**Residential Services (Optional Method)**

There is an alternate method of calculating single family dwelling services and feeders. It’s called the “Optional Calculation.”

There are some limitations in using the optional method. (1) The service size has to be a minimum of 100 amps. (2) The general method must be used to calculate the neutral size, there is no optional method as far as the neutral is concerned. On the positive side, the major difference in using the optional method is that instead of applying demand factors to each individual area we apply one big demand factor to everything, except heating and air-conditioning...

**.***220.82(A) Dwelling Unit Feeder and Service Load (Optional Method): 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 or greater. The calculated load shall be the result of adding the loads from 220.82(B) and (C). Feeder and service-entrance conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.61*There are some limitations in using the optional method. (1) The service size has to be a minimum of 100 amps. (2) The general method must be used to calculate the neutral size, there is no optional method as far as the neutral is concerned. On the positive side, the major difference in using the optional method is that instead of applying demand factors to each individual area we apply one big demand factor to everything, except heating and air-conditioning...

General Loads

Because we’re applying only one major demand factor we use only the nameplate ratings for all our loads. We must not apply any of the individual demand factors that we used in the general method.

Loads identified as “General Loads” include everything except air-conditioning and heating equipment...

(no 75% demand factor for 4 or more)

The Code specifically requires us to total the nameplate ratings of all of these loads, then apply the following demand factor...

*220.82(B) General Loads: 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 loads:**(1) 3 volt-amperes per square ft. for general lighting and general-use receptacles. The floor*

area for each floor shall be calculated from the outside dimensions of the dwelling unit. The

calculated floor area shall not include open porches, garages, or unused or unfinished

spaces not adaptable for future use.area for each floor shall be calculated from the outside dimensions of the dwelling unit. The

calculated floor area shall not include open porches, garages, or unused or unfinished

spaces not adaptable for future use.

**(2) 1500 volt-amperes for each 2-wire, 20-ampere small appliance branch circuit and each**

laundry branch circuit covered in 210.11(C)(1) and (C)(2).laundry branch circuit covered in 210.11(C)(1) and (C)(2).

*(3) The nameplate rating of the following...**a. All appliances that are fastened in place, permanently connected, or located to be on a*

specific circuit.specific circuit.

*b. Ranges, wall-mounted ovens, counter-mounted cooking units.**c. Clothes dryers that are not connected to the laundry branch circuits.**d. Water heaters.*Loads identified as “General Loads” include everything except air-conditioning and heating equipment...

**(1)**General Lighting and Receptacles at 3 vA per square foot**(2)**Small Appliances at 1,500 vA (minimum of two)**(3)**Laundry at 1,500 vA (minimum of one)**(4)**Fixed Appliances; including: water heaters, dishwashers, disposals, trash compactors, etc.(no 75% demand factor for 4 or more)

**(5)**Cooking Appliances; including: ranges, ovens, cooktops (no Table 220.55)**(6)**Dryers (nameplate rating, no 5,000 vA minimum)**(7)**Motors and General Loads including; sprinkler and pool pumps (nameplate rating)

The Code specifically requires us to total the nameplate ratings of all of these loads, then apply the following demand factor...

**First 10 kVa at . . . . . . .100%**

__Optional Demand (220-82(B))__Remainder at. . . . . . . . 40%

**For example:**let’s calculate the following single-family dwelling using the optional method (240/120v, single-phase):

Air-Conditioning Versus Heat

*220.82(C): Heating and Air-Conditioning Load. The largest of the following six selections (load in kVA) shall be included:*

*(1) 100 percent of the nameplate rating(s) of the air conditioning and cooling.*

(2) 100 percent of the nameplate rating(s) of the heat pump when the heat pump is used

without any supplemental electric heating.

(2) 100 percent of the nameplate rating(s) of the heat pump when the heat pump is used

without any supplemental electric heating.

*(3) 100 percent of the nameplate rating(s) 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 supplementary heat, it does

not need to be added to the supplementary heat for the total central space heating load.

supplemental electric heating for central electric space-heating systems. If the heat pump

compressor is prevented from operating at the same time as the supplementary heat, it does

not need to be added to the supplementary heat for the total central space heating load.

*(4) 65 percent of the nameplate rating(s) of electric space heating if less than four separately*

controlled units.controlled units.

*(5) 40 percent of the nameplate rating(s) of electric space heating if four or more separately*

controlled units.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.systems where the usual load is expected to be continuous at the full nameplate value.

Typically the competition is between the air-conditioner at 100% versus central heat at 65%...

**(1)**100% of the nameplate rating(s) of the air conditioning and cooling, including heat pump

compressors.

**(2)**65% of the nameplate rating(s) of the central electric space heating including integral

supplemental heating in heat pumps.

Let’s calculate the following air-conditioning and heating loads using the optional method...

**Air-Conditioner (5 horsepower) vs. Central Heat (9.5 kW)**

Use The Largest Of...

*230.79(C) For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire.*

Main Breaker Size

**= (230.79(C)) 106.2A x .83 = 88.1A (100 amps) (240.6(A))**

Feeder Size

**(Table 310.15(B)(7)) = #3 THW**

Residential Services (Optional Method) Sample Problem

What are the service requirements for a 230/115 volt, single-family dwelling with the following loads...