Formula Used
Lighting VA per unit = floor area × lighting VA per square foot.
Small appliance VA = small appliance circuits × VA per circuit.
Laundry VA = laundry circuits × VA per circuit.
Unit connected VA = lighting + small appliance + laundry + fixed appliances + range + dryer + water heater + larger of heating or cooling + EV load.
Total connected dwelling VA = unit connected VA × number of units.
Demanded dwelling VA = total connected dwelling VA × dwelling demand factor.
Common demanded VA = common connected VA × common demand percent.
Total demand VA = demanded dwelling VA + common demanded VA + 25% largest motor adder + reserve load.
Single phase amps = total demand VA ÷ voltage ÷ power factor.
Three phase amps = total demand VA ÷ 1.732 ÷ voltage ÷ power factor.
How to Use This Calculator
Enter the number of dwelling units first. Add the average area and the lighting load basis. Then enter appliance, laundry, dryer, range, heating, cooling, and water heater loads for one typical unit.
Add common building loads next. Include corridor loads, elevators, pumps, common HVAC, site lighting, and other shared loads. Choose the demand table or enter your own demand percent.
Select phase, voltage, power factor, and reserve percent. Press the calculate button. The result appears above the form and below the header. Use the CSV or PDF buttons to save the report.
Example Data Table
| Units |
Area per Unit |
Connected VA per Unit |
Demand Factor |
Common Load |
Estimated Use |
| 4 |
750 sq ft |
36,000 VA |
45% |
3,000 VA |
Small apartment block |
| 8 |
850 sq ft |
39,750 VA |
43% |
5,000 VA |
Medium dwelling project |
| 18 |
900 sq ft |
42,000 VA |
38% |
12,000 VA |
Large multifamily service |
| 40 |
780 sq ft |
35,500 VA |
30% |
25,000 VA |
Apartment feeder planning |
Multi Dwelling Load Planning
A multi dwelling load calculator helps estimate service demand for apartments, flats, duplex groups, and mixed dwelling projects. It turns connected loads into a practical planning number. The result is useful during early design. It is also helpful before a formal electrical review.
Why This Calculation Matters
Each dwelling has lighting, receptacles, laundry circuits, cooking equipment, dryers, water heating, and comfort systems. The full connected total can look very high. Buildings rarely use every load at once. Demand factors adjust the total for likely simultaneous use. This gives a better feeder and service estimate.
The calculator separates unit loads from shared building loads. Unit loads receive a dwelling demand factor. House panels, corridors, elevators, pumps, offices, site lighting, and similar common loads are added after that step. This approach makes the result easier to check. It also shows which part of the project drives capacity.
Important Inputs
Start with the number of dwelling units. Add the average floor area if lighting load is based on area. Enter small appliance and laundry circuits per unit. Include fixed appliances, ranges, dryers, water heaters, and other permanent equipment. For heating and cooling, the larger value is usually carried forward for each unit. Enter shared building loads separately.
The demand factor table used here is a planning aid. It follows a common multifamily style. Local codes, utility rules, and engineer judgment may require a different method. Large ranges, EV chargers, commercial spaces, fire pumps, or unusual equipment should be reviewed with care.
Using the Results
The calculator shows connected load, demanded unit load, common load, total demand, amperage, and recommended service size. Amperage is calculated from voltage and power factor. A reserve percentage can be added for spare capacity. This makes early budgeting safer.
Use the CSV file for spreadsheets. Use the PDF file for client notes or permit discussions. Keep all assumptions with the report. Update the calculator when unit counts or equipment schedules change. Final construction documents should be checked by a qualified professional.
Design Notes
Early load planning also supports transformer sizing, riser space, switchgear budgets, and utility coordination. It can reveal when equipment rooms need more clearance. It can show when phased construction needs temporary capacity.
FAQs
What is a multi dwelling load calculator?
It estimates electrical service demand for buildings with multiple dwelling units. It combines unit loads, shared loads, demand factors, reserve capacity, voltage, phase, and power factor.
Can this calculator replace an engineer?
No. It is a planning tool. Final construction design should be checked by a qualified electrical professional and the local authority.
Why is demand load lower than connected load?
Connected load assumes everything runs at once. Demand load applies a factor for likely simultaneous use. This gives a more realistic service estimate.
Should heating and cooling be added together?
Usually the larger of heating or cooling is carried for each dwelling. Some systems need special treatment. Always check local rules and equipment schedules.
What common loads should I include?
Include corridors, stairs, site lighting, elevators, pumps, common HVAC, offices, access systems, and other shared building equipment.
What does the largest motor adder mean?
The calculator adds 25% of the largest motor VA. This helps account for extra motor starting and sizing allowance in planning estimates.
Why add reserve capacity?
Reserve capacity allows for future changes, design uncertainty, and spare capacity. It is useful during early budgeting and utility coordination.
Can I use a custom demand factor?
Yes. Select the custom method and enter your own percent. Use this when your local code, utility, or engineer requires a different value.