Multifamily Dwelling Load Calculator
Size an apartment building feeder or service by the NEC 220.84 optional method. Enter one dwelling unit's connected load and the number of units, and the calculator applies the Table 220.84 demand factor for that unit count, adds house loads in the right place, and reports the calculated load in amperes with the standard service rating that covers it.
How to use this calculator
- Enter the number of dwelling units and the floor area of one unit. The unit count picks the Table 220.84 row; the area drives the 3 VA/ft² general lighting and receptacle load.
- Pick the service: 120/240 V or 208Y/120 V single-phase, or 208Y/120 V or 480Y/277 V three-phase. Three-phase divides by the square root of 3 times the voltage.
- Enter the per-unit circuits and appliance nameplates: two small-appliance circuits minimum, laundry circuits (zero if the building has a common laundry), and the range, dryer, water heater and other fastened-in-place loads at full nameplate.
- Enter the per-unit air conditioning and electric heat. Only the larger counts, per NEC 220.84(C)(5), and the tool shows which governed.
- Enter the house load in VA if you have it. It is added after the demand factor, per 220.84(B). Uncheck any 220.84(A) condition the building fails and the tool flags that the optional method does not apply.
NEC reference
This calculator implements NEC 2020 220.84 (Multifamily Dwelling) and Table 220.84, the optional calculation for a feeder or service supplying three or more dwelling units. The per-unit connected load follows 220.84(C): 3 VA/ft² for general lighting and general-use receptacles, 1500 VA for each 2-wire 20 A small-appliance branch circuit and each laundry branch circuit per 210.11(C), the nameplate rating of fastened-in-place appliances, ranges, dryers and water heaters, the nameplate of permanently connected motors, and the larger of the air-conditioning or fixed electric space-heating load. Every one of those is taken at 100%. The summed connected load of all units is then multiplied by the Table 220.84 demand factor for the unit count, which runs from 45% at 3 to 5 units to 23% at 62 units and over. House loads are calculated under Part III and added after the demand factor per 220.84(B). Recommended service ratings are the NEC 240.6(A) standard sizes. Table 220.84 is unchanged in the 2017, 2020 and 2023 editions.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Worked example: 12 unit building, 1200 ft² units
Twelve identical 1200 ft² apartments, each with a 12 kW range, 5.5 kW dryer, 4.5 kW water heater, 1.2 kW dishwasher, a 1.1 kW disposal, a 6 kW air conditioner and 5 kW of electric heat, on a 120/240 V service. Every unit has electric cooking and electric heat or air conditioning and is fed by one feeder, so 220.84 applies.
| Line (per dwelling unit) | VA |
|---|---|
| General lighting and receptacles: 1200 ft² × 3 VA | 3,600 |
| Two small-appliance circuits × 1500 VA | 3,000 |
| Laundry circuit | 1,500 |
| Range 12 kW + dryer 5.5 kW + water heater 4.5 kW + dishwasher 1.2 kW + disposal 1.1 kW | 24,300 |
| Air conditioning (larger of AC 6 kW and heat 5 kW) | 6,000 |
| Connected load per unit | 38,400 |
| × 12 units | 460,800 |
| Table 220.84 demand factor, 12 to 13 units: 41% | × 0.41 |
| Calculated load | 188,928 |
188,928 VA ÷ 240 V = 787.2 A, so an 800 A service covers the dwelling units. These are the calculator's default inputs, so you can check the tool against the worked example directly. Any house load, the corridor and exterior lighting, the house panel, the common laundry, would be calculated separately under Part III and added to the 188,928 VA before converting to amperes.
Table 220.84 demand factors
The factor applies to the total connected load of all dwelling units. Values only; consult the published NEC for the table itself.
| Units | Factor | Units | Factor |
|---|---|---|---|
| 3 to 5 | 45% | 28 to 30 | 33% |
| 6 to 7 | 44% | 31 | 32% |
| 8 to 10 | 43% | 32 to 33 | 31% |
| 11 | 42% | 34 to 36 | 30% |
| 12 to 13 | 41% | 37 to 38 | 29% |
| 14 to 15 | 40% | 39 to 42 | 28% |
| 16 to 17 | 39% | 43 to 45 | 27% |
| 18 to 20 | 38% | 46 to 50 | 26% |
| 21 | 37% | 51 to 55 | 25% |
| 22 to 23 | 36% | 56 to 61 | 24% |
| 24 to 25 | 35% | 62 and over | 23% |
| 26 to 27 | 34% |
Frequently asked questions
Do the range and dryer demand factors apply here?
No, and this is the error worth watching for. Under 220.84(C) every load enters at 100% of nameplate, including the full range and dryer ratings. The Part III demand factors of Table 220.55 (ranges), 220.54 (dryers) and Table 220.42 (general lighting) belong to the standard calculation, not to this one. The single Table 220.84 factor is the only reduction taken. Applying both stacks two diversity allowances on the same loads and undersizes the service.
How do house loads fit in?
NEC 220.84(B) is explicit: house loads are calculated under Part III and added to the dwelling-unit load. They never get the multifamily demand factor, because corridor lighting, elevators and common-area equipment do not diversify the way apartments do. This calculator takes the house-load figure as an input and adds it after the factor. It does not compute the house load itself, which is an ordinary Part III calculation on the common-area loads.
What if the units are not identical?
Table 220.84 applies to the summed connected load of all dwelling units, so different unit types are handled by calculating each type's 220.84(C) connected load, adding them, and applying the factor for the total number of units. This calculator takes one unit profile times a count, which covers the common case of a building of one or two repeated plans. For a mixed building, run each plan, sum the connected loads by hand, and read the factor from the table above.
What if the units have gas cooking?
Then 220.84(A)(2) is not satisfied and the optional method does not apply as written. There is an exception: where the Part III calculated load for a multifamily dwelling without electric cooking exceeds the Part IV calculation for the identical load plus electric cooking at 8 kW per unit, the lesser of the two may be used. That comparison needs a full Part III calculation of the building and is not performed here. Uncheck the electric cooking condition and the calculator flags the building as ineligible while still showing the arithmetic for comparison.
Is this the same as the 220.82 dwelling optional method?
No. 220.82 sizes a single dwelling unit's service and uses a 10 kVA breakpoint with 40% on the remainder plus the 220.82(C) heating factors. 220.84 sizes the feeder or service for the whole multifamily building and uses one flat demand factor chosen by unit count, with no breakpoint at all. If you are sizing the feeder to one apartment rather than the building service, 220.82 is the tool.
Related calculators
- Electrical Load Calculator: the NEC 220.82 optional method for a single dwelling unit service.
- Residential Electrical Load Calculator: the Article 220 Part III standard method, category by category, which is what house loads use.
- Existing Dwelling Load Calculator: the NEC 220.83 method for adding a load to an existing dwelling unit.
- Service Size Calculator: turn the calculated load into a service disconnect, conductors, and GEC.
- Neutral Load Calculator: the feeder or service neutral load per NEC 220.61, which 220.84 permits.
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