA | 110,000 VA |
| 120 | 10% | 1,200,000 VA | 120,000 VA |
The examples above demonstrate the mathematical application of the percentage. They do not mean that every connected load in an apartment building can automatically be reduced by the listed percentage.
For a complete design, lighting, receptacles, appliances, heating, air conditioning, elevators, common areas, EV chargers, fire pumps, and other building loads may require separate treatment.
How to Calculate Multifamily Electrical Demand
A basic calculation can be expressed as:
Demand Load = Connected Load × Demand Factor
Suppose a multifamily building has 20 dwelling units and the applicable calculated residential load is 200,000 VA.
The applicable factor is 27%.
Therefore:
200,000 VA × 0.27 = 54,000 VA
The resulting calculated demand is 54 kVA.
If the service is 120/208 V, three phase, the approximate current can be calculated as:
I = VA ÷ (√3 × V)
I = 54,000 ÷ (1.732 × 208)
I ≈ 150 A
This value is then considered with the other applicable building loads and NEC requirements when selecting the service rating.
For more electrical load calculations, you can also use our demand factor in power system calculator to simplify repetitive calculations and check different load scenarios.
What Loads Are Considered in a Multifamily Building?
A residential service calculation normally includes several different categories of electrical loads. The exact treatment depends on the NEC calculation method being used.
Common loads include:
- General lighting
- General-use receptacles
- Small-appliance circuits
- Laundry equipment
- Electric ranges and ovens
- Electric water heaters
- HVAC equipment
- Space heating
- Electric dryers
- Garage and parking loads
- Common-area lighting
- Elevators
- Pumps
- EV charging equipment
- Fire protection equipment
- Other building services
One important point is that the multifamily dwelling-unit demand factor should not be treated as a universal discount applied to every electrical load.
Large commercial-style loads and building services may need to be calculated separately. For example, an apartment complex with several electric vehicle chargers may have a substantial additional load that cannot simply be absorbed into the residential dwelling-unit calculation.
Example: 10-Unit Apartment Building
Consider a 10-unit apartment building where the applicable residential connected load is calculated at 100,000 VA.
According to the table, 10 dwelling units have a 37% demand factor.
Calculation:
100,000 VA × 37% = 37,000 VA
Therefore, the calculated residential demand is:
37 kVA
At 120/208 V, three phase:
37,000 ÷ (1.732 × 208) ≈ 103 A
The designer would then evaluate all other applicable loads before selecting the final service rating.
For projects with many units, using a structured load calculation can help avoid both oversizing and undersizing. Our demand factor in power system guide can be useful when comparing connected load and estimated demand across different project sizes.
Demand Factor vs Diversity Factor
These terms are often confused, but they describe different concepts.
| Term | Meaning | Basic Relationship |
|---|---|---|
| Demand Factor | Maximum demand compared with connected load | Maximum Demand ÷ Connected Load |
| Diversity Factor | Sum of individual maximum demands compared with system maximum demand | Sum of Individual Maximum Demands ÷ System Maximum Demand |
| Load Factor | Average load compared with maximum demand | Average Load ÷ Maximum Demand |
| Utilization Factor | Maximum demand compared with equipment capacity | Maximum Demand ÷ Rated Capacity |
For apartment buildings, diversity is an important reason why the maximum building demand can be much lower than the sum of individual connected loads.
Why Demand Factors Matter in Apartment Electrical Design
Using the correct demand factor can have a major effect on the size of electrical infrastructure.
An oversized service can increase:
- Transformer capacity requirements
- Switchgear cost
- Feeder conductor size
- Conduit size
- Voltage-drop considerations
- Installation cost
An undersized service, on the other hand, can create excessive loading, nuisance trips, voltage problems, and difficulty accommodating future loads.
The goal is not simply to obtain the smallest possible service. The goal is to calculate a realistic maximum demand while complying with the applicable NEC requirements.
For larger multifamily developments, it is useful to compare several scenarios before selecting the final equipment. You can use our demand factor in power system guide to quickly test different connected loads and demand percentages.
Important NEC Considerations
NEC 220.84 provides an optional method for calculating feeder and service loads for certain multifamily dwelling applications. The method has specific conditions and exclusions, so the table should not be used independently of the surrounding NEC requirements.
The designer should also check:
- The NEC edition adopted by the authority having jurisdiction.
- Local amendments to the NEC.
- Whether the building qualifies for the optional multifamily calculation.
- Heating and air-conditioning load requirements.
- Appliance calculation rules.
- Common-area electrical loads.
- EV charging loads.
- Elevator and mechanical equipment.
- Fire pumps and other special loads.
- Continuous-load requirements where applicable.
The final service size should be based on the complete calculated load, not just the residential dwelling-unit portion.
Demand Factor for Multi Dwelling Unit: Common Mistakes
Several mistakes can lead to an incorrect service calculation.
Applying the Factor to Every Load
The listed percentage is not a blanket reduction for every electrical load in the property. Individual load categories may have their own NEC calculation requirements.
Using the Wrong Number of Units
The applicable percentage is based on the number of dwelling units supplied by the feeder or service under the relevant calculation method.
Ignoring Common Areas
Hallways, parking areas, exterior lighting, mechanical rooms, elevators, laundry rooms, and other common facilities may add significant demand.
Forgetting EV Chargers
Modern apartment buildings increasingly include Level 2 and DC charging equipment. These loads require careful treatment and can substantially affect the service calculation.
Ignoring Local Amendments
The NEC is adopted and amended by jurisdictions. A calculation that follows the base NEC may still require changes to satisfy the local authority having jurisdiction.
Frequently Asked Questions
What is the demand factor for 10 dwelling units?
For the NEC optional multifamily dwelling calculation, 10 dwelling units correspond to a 37% demand factor. The factor must be applied only where the applicable NEC calculation method permits it.
What is the demand factor for 20 apartment units?
For 20 dwelling units, the listed factor is 27%. For example, a qualifying 200,000 VA residential load would produce 54,000 VA after applying 27%.
What is the demand factor for 50 dwelling units?
For 50 dwelling units, the applicable factor is 21% under the referenced NEC multifamily dwelling table. Other building loads still need to be evaluated separately.
Can I apply the multifamily demand factor to HVAC loads?
Not automatically. Heating and air-conditioning loads have specific NEC calculation provisions. The applicable equipment and operating conditions must be reviewed before applying any demand factor.
Which NEC section covers multifamily dwelling demand factors?
NEC Article 220 contains the relevant load calculation provisions. Section 220.84 is the optional method specifically associated with certain multifamily dwelling services and feeders. Always verify the current NEC edition and local amendments before using the calculation for construction.
Final Takeaway
The demand factor for multi dwelling unit calculations allows qualifying multifamily electrical services and feeders to account for the lower probability that every dwelling will reach its maximum load at the same time. The NEC table ranges from 100% for one unit to 10% for more than 100 units under the applicable optional method.
However, the percentage is only one part of a complete electrical load calculation. HVAC, appliances, common areas, EV chargers, elevators, pumps, and other building services may require separate calculations. A properly prepared multifamily service calculation should therefore combine the applicable NEC demand rules with the actual characteristics of the building.
For faster electrical design checks, use our demand factor in power system guide to compare connected loads, demand percentages, and estimated maximum demand before sizing the feeder or service.
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