Branch-Circuit, Feeder and Service Calculations, Part XVI

by Charles R. Miller
Published: June 2007

Article 220 – Load Calculations

220.52 Small Appliance and Laundry Loads—Dwelling Unit

The National Electrical Code (NEC) contains an introduction, nine chapters and annexes. An outline showing Code arrangement is in 90.3. Chapters 1 through 4 apply generally; Chapters 5 though 7 apply to special occupancies, special equipment or other special conditions. Chapters 5 though 7 supplement or modify the general rules in the first four chapters. Chapters 1 through 4 apply except as amended by Chapters 5, 6 and 7 for the particular conditions. For the most part, Chapter 8 is an article unto itself. It covers communications systems and is not subject to the requirements of Chapters 1 through 7 except where the requirements are specifically referenced in Chapter 8. Chapter 9 consists of tables and is followed by annexes.

Annexes are not part of the requirements of the NEC, but are included for informational purposes. Annex C and D can be especially helpful. Annex C contains conduit and tubing fill tables for conductors and fixture wires of the same size. Tables 4 and 5 in Chapter 9 can be used to determine the maximum number of conductors permitted in raceways or to determine the minimum size raceway needed for certain conductors. If all the conductors are the same size and same type of insulation, simply look at Annex C. No calculation is necessary. Annex D is a great resource because it contains examples of calculation methods from the Code.

Load-calculation requirements are in article 220 of the National Electrical Code. Last month’s Code in Focus concluded by covering demand factors for nondwelling receptacle loads in 220.44 and Table 220.44. This month, the topic continues with motor loads as specified in 220.50.

Motor-load calculations are referenced twice in part II of Article 220. Both of these references contain requirements for calculating branch circuits supplying motor loads. Part III, titled “Feeder and Service Load Calculations,” also contains requirements for motor loads. Motor loads shall be calculated in accordance with 430.24, 430.25 and 430.26 and with 440.6 for hermetic refrigerant motor compressors [220.50].Unless calculating the motor overload protection (heaters and overloads), do not use the actual current rating marked on the nameplate. When calculating motor loads, use the values given in Tables 430.247 through 430.250. The values given in those tables shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate [430.6(A)(1)]. This provision does not apply to motors built for low speeds (less than 1,200 rpm) or high torques for multispeed motors. Separate motor overload protection shall be based on the motor nameplate current rating [430.6(A)(1)] (see Figure 1).

Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads [430.24]. When performing load calculations for feeders and services, if there is more than one motor, start by finding the full-load current of each motor. Next, multiply the highest full-load current by 125 percent. Finally, add the full-load currents of the other motor(s) into the calculation. For example, what is the minimum rating in amperes for conductors supplying a 5 hp, 230-volt, 3-phase motor; a 10 hp, 230-volt, 3-phase motor; a 15 hp, 230-volt, 3-phase motor; and a 10 hp, 230-volt, single-phase motor? Full-load currents for 3-phase motors are in Table 430.250, and full-load currents for single-phase motors are in 430.248. The full-load current for a 5 hp, 230-volt, 3-phase motor is 15.2 amperes; the full-load current for a 10 hp, 230-volt, 3-phase motor is 28 amperes; the full-load current for a 15 hp, 230-volt, 3-phase motor is 42 amperes; and the full-load current for a 10 hp, 230-volt, single-phase motor is 50 amperes (see Figure 2).

Although the 15 hp motor has the largest amount of horsepower, the motor with the highest full-load current rating is the single-phase motor. Multiply 50 amperes by 125 percent (50 × 125% = 62.5). Now add to this value to the full-load currents of the other motors (62.5 + 15.2 + 28 + 42 = 147.7 = 148). The minimum rating in amperes for conductors supplying these motors is 148 amperes (see Figure 3).

As specified in 430.24, conductors supplying two or more motors must have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group or on the same phase. It may not be necessary to include all the motors into the calculation. It is permissible to balance the motors as evenly as possible between phases before performing motor-load calculations. For example, what is the minimum rating in amperes for conductors supplying a 10 hp, 208-volt, 3-phase motor and three 3 hp, 120-volt, single-phase motors? First, find the full-load currents for the motors. The full-load current for a 10 hp, 208-volt, 3-phase motor is 30.8 amperes [Table 430.250]. Full-load currents for single-phase motors are in 430.248. Note that the currents listed in the 115-volt column are permitted for system voltage ranges of 110 to 120 volts. The full-load current for a 3 hp, 120-volt motor is 34 amperes. Next, balance the motors as evenly as possible between phases. Connect the 3-phase motor to each of three ungrounded (hot) conductors. Because the 3 hp motors are 120-volts, connect each motor to the grounded conductor and one ungrounded conductor. One motor will be on phase A, one motor on B and one on phase C. Although there are four motors total, there are only two motors on each phase (see Figure 4).

Because the motors are balanced between phases, the full-load current on each phase is 64.8 amperes (30.8 + 34 = 88.8). Multiply 34 amperes by 125 percent (34 × 125% = 42.5). Now add to this value the full-load currents of the other motor on the same phase (42.5 + 30.8 = 73.3 = 73). The minimum rating in amperes for conductors supplying these motors is 73 amperes (see Figure 5).

The next motor reference in 220.50 pertains to multimotor and combination-load equipment. The ampacity of the conductors supplying multimotor and combination-load equipment shall not be less than the minimum circuit ampacity marked on the equipment in accordance with 430.7(D). Where the equipment is not factory-wired and the individual nameplates are visible in accordance with 430.7(D)(2), the conductor ampacity shall be determined in accordance with 430.24 [430.25]. Hermetic refrigerant motor compressor loads on feeders, and services must be calculated in accordance with 440.6.

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.