8.5 Optional Dwelling and Multifamily Calculations

When the Optional Method May Be Used — 220.82(A)

This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 amperes or greater. It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Part III of this article.

Three gates: one dwelling unit, a single 3-wire 120/240 V or 208Y/120 V supply, and 100 amperes or greater. A 60-ampere cabin service must use the standard method.

The General Load — 220.82(B)

The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following:

1. 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit;
2. 3 volt-amperes per square foot of floor area for general lighting and general-use receptacles, using outside dimensions and excluding open porches, garages, and unused or unfinished spaces not adaptable for future use;
3. The nameplate rating of the following:
   • all appliances that are fastened in place, permanently connected, or located on a specific circuit;
   • ranges, wall-mounted ovens, counter-mounted cooking units;
   • clothes dryers not connected to the laundry branch circuit;
   • water heaters;
4. The nameplate ampere or kVA rating of all permanently connected motors not included in item 3.

Notice how much simpler this is: nameplate ratings, not Table 220.55 demand values. The range goes in at its full nameplate, and the 40-percent factor does the reducing.

Heating and Air Conditioning — 220.82(C)

Include the largest of the following six:

1. 100 percent of the nameplate rating of the air conditioning and cooling;
2. 100 percent of the nameplate rating of the heat pump when the heat pump is used without any supplemental electric heating;
3. 100 percent of the nameplate rating of the heat pump compressor and 65 percent of the supplemental electric heating for central electric space-heating systems (if the heat pump compressor is prevented from operating at the same time as the supplemental heat, it does not need to be added);
4. 65 percent of the nameplate rating of electric space heating if less than four separately controlled units;
5. 40 percent of the nameplate rating of electric space heating if four or more separately controlled units;
6. 100 percent of the nameplate ratings of electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value.

Items 4 and 5 matter greatly in Alaska: four or more separately controlled baseboard heaters drop the heating load to 40 percent, whereas one or two zones sit at 65 percent. Baseboard heat with individual thermostats in every room is common in Alaska housing, and it qualifies for the 40 percent factor.

Worked Example — Optional Method

The same 2,400-square-foot Anchorage dwelling from section 8.4: two small-appliance circuits, one laundry circuit, a 12 kW range, a 5 kW dryer, a 4.5 kW water heater, a 1.2 kW dishwasher, a 0.9 kW disposal, a 1.5 kW trash compactor, and 12 kW of electric baseboard heat in six separately controlled zones.

GENERAL LOADS, 220.82(B)
  Small appliance + laundry  3 x 1,500     =   4,500 VA
  General lighting  2,400 x 3              =   7,200 VA
  Range (nameplate)                        =  12,000 VA
  Dryer (nameplate)                        =   5,000 VA
  Water heater                             =   4,500 VA
  Dishwasher                               =   1,200 VA
  Disposal                                 =     900 VA
  Trash compactor                          =   1,500 VA
                                              ---------
  Total general load                          36,800 VA

  First 10,000 VA at 100%                  =  10,000 VA
  Remaining 26,800 VA at 40%               =  10,720 VA
                                              ---------
  General load demand                         20,720 VA

HEATING, 220.82(C)(5)
  Six separately controlled units -> 40%
  12,000 x 0.40                            =   4,800 VA
                                              ---------
TOTAL CALCULATED LOAD                         25,520 VA

Service current = 25,520 / 240             =   106.3 A

The standard method gave 154.7 amperes for the same house; the optional method gives 106.3 amperes. The optional method is nearly always smaller, which is why it dominates residential practice. Both are legal; the designer chooses.

A 125-ampere service would satisfy the optional calculation. In practice most builders install 200 amperes for future capacity, which is a design choice rather than a Code requirement — recall 90.1(B): Code minimum is not necessarily adequate for future expansion.

Existing Dwelling Units — 220.83

This section applies where additional loads are being added to an existing dwelling unit, and the existing service or feeder is being evaluated.

220.83(A) Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed. The following percentages shall be used for existing and additional new loads:

Load calculation shall include: general lighting and receptacle loads at 3 VA per square foot; 1500 VA for each 2-wire, 20-ampere small-appliance and laundry branch circuit; the nameplate rating of all appliances fastened in place, ranges, ovens, cooking units, clothes dryers, and water heaters.

220.83(B) Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed. The following percentages apply:

Note how the percentages harden when new heating or cooling is added — the Code stops assuming diversity for the equipment being added.

Multifamily Dwellings — 220.84

220.84(A) Feeder or Service Load. It shall be permissible to calculate the load of a feeder or service that supplies three or more dwelling units of a multifamily dwelling in accordance with Table 220.84 instead of Part III of this article if all of the following conditions are met:

1. No dwelling unit is supplied by more than one feeder;
2. Each dwelling unit is equipped with electric cooking equipment;
3. Each dwelling unit is equipped with either electric space heating, air conditioning, or both.

Exception to (2): where the calculated load for multifamily dwellings without electric cooking in Part III of this article exceeds that calculated under Part IV for the identical load plus electric cooking (based on 8 kW per unit), the lesser of the two loads shall be permitted.

220.84(C) Calculated Loads. The calculated load to which the demand factors of Table 220.84 apply shall include:

1. 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit;
2. 3 volt-amperes per square foot for general lighting and general-use receptacles;
3. The nameplate rating of all appliances fastened in place, permanently connected, or located on a specific circuit, plus ranges, ovens, cooking units, clothes dryers, and water heaters;
4. The nameplate ampere or kVA rating of all permanently connected motors not included in item 3;
5. The larger of the air-conditioning load or the fixed electric space-heating load.

Table 220.84 Demand Factors decline steeply with unit count:

220.85 Two Dwelling Units. Where two dwelling units are supplied by a single feeder and the calculated load under Part III exceeds that for three identical units calculated under 220.84, the lesser of the two loads shall be permitted.

Choosing a Method

Nothing in the Code requires you to use the smaller result. Both the standard and optional methods are legal, and the calculated load is a minimum. On an exam, read the question carefully: it will specify which method to use, and the answers will usually include the result of the other method as a distractor.