220.14 Other Loads—All Occupancies
Knowing how to calculate loads in accordance with the National Electrical Code (NEC) is a fundamental function of being an electrician. Requirements for calculating branch-circuit, feeder and service loads are in Article 220. Article 220 is divided into five parts. General requirements are in Part I. Part II covers branch-circuit load calculation methods. Calculation methods for feeders and services are covered in Part III and IV, and calculation methods for farms are covered in Part V.
Knowing how to perform calculations in Part II is essential in determining branch-circuit conductors and branch-circuit overcurrent protection. Branch-circuit load calculation procedures are also necessary when calculating feeders and services. Last month’s column concluded by covering specific appliance loads for all occupancies in 220.14(A). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination.
Although clothes dryers, ranges, cooking appliances and outlets for motor loads are listed in 220.14, the actual calculation procedures are not specified here. Calculation procedures for these types of branch-circuit loads are specified elsewhere.
This section does provide references to the sections containing the stipulations. Load calculation procedures for electric clothes dryers are in 220.54. Load calculations are permitted as specified in 220.55 for electric ranges and other cooking appliances [220.14(B)]. Load calculation procedures for motor loads must be calculated in accordance with the requirements in 430.22, 430.24 and 440.6 [220.14(C)].
Understanding terms in the NEC is an essential element in reading and comprehending it. Article 100 contains selected definitions that are vital to the proper application of the Code. Although it does not include commonly defined general terms or commonly defined technical terms from related codes and standards, it does include terms that are used in two or more articles.
Other terms are defined in the article in which they appear but may be referenced in Article 100. The term “luminaire” was added to Article 100 in the 2002 edition. Article 100 defines luminaire as a complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and ballast (where applicable), and to connect the lamps to the power supply (see Figure 1).
Please note that “luminaire” is the international term for “lighting fixture.” Throughout the Code, luminaire is the main term and “fixture” or “lighting fixture” follows in parentheses. However, for our purposes here, “lighting fixture” has been omitted.
Luminaires that are not part of the general lighting loads, as specified in 220.12, must be calculated in accordance with 220.14(D). An outlet supplying luminaire(s) must be calculated based on the maximum volt-ampere rating of the equipment and lamps for which the luminaire(s) is rated (see Figure 2). Prior to the 2005 NEC, only recessed luminaires were referenced in the section between motor loads and heavy-duty lampholders.
Outlets supplying other types of luminaires (surface, suspended, etc.) were not specifically covered, and therefore had to be calculated in accordance with 220.3(B)(11). Previously, load calculations for other types of luminaires were based on 180 volt-amperes per outlet. Now, regardless of the luminaire type, branch-circuit load calculations must be based on maximum volt-ampere rating of the equipment and lamps for which the luminaire(s) is rated.
Luminaires are marked with a maximum wattage for the lamps. Even if the electrical design calls for lamps rated less than the marked maximum wattage (or volt-amperes), the maximum volt-ampere must be inserted into the load calculation.
For example, branch-circuit loads are needed for recessed luminaires in a store. The maximum rating marked on each luminaire is 150 watts. The rating of the installed lamp in each fixture is only 100 watts. Although 100-watt lamps will be installed, the load calculation must be based on the maximum wattage or volt-amperes. Each of these recessed luminaires must be calculated at 150 volt-amperes (see Figure 3).
If the calculation for the branch-circuit load were based on the installed lamp rating instead of the maximum volt-ampere rating, there could be a problem in the future.
For example, the branch-circuit load for recessed luminaires in a store was calculated incorrectly. The loads were based on the installed lamp and not the maximum rating marked on the luminaire. While the original installation called for 100-watt lamps, the maximum rating marked on each luminaire was 150 watts at 120 volts.
The incorrect calculation was as follows: each luminaire was calculated at 100 watts (volt-amperes). Because the maximum current of the luminaires was expected to continue for three hours or more, the load was considered continuous. The load of each luminaire was multiplied by 125 percent (100 x 125 percent = 125 volt-amperes).
The recessed luminaires were fed from 20-ampere circuits. A 20-ampere, 120-volt circuit is good for 2,400 volt-amperes. Nineteen luminaires were installed on each branch circuit (2,400 ÷ 125 = 19, see Figure 4). A year after the recessed luminaires were installed, in the previous example, the lamps were replaced. Instead of relamping with 100-watt lamps, 150-watt lamps were installed. In the original installation, the volt-amperes of the installed lamps totaled 1,900 volt-amperes (100 x 19 = 1,900). The current on the branch circuit was 15.8 amperes (1,900 ÷ 120 = 15.8). Because the load was incorrectly calculated, the branch circuit is now overloaded. With the new lamps installed, the load on the circuit is now 2,850 volt-amperes (150 x 19 = 2,850).
The current on the branch circuit is now 23.8 amperes (2,850 ÷ 120 = 23.8). With the new lamps, the circuit is now overloaded (see Figure 5). Note, these last two examples were not real installations; they were created to illustrate the potential hazards of not following the specified calculation procedures in Article 220.
The correct calculation for luminaires with a maximum rating of 150 watts is as follows: multiply the maximum rating of each luminaire by 125 percent (150 x 125 percent = 188 volt-amperes).
The recessed luminaires were fed from 20-ampere circuits. A 20-ampere, 120-volt circuit is good for 2,400 volt-amperes. A maximum of 12 luminaires (fixtures) should have been installed on each branch circuit (2,400 ÷ 188 = 12, see Figure 6).
As a general rule in dwelling units, all luminaires are included in the general lighting load of 3 volt-amperes per square foot as specified in Table 220.12. Therefore, it is not necessary to calculate additional loads for luminaires in dwelling units.
Next month’s column continues the discussion of load calculations. EC
(Figures available upon request)
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 NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com, or www.charlesRmiller.com.