When can I use the NEC 220.84 optional method?

The optional method is permitted only when there are three or more dwelling units, every unit is equipped with electric cooking, and every unit has electric space heating, electric air conditioning, or both. If any of these is missing, you must use the standard method in 220.42 through 220.55.

What demand factor does a 3-to-5 unit building get?

Buildings with 3 to 5 dwelling units use a 45 percent demand factor under Table 220.84. The factor drops as the unit count rises because diversity increases, reaching about 28 percent at 40 or more units.

Why use the larger of A/C or heating instead of both?

NEC 220.60 lets you omit the smaller of two noncoincident loads because a unit will not run full heating and full cooling at the same time. The calculator automatically takes the larger of the two values you enter.

Does the small-appliance allowance still apply?

Yes. Each unit gets at least two 1500 VA small-appliance branch circuits plus one 1500 VA laundry circuit. The connected load includes these before the demand factor is applied.

What voltage should I divide by?

For a standard single-phase 120/240 V service, divide the demand load in VA by 240 to get amps. For a three-phase line-to-line service use 208 V; a full three-phase calculation also multiplies by the square root of three for balanced loads.

Multifamily Dwelling Load Calculation (NEC 220.84)

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 voltage

The 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.