# Branch-Circuit, Feeder and Service Calculations, Part LII

Last month’s Code in Focus concluded by covering optional feeder and service load calculation requirements for one-family dwellings in 220.82. This month, the discussion continues with optional feeder or service load calculations for existing dwelling units as specified in 220.83.
Section 220.83 pertains to an existing dwelling that will be getting some type of additional electrical load. The additional load could be from building a new addition onto an existing dwelling, replacing the existing heating and air conditioning system with a system that draws more volt-amperes (VA), adding a swimming pool, etc. Regardless of the type of load added, it may be permissible to use the calculations in this section to determine if the existing service or feeder is of sufficient capacity to serve additional loads. As the title states, this section only applies to existing one-family dwellings. Optional method calculations for two dwelling units are in 220.85 and calculations for multifamily dwellings are in 220.84. This optional calculation for an existing dwelling can only be used if the dwelling is supplied by a single-phase service. The service can be fed from a 120/240-volt (V) or 208Y/120V system but must be a 3-wire system.

Formulas from 220.83 are divided into two groups. The first formula shall be used for existing and additional new loads where additional air conditioning equipment or electric space-heating equipment is not to be installed. This calculation method is almost identical to the calculation method in 220.82(B). The only difference is the amount of load that is rated at 100 percent. Include all of the existing loads and the new loads when using this formula. For example, an addition will be built onto an existing one-family dwelling; the existing kitchen will also be renovated. The area, as determined by 220.12, of the existing dwelling is 1,500 square feet. The existing dwelling has the following loads: two small-appliance branch circuits; one laundry branch circuit; two attic roof ventilators rated 506 VA each; one motor rated 1,176 VA; a range rated 8,000 VA; a clothes dryer rated 5,000 VA; and a water heater rated 4,500 VA. The service supplying this existing dwelling is 120/240V, single phase, and is rated 100 amperes (A) (see Figure 1).

The addition built onto this dwelling will add 500 square feet (as determined by 220.12) of floor area. Therefore, the total calculated floor area will be 2,000 square feet. The existing range will be replaced by a new 12,000 VA range. The kitchen renovation will also include two additional appliance loads: a dishwasher rated 1,380 VA and a kitchen waste disposer rated 864 VA. Two more small-appliance branch circuits will be installed, thus making a total of four small-appliance branch circuits. No additional air conditioning equipment or electric space-heating equipment will be installed (see Figure 2).

Is this 100A service of sufficient capacity to serve these additional loads? If not, what size service is required?
When calculating an existing dwelling unit, include all the existing loads and all the additional new loads. Start by calculating the general lighting and general--use receptacles at 3 VA per square foot [220.83(A)(1)]. The general lighting and general-use receptacle load is 6,000 VA (2,000 sq. ft. 3 VA per sq. ft. = 6,000). The small-appliance branch circuit and laundry branch circuit load is 7,500 VA (1,500 5 = 7,500). The appliance load, as specified in 220.83(A)(3)(a), is 4,432 VA (506 + 506 + 1,176 + 1,380 + 864 = 4,432). Because of the new range, the range load is 12,000 VA. The clothes dryer load is 5,000 VA and the water heater load is 4,500 VA. Before applying the demand factor, the total for the existing loads and the new loads is 39,432 VA (6,000 + 7,500 + 4,432 + 12,000 + 5,000 + 4,500 = 39,432) (see Figure 3).

In accordance with 220.83(A), calculate the first 8 kVA (8,000 VA) of load at 100 percent and the remainder of load at 40 percent. The first 8,000 VA calculated at 100 percent is 8,000 VA (8,000 100% = 8,000). After deducting 8,000, the remainder is 31,432 VA (39,432 – 8,000 = 31,432). The remainder calculated at 40 percent is 12,573 VA (rounded to the nearest whole number) (31,432 40% = 12,572.8 = 12,573). The total load after applying demand factors is 20,573 VA (12,573 + 8,000 = 20,573). Find the minimum ampere rating by dividing volt-amperes by voltage. The minimum ampere rating required for this service is 86A (20,573 ÷ 240 = 85.7 = 86). Since the service supplying this dwelling is rated 100A, it is of sufficient capacity to serve these additional loads (see Figure 4).

Before the job starts, the homeowners change their mind. They decide to install a central air conditioning system that is rated 7,200 VA. Is this 100A service of sufficient capacity to serve these additional loads plus the central air conditioning unit? If not, what size service is required? Use the second formula, 220.83(B), for existing and additional new loads where additional air conditioning equipment or electric space-heating equipment is to be installed. The calculations in 220.83(A) and (B) are identical except for the demand factors for air conditioning and heating equipment. The larger connected load of air conditioning or space heating, but not both, shall be used. The following loads must be added to the calculation at 100 percent of nameplate rating: air conditioning equipment, central electric space heating, and less than four separately controlled space-heating units. Do not apply the 40 percent demand factor to heating and air conditioning equipment, this equipment must be added to the calculation at 100 percent. Since this existing dwelling unit has already been calculated with everything except the air conditioning system, add the total (100 percent) load of the air conditioning system to the calculated load of 20,573 VA. The total load including the air conditioning equipment is 27,773 VA (20,573 + 7,200 = 27,773). The minimum ampere rating required for this service is 116A (27,773 ÷ 240 = 115.7 = 116). In accordance with 240.6(A), the next standard ampere rating above 116 is 125 amperes. Therefore, the minimum size service for this dwelling is 125 amperes (see Figure 5).

Next month’s Code in Focus continues the discussion of feeder and service load calculations.

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, charles@charlesRmiller.com and www.charlesRmiller.com.