Two sections in the National Electrical Code (NEC) provide methods of calculating loads for existing installations. One deals exclusively with existing dwelling units, while the other deals with any existing dwellings, commercial and industrial installations. They are fairly easy to use and understand, assuming the inspector, engineer, electrician or electrical contractor is familiar with these feeder or service applications.
As a design/build EC in the 1970s and ’80s, I used these methods to save my customers money while providing safe installations. Both of these sections are in Part IV of Article 220, providing optional feeder and service load calculations. Careful study and familiarity of these sections can help you and your customer add new loads without enlarging the feeder or service.
Section 220.83 applies to existing dwelling units with 120/240-volt (V) three-wire, or 208Y/120V three-wire services and is used to determine if the existing service or feeder is of sufficient capacity to add loads. There are two subsections: (A) where additional air conditioning or electrical space heating equipment is not to be installed and (B) where additional air conditioning or electrical space heating equipment will be installed or will be replaced with larger units.
Let’s start with (A), where the following percentages shall be used for existing and new loads but not air conditioning or electric heating loads: 100 percent of the first 8 kilovolt-amperes (kVA) of load and the remainder of the existing and new loads at 40 percent. The load calculations must include the following: (1) general lighting and general-use receptacles at 3 volt-amperes (VA) per square foot or area, as determined by 220.12; (2) 1,500 VA for each two-wire small-appliance branch circuit and each laundry branch circuit covered in 210.11(C)(1) and (C)(2); and (3) the nameplate rating of all appliances fastened in place, permanently connected, or located on a specific circuit, ranges, wall-mounted ovens, or counter—mounted cooking units, clothes dryers not connected to the laundry branch circuit specified in (2), and water heaters. Take all of these that are existing loads, add any new loads, and multiply 100 percent for the first 8 kVA and the remainder at 40 percent.
In (B), where adding air conditioning or electric heating loads, use the following percentages, and then add the larger connected load of air conditioning or space heating. However, do not add both, because these loads would be noncoincident loads (not operating at the same time) as covered in 220.60. The percentages are: 100 percent of any air-conditioning equipment, 100 percent of central electric space heating, or 100 percent of less than four separately controlled space heating equipment, and 100 percent for the first 8 kVA of all other loads and 40 percent of the remainder of all other loads.
These other calculated loads would be the same as the “other loads” in (A) above. The 100 percent calculation for the air conditioning or electric heating load as well as the 100 percent of the first 8 kVA and the 40 percent of the remainder load will provide a large enough calculation for a feeder or service with the diversity of operation of these loads.
Before Section 220.87 was inserted into the 1981 NEC in 220-35, it was first located in the City of Phoenix addendum to the NEC. There have been only minor changes since it was inserted into the NEC, and it has been a proven method for calculating new loads to existing feeders or services.
Section 220.87 consists of three numbered requirements and one exception. It requires the maximum demand data for the feeder or service is available for one year. An exception permits the calculated load, if the demand data is not available for one year, to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the service or feeder, based on the initial loading at the start of the recording. This shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include, by measurement or calculation, the larger of the heating or cooling equipment load and other loads that may be periodic in nature due to seasonal or similar conditions.
Section 220.87 also requires the maximum demand at 125 percent and the new load at or below the ampacity of the feeder or service. The third requirement states that the feeder must have overcurrent protection in accordance with 240.4 and the service must have overload protection based on 230.90.
These two calculation methods can save a lot of money while still maintaining safety.