Article 220 of the National Electrical Code (NEC) contains requirements for calculating branch-circuit, feeder and service loads. Knowing how to perform load calculations is an essential part of being an electrician. While calculating feeders and services may not be needed as often as branch circuits, it is still important to know how such calculations are performed. Results from calculations in Article 220 must be applied to other sections for a Code-compliant installation. Results can be used in conjunction with provisions in Article 240 to find the minimum size overcurrent protective devices. Results can also be used along with provisions in Article 210 to find minimum sizes of conductors. Last month’s column concluded by covering lighting loads for specified occupancies in 220.12. This month, the discussion continues with requirements for outlets in all occupancies.
Calculation provisions for general lighting are covered in 220.12 and Table 220.12. While 220.12 includes outlets used for general illumination, it does not include calculation provisions for general-use receptacles. In all occupancies, the minimum load for each outlet for general-use receptacles shall not be less than that calculated in 220.14(A) through (L).
Outlets that do not provide power for general illumination also fall under the provisions in 220.14. Loads shown in this section are based on nominal branch-circuit voltages, which are listed in 220.5(A). Because of an exception to this section, calculating loads for outlets serving switchboards and switching frames in telephone exchanges is not required.
Specific appliances (and loads not covered in the rest of this section) are addressed first in this section. An outlet for a specific appliance or other load not covered in 220.14(B) through (L) must be calculated based on the ampere rating of the appliance or load served. [220.14(A)] The load for an appliance is simply the ampere rating of that appliance. Appliance nameplates contain certain information. Current and voltage ratings may be part of the information provided on the nameplate. If the appliance’s nameplate shows the current in amperes, no calculation is necessary. For example, what is the branch-circuit load for a 10-ampere, 120-volt dishwasher in a dwelling? Since the rating on the nameplate is 10 amperes, the branch-circuit load for this dishwasher is 10 amperes (see Figure 1). Although the appliance in this example is in a dwelling, 220.14(A) pertains to appliances in all occupancies.
An appliance’s nameplate may or may not contain the rating in amperes. Each electrical appliance shall be provided with a nameplate giving the identifying name and the rating in volts and amperes or in volts and watts. [422.60(A)] If volts and watts (or volt-amperes) are provided and amperes are needed, a calculation will be necessary. Use Ohm’s Law to find an unknown factor (or quantity) when there are at least two known factors. Ohm’s Law states that the current in a circuit is equal to the circuit’s voltage divided by the resistance of the circuit. An Ohm’s Law wheel containing 12 formulas is shown in Figure 2. Terms that make up the Ohm’s Law wheel include current (I), voltage (E), resistance (R) and power (W) (see Figure 2).
When choosing a formula from the Ohm’s Law wheel, start with the unknown factor and select the appropriate letter from the inner circle. Next, out of the three formulas for that particular letter in the inner circle, select the formula containing the two known factors. Finally, insert the known factors into the formula and solve for the unknown factor. For example, an appliance has a nameplate rating of 1,200 watts at 120 volts. This appliance’s current rating is not on the nameplate. If current is needed, a calculation is necessary. Since the unknown factor is current, find the symbol “I.” The lower left quadrant of the Ohm’s Law wheel contains three formulas to solve for amperes. In this example, the two known factors are watts and volts. Next, select the formula containing “W” for watts and “E” for volts. To find amperes, divide watts by volts (I = W ÷ E). The appliance in this example draws 10 amperes (1,200 ÷ 120 = 10) (see Figure 3).
Two alternative wheels may be easier to remember than one Ohm’s Law wheel. “PIE” and “EIR” charts are shown in Figure 4. Note the symbol “W” for watts has been replaced by the symbol “P” for power. This makes it easier to memorize it as the “PIE” chart (see Figure 4). Most unknown factors can be calculated by using one or a combination of both charts.
When using the “PIE” and “EIR” charts, start by selecting the chart containing both known factors and the unknown factor. Find the formula by covering the unknown factor. If the two remaining factors are side by side, multiply. If one known factor is on top and one on the bottom, divide the top known factor by the bottom known factor. For example, volt-amperes (VA) are needed for an appliance in a service calculation.
The appliance has a nameplate rating of 120 volts and 10 amperes. Because there is no wattage or volt-ampere rating on the nameplate, a calculation is necessary. Since the two known factors are volts and amperes and the unknown factor is volt-amperes, select the “PIE” chart. Next, cover the “P” because it is the unknown factor. Because “I” and “E” are side by side, multiply the two known factors (volts and amperes.) The power required for the appliance in this example is 1,200 watts (120 x 10 = 1,200) (see Figure 5).
Calculating appliance loads in accordance with 220.14(A) is the first step in determining ratings of branch-circuits, feeders, and services. Knowing which other sections of the NEC to apply is as important as knowing how to calculate the load. Appliance branch circuits must not only be installed in accordance with Article 210, but also with Article 422. Article 422 covers electric appliances used in any occupancy. While some of the requirements are general appliance requirements, other requirements in Article 422 pertain to specific types of appliances. Before installing an appliance, search for all applicable requirements.
Next month’s Code in Focus column continues the discussion of load calculations. EC
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.