Sizing the electrical service for an apartment or condo building one unit at a time wildly overshoots, because not every unit peaks at the same moment. NEC 220.84 gives a legal shortcut: sum the full connected load of every unit, then multiply by a single diversity factor from Table 220.84 that shrinks as the building grows.
How it works
The optional method first builds the connected load of one representative unit, then scales it to the whole building and applies the table factor:
per unit = 3 VA/ft² × area
+ 1500 VA × small-appliance circuits (min 2)
+ 1500 VA laundry
+ range nameplate
+ larger of (A/C, heating)
+ other appliance nameplate
total = per unit × number of units
demand VA = total × Table 220.84 factor
amps = demand VA / service voltageThe demand factor starts at 45 percent for 3 to 5 units and falls step by step to about 28 percent for 40 or more units. Below 3 units the optional method is not allowed and no factor may be applied.
Example and notes
An 8-unit building of 900 ft² units, each with a 12 kVA range, 9 kVA electric heat, 5 kVA A/C, and 4.5 kVA of other appliances, has a per-unit connected load near 33 kVA. Across 8 units that is about 264 kVA, and the 42 percent factor for 8 units brings the demand to roughly 111 kVA, or about 462 A at 240 V — round up to a 600 A service. Always confirm each unit truly has electric cooking plus heating or cooling before relying on this method, and compare against the standard method if any unit is gas-fired.
Demand factor table (NEC Table 220.84)
The demand factor decreases as the building grows, because statistical diversity means fewer units peak simultaneously:
| Number of units | Demand factor |
|---|---|
| 3 – 5 | 45% |
| 6 – 7 | 44% |
| 8 – 10 | 43% |
| 11 – 13 | 42% |
| 14 – 15 | 41% |
| 16 – 17 | 40% |
| 18 – 20 | 39% |
| 21 – 22 | 38% |
| 23 – 25 | 37% |
| 26 – 27 | 36% |
| 28 – 30 | 35% |
| 31 – 40 | 34% |
| 41 – 50 | 33% |
| 51 – 60 | 32% |
| 61 + | 28% |
These figures match the percentages in NEC 2023 Table 220.84. At very large scale (61+ units) the factor stabilizes near 28%, reflecting the high diversity of a large apartment community.
Common pitfalls when using the optional method
Missing the eligibility check. The optional method is only valid when all three qualifying conditions hold: three or more units, electric cooking in every unit, and electric heating or cooling (or both) in every unit. If even one unit has gas cooking or no air conditioning, the optional method does not apply and you must use the standard method.
Using nameplate instead of calculated load. The range load entered should be the nameplate demand rating per NEC 220.55 or the calculated 8 kVA minimum for a single range. For cooking equipment rated above 1.75 kW, use the full nameplate. Do not estimate or round down nameplate kVA to save service amps.
Forgetting laundry. Each unit must include a 1500 VA laundry circuit. In a building with central laundry rather than in-unit hookups, consult 220.84 and local interpretation — some inspectors still require the per-unit laundry load, while others allow it to be replaced by the central laundry equipment load on a separate feeder.