Published In September 2006
Article 220 C Load Calculations
220.14 Other Loads—All Occupancies
Knowing how to perform load calculations in accordance with the National Electrical Code (NEC) plays a significant role in an electrician’s professional career. Before installing branch circuits, feeders or services on a job, loads must be calculated. Branch-circuit load calculation requirements are in Part II of Article 220.
After calculating branch-circuit loads, conductor sizes and ratings for overcurrent protection must be determined. Results from calculations in Part II of Article 220 are used in conjunction with specifications from 210.19 to size branch-circuit conductors. Sizing of branch-circuit overcurrent protective devices must be done in accordance with 210.20 and Part II of Article 220.
To size feeder (and service) conductors and overcurrent protection, loads must first be calculated in accordance with Part III or Part IV of Article 220. Last month’s column concluded by covering fixed multioutlet assemblies in 220.14(H). This month, the discussion continues with more requirements for general-use receptacles and outlets not used for general illumination.
Load calculations for receptacle outlets are covered in 220.14(I), (J), and (K). A receptacle, as defined in Article 100, is a contact device installed at the outlet for the connection of an attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke (see Figure 1). Sometimes there is confusion pertaining to a single duplex receptacle on a branch circuit with no other devices.
Although a duplex receptacle is installed and mounted by one strap or yoke, it is considered two receptacles. A branch circuit supplying only a duplex receptacle and no other device is not an individual branch circuit. An individual branch circuit, as defined in Article 100, is a branch circuit that supplies only one utilization equipment.
Except for dwelling occupancies and, under certain conditions, banks and office buildings, the calculated load for receptacle outlets is 180 volt-amperes for each single or for each multiple receptacle on one yoke. The load calculation for a single receptacle is 180 volt-amperes. The load calculation for a duplex receptacle is 180 volt-amperes. The load for three receptacles on one yoke or strap is also calculated at 180 volt-amperes (see Figure 2).
To calculate receptacles in accordance with 220.14(I), multiply the number of receptacles by 180 volt-amperes. For example, what is the calculated load for 30 15-ampere duplex receptacles in a retail store? Multiply the number of receptacles by 180 (30 × 180 = 5,400). The minimum calculated load for 30 15-ampere duplex receptacles in a retail store is 5,400 volt-amperes. The calculated load for 20-ampere receptacle outlets is no different than the calculated load for 15-ampere receptacle outlets. For example, what is the calculated load for 30 20-ampere duplex receptacles in a retail store? Although 20-ampere receptacles have a higher rating than 15-ampere receptacles, the calculated load is exactly the same. The minimum calculated load for 30 20-ampere duplex receptacles in a retail store is 5,400 volt-amperes (30 × 180 = 5,400) (see Figure 3).
The calculated load is used to determine the maximum number of receptacles permitted on a branch circuit in all but dwelling occupancies. The ampere rating of the overcurrent protective device is what determines the maximum number of receptacles on a branch circuit. For example, the maximum number of receptacles on a 15-ampere breaker (or fuse), supplied by a nominal source voltage of 120, is 10.
The calculation can be performed either by converting the ampacity rating to volt-amperes or by converting volt-amperes to amperes. Use Ohm’s Law to find amperes when volt-amperes and voltage are known (I = W ÷ E). Divide 180 by 120. The calculated load for one receptacle supplied by 120 volts is 1.5 amperes (180 ÷ 120 = 1.5).
To find the maximum number of receptacles permitted on a 15-ampere breaker, divide the rating of the breaker by 1.5 amperes (15 ÷ 1.5 = 10). The maximum number of receptacles permitted on a 15-ampere, 120-volt breaker is 10 (see Figure 4). Because of provisions in Table 210.21(B)(3) and Table 210.24, 20-ampere receptacles are not permitted on a branch circuit having a rating of 15-amperes.
Because of the higher rating on a 20-ampere breaker, more receptacles are permitted than on 15-ampere overcurrent devices. The calculated load per receptacle is the same, 1.5 amperes. To find the maximum number of receptacles permitted on a 20-ampere breaker, divide the rating of the breaker by 1.5 amperes (20 ÷ 1.5 = 13.3 = 13). The maximum number of receptacles permitted on a 20-ampere, 120-volt breaker is 13 (see Figure 5). In accordance with Tables 210.21(B)(3) and 210.24, these receptacles can be 15-ampere, 20-ampere or any combination thereof.
Although a single receptacle and duplex receptacle do not share the exact same definition, they are counted the same in a load calculation. Unless specifically stated in 220.14(J) and (K), receptacle outlets shall be calculated at not less than 180 volt-amperes for each single or for each multiple receptacle on one yoke [220.14(I)]. For example, what is the branch-circuit load calculation for 30 15-ampere single receptacles in a retail store? The calculated load for 30 15-ampere single receptacles is the same as it would be for 30 15-ampere duplex receptacles, 5,400 volt-amperes (30 × 180 = 5,400) (see Figure 6).
Some companies manufacture a single device containing four receptacles. Since there are four receptacles associated with this single piece of equipment, the load calculation is different. A single piece of equipment consisting of a multiple receptacle composed of four or more receptacles must be calculated at not less than 90 volt-amperes per receptacle [220.14(I)]. For example, what is the load calculation for a quad receptacle manufactured as a single device? Multiply the number of receptacles by 90 volt-amperes (4 × 90 = 360). Because there are four outlets in this single piece of equipment, the calculated load is 360 volt-amperes (see Figure 7).
Two duplex receptacles in the same box and under one double-duplex receptacle cover plate, also has a calculated load of 360 volt-amperes. Not because it is one piece of equipment, but because the receptacle outlets are on two different yokes (2 × 180 = 360). Likewise, two single receptacles in the same box and under one cover must be calculated at 360 volt-amperes.
The last sentence in 220.14(I) states that this load calculation provision does not apply to receptacles on small-appliance and laundry branch circuits in dwelling units. Receptacle outlets of 15- and 20-ampere ratings in dwellings are included in the general lighting-load calculations of 220.12. No additional load calculation is required for such outlets. Next month’s column continues the discussion of load calculations.
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.