Maximum Loads:

Last month’s Code in Focus concluded by covering loads for additions to existing installations; this month, the discussion continues with maximum loads as specified in 220.18.

Load calculation requirements are in Article 220 of the National Electrical Code, which provides requirements for calculating branch-circuit, feeder and service loads. The requirements are divided into five parts:

  • Part I covers general requirements for calculation procedures.
  • Part II provides calculation provisions for branch circuits.
  • Part III covers feeder and service calculation requirements—sometimes referred to as the standard method.
  • Part IV specifies optional feeder and service load calculations.
  • Part V designates requirements for the calculation of farm loads.

Results from calculations in Article 220 are used in conjunction with provisions from other articles when sizing conductors and overcurrent protection. For example, results from calculations in Parts III, IV and V of Article 220 are used with the provision in 215.2(A)(1) to find the minimum feeder-circuit conductor size. Likewise, results (from calculations in Article 220) are used with provisions in 215.3 to find the minimum size fuse or breaker permitted for feeders.

As specified in 220.18, the total load must not exceed the rating of the branch circuit. Motors, air-conditioning equipment, motor-operated utilization equipment, inductive lighting and ranges are mentioned in 220.18(A), (B) and (C).

Section 220.18 refers Code users to Article 430 for motor-operated loads and Article 440 for air conditioning and refrigerating equipment. Article 430 covers motors, motor branch-circuit and feeder conductors and their protection, motor-overload protection, motor-control circuits, motor controllers and motor-control centers. [430.1] Where a circuit supplies only air conditioning equipment, refrigeration equipment or both, provisions in Article 440 are applicable. As stated in 440.1, the provisions in Article 440 apply to electric motor-driven air conditioning and refrigerating equipment and to the branch circuits and controllers for such equipment. Article 440 also contains considerations necessary for circuits supplying hermetic refrigerant motor compressors and for any air conditioning or refrigerating equipment that is supplied from a branch circuit that supplies a hermetic refrigerant motor compressor (see Figure 1).

With circuits supplying loads consisting of motor-operated utilization equipment that is fastened in place and has a motor larger than 1/8 horsepower (hp) in combination with other loads, the total calculated load shall be based on 125 percent of the largest motor load plus the sum of the other loads [220.18(A)]. This is not the first reference to a measurement of 125 percent pertaining to motor loads. While there is no measurement of 125 percent mentioned in 220.14(C), it is within the sections referenced therein: Outlets for motor loads must be calculated in accordance with requirements in 430.22, 430.24 and 440.6 [220.14(C)]. While specifications for conductors supplying a single motor are in 430.22, specifications for conductors supplying more than one motor are in 430.24. Section 440.6 pertains to air conditioning and refrigeration equipment.

Conductors that supply a single motor used in a continuous- duty application shall have an ampacity of not less than 125 percent of the motor’s full-load current rating as determined by 430.6(A)(1) [430.22(A)]. When calculating motor loads, use the values given in Tables 430.247 through 250, not the actual current rating marked on the nameplate. Use the tables in the back of Article 430 to determine the ampacity of conductors or ampere ratings of switches, branch-circuit and ground-fault protection. Prior to the 2005 edition, these tables were 430.147 through 430.150.

For example, what is the minimum rating in amperes for conductors supplying a 2 hp, 230-volt, single-phase motor? First, find the full-load current in amperes for the given motor. As shown in Table 430.248, the current for a 2 hp, 230-volt motor is 12 amperes. Next, multiply the full-load current of the motor by 125 percent (12 x 125 percent = 15). The minimum rating in amperes for conductors supplying a 2 hp, 230-volt, single-phase motor is 15 amperes (see Figure 2).

The current rating marked on the motor’s nameplate shall not be used to find branch-circuit conductors; branch-circuit, short-circuit and ground-fault protection; and feeder short-circuit and ground-fault protection. For example, an outlet is needed for a 10 hp, 208-volt, 3-phase motor. The full-load current marked on this motor’s nameplate is 27 amperes. What is the minimum rating in amperes for conductors feeding this motor? Although known, the actual current rating of this motor cannot be used for this calculation. Use the full-load current shown in Table 430.250. The full-load current (from Table 430.250) for a 10 hp, 208-volt, 3-phase motor is 30.8 amperes. Now multiply the full-load current of the motor by 125 percent (30.8 x 125 percent = 38.5.) The minimum rating in amperes for conductors supplying a 10 hp, 208-volt, 3-phase motor is 38.5 amperes (see Figure 3). The current rating of the motor on the nameplate is permissible to use when finding motor overload protection (heaters and overloads).

Additionally, conductors supplying several motors or a motor 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 calculating loads for circuits having more than one motor, start by finding the full-load current of each motor. Next, multiply the largest full-load current by 125 percent. Finally, add the other motor or motors into the calculation. For example, what is the minimum rating in amperes for conductors supplying a 10 hp, 208-volt, 3-phase motor and a 7½ hp, 208-volt, 3-phase motor? The full-load current from Table 430.250 for a 10 hp, 208-volt, 3-phase motor is 30.8 amperes. The full-load current from the same table for a 7½ hp, 208-volt, 3-phase motor is 24.2 amperes. Multiply the highest full-load current by 125 percent (30.8 x 125 percent = 38.5). Now add to this value to the full-load current of the other motor (38.5 + 24.2 = 62.7). The minimum rating in amperes for conductors supplying a 10 hp, 208-volt, 3-phase motor and a 7½ hp, 208-volt, 3-phase motor is 62.7 amperes (see Figure 4).

Where a circuit supplies air conditioning equipment, refrigerating equipment or both, the loads must be calculated in accordance with the provisions in Article 440. Branch-circuit conductor sizing requirements for air conditioning and refrigerating equipment are in Part IV (440.31 through 440.35). Branch-circuit conductors supplying a single motor-compressor must have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater [440.32]. Definitions for branch-circuit selection current and rated-load current are in 440.2.

Conductors supplying one or more motor-compressors with or without an additional load shall have an ampacity not less than the sum of the rated-load or branch-circuit selection current ratings, whichever is larger, of all the motor-compressors plus the full-load currents of the other motors, plus 25 percent of the highest motor or motor-compressor rating in the group [440.33] (see Figure 5).

Next month’s Code in Focus continues the discussion of load calculations.             EC

Miller, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code and NFPA’s Electrical Reference. For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at 615.333-3336, or via e-mail at charles@charlesRmiller.com.