Section 3: Load Planning Bill Brown, P.E., Square D Engineering Services

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1 Section 3: Load Planning Bill Brown, P.E., Square D Engineering Services Basic Principles The most vital, but often the last to be acquired, pieces of information for power system design are the load details. An important concept in load planning is that due to non-coincident timing, some equipment operating at less than rated load, and some equipment operating intermittently rather than continuously, the total demand upon the power source is always less than the total connected load [1]. This concept is known as load diversity. The following standard definitions are given in [1] and [2] and are tools to quantify it: Demand: The electric load at the receiving terminals averaged over a specified demand interval. of time, usually 15 min., 30 min., or 1 hour based upon the particular utility s demand interval. Demand may be expressed in amperes, kiloamperes, kilowatts, kilovars, or kilovoltamperes. Demand Interval: The period over which the load is averaged, usually 15 min., 30 min., or 1 hour. Peak Load: The maximum load consumed or produced by a group of units in a stated period of time. It may be the maximum instantaneous load or the maximum average load over a designated period of time. Maximum Demand: The greatest of all demands that have occurred during a specified period of time such as one-quarter, one-half, or one hour. For utility billing purposes the period of time is generally one month. Coincident Demand: Any demand that occurs simultaneously with any other demand. Demand Factor: The ratio of the maximum coincident demand of a system, or part of a system, to the total connected load of the system, or part of the system, under consideration, i.e., (3-1) Diversity Factor: The ratio of the sum of the individual maximum demands of the various subdivisions of a system to the maximum demand of the whole system, i.e., (3-2) where D i D G = maximum demand of load i, regardless of time of occurrence. = coincident maximum demand of the group of n loads. Using (1), the relationship between the diversity factor and the demand factor is (3-3) where TCL i DF i = total connected load of load group i = the demand factor of load group i 1

2 Load Factor: The ratio of the average load over a designated period of time to the peak load occurring in that period, i.e., (3-4) If T is the designated period of time, an alternate formula for the load factor may be obtained by manipulating (3-4) as follows: (3-5) These quantities must be used with each type of load to develop a realistic picture of the actual load requirements if the economical sizing of equipment is to be achieved. Further, they are important to the utility rate structure (and thus the utility bill). As stated in [2], the following must be taken into account in this process: Load Development/Build-Up Schedule Peak load requirements, temporary/construction power requirements, and timing Load Profile Load magnitude and power factor variations expected during low-load, average load, and peak load conditions Expected Daily and Annual Load Factor Large motor starting requirements Special or unusual loads such as resistance welding, arc welding, induction melting, induction heating, etc. Harmonic-generating loads such as variable-frequency drives, arc discharge lighting, etc. Forecasted load growth over time Reference [4] and individual engineering experience on previous projects are both useful in determining demand factors for different types of loads. In addition, the National Electrical Code [3] gives minimum requirements for the computation of branch circuit, feeder, and service loads. NEC Basic branch circuit requirements NEC [3] Article 220 gives the basic requirements for load calculations for branch circuits, feeders, and services. In order to understand these requirements, the basic NEC definitions of branch circuit, feeder, and service must be understood, along with several other key terms: Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s). Feeder: All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. Service: The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served. Outlet: The point on the wiring system at which current is taken to supply utilization equipment. 2

3 Receptacle: A receptacle 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. Continuous Load: A load where the maximum current is expected to continue for three hours or more. The NEC definition of Demand Factor is essentially the same as given above. Minimum lighting load (Article ): Minimum lighting load must not be less than as specified in table 3-1 (NEC Table ): Table 3-1: General lighting loads by occupancy (NEC [3] table ) Type of Occupancy a See NEC Article (J) b See NEC Article (K) Unit Load Volt-Amperes Per quare Meter Unit Load Volt-Amperes per Square Foot Armories and auditoriums 11 1 Banks 39 b 3.5 b Barber shops and beauty parlors 33 3 Churches 11 1 Clubs 22 2 Court Rooms 22 2 Dwelling Units a 33 3 Garages commercial (storage) Hospitals 22 2 Hotels and motels, including apartment houses without provision 22 2 for cooking by tenants a Industrial commercial (loft) buildings 22 2 Lodge rooms Office buildings 39 b 3.5 b Restaurants 22 2 Schools 33 3 Stores 33 3 Warehouses (storage) In any of the preceding occupancies except one- family dwellings and individual dwelling units of two-family and multi-family dwellings: Assembly halls and auditoriums 11 1 Halls, corridors, closets, stairways Storage Spaces Motor Loads (Article (C)): Motor loads must be calculated in accordance with Articles , , and 440.6, summarized as follows:): The full load current rating for a single motor used in a continuous duty application is 125% of the motor s full-load current rating as determined by Article 430.6, which refers to horsepower/ampacity tables , , , or as appropriate (Article ). The load calculation for several motors, or a motor(s) and other loads, is 125% of the full load current rating of the highest rated motor per a.) above plus the sum of the full-load current ratings of all the other motors in the group, plus the ampacity required for the other loads (Article ). 3

4 For hermetic refrigerant motor compressors or multi-motor equipment employed as part of air conditioning or refrigerating equipment, the equipment nameplate rated load current should be used instead of the motor horsepower rating (Article 440.6). Luminaires (lighting fixtures) (Article (D)): An outlet supplying luminaire(s) shall be calculated based upon the maximum volt-ampere rating of the equipment and lamps for which the luminaire(s) is rated. Heavy-Duty Lampholders (Article (E)): Loads f for heavy-duty lampholders must be calculated at a minimum of 600 volt-amperes. Sign and outline lighting (Article (F)): Sign and outline lighting loads shall be calculated at a minimum of 1200 volt-amperes for each required branch circuit specified in article 600.5(A). Show windows (Article (G)): Show windows can be calculated in accordance with either: The unit load per outlet as required in other provisions of article volt-amperes per 300mm (1ft.) of show window. Loads for fixed multioutlet assemblies in other than dwelling units or the guest rooms and guest suites of hotels or motels must be calculated as follows (Article (H)): Where appliances are unlikely to be used simultaneously, each 1.5m (5 ft.) or fraction thereof of each separate and continuous length must be considered as one outlet of 180 volt-amperes. Where appliances are likely to be used simultaneously, each 300mm (1 ft.) or fraction thereof must be considered as an outlet of 180 volt-amperes. Receptacle outlets (Articles (I), (J), (K), (L)): Loads for these are calculated as follows: Dwelling occupancies (Article (J)): In one-family, two-family, and multifamily dwellings and in guest rooms or guest suites of hotels and motels, general-use receptacle outlets of 20A rating or less are included in the general lighting load per above. No additional load calculations are required for these. Banks and office buildings (Article (K)): Receptacle outlets must be calculated to be the larger of either the calculated value per c.) below or 11 volt-amperes/square meter (1 volt-ampere per square ft.). All other receptacle outlets (Article (I)): Each receptacle on one yoke must be calculated as 180 volt-amperes. A multiple receptacle consisting of four or more receptacles must be calculated at 90 volt-amperes per receptacle. Sufficient branch circuits must be incorporated into the system design to serve the loads per Article (summarized 1.) 8.) above), along with branch circuits for any specific loads not covered in Article The total number of branch circuits must be determined from the calculated load and the size or rating of the branch circuits used. The load must be evenly proportioned among the branch circuits (Article (C)). In addition, Article (C) requires several dedicated branch circuits as follows for dwelling units: Two or more 20A small-appliance branch circuits (Article (C)(1)). One or more 20A laundry branch circuits (Article (C)(2)). One or more bathroom branch circuits (Article (C)(3)). Continuous Loads (Article ): The rating of the overcurrent protection for a branch circuit must be at least the sum of the non-continuous load +125% of the continuous load unless the overcurrent device is 100%-rated. Because the rating of the overcurrent protection determines the rating of the branch circuit (Article 210.3), the branch circuit must be sized for the non-continuous load +125% of the continuous load. In load calculations, continuous loads should therefore be multiplied by 1.25 unless the circuit overcurrent device is 100% rated. Note that motor loads are not included in this calculation as the 125% factor is already included in the applicable sizing per above. 4

5 NEC Basic Feeder Circuit Sizing Requirements Once the branch circuit loads are calculated, the feeder circuit loads may be calculated by applying demand factors to the branch circuit loads. General Lighting Loads (Article ): The feeder general lighting load can be calculated by multiplying the branch circuit general lighting load calculated per B.) 1.) above, for those branch circuits supplied by the feeder, by a demand factor per table 3-2 (NEC table ). Table 3-2: Lighting load feeder demand factors (NEC [3] table ) Type of Occupancy Dwelling units Hospitals* Hotels and motels, including apartment houses without provision for cooking by tenants* Warehouses (storage) Portion of Lighting Load to Which Demand Factor Applies (Volt-Amperes) First 3,000 or less at From 3,001 to 120,000 at Remainder over 120,000 at First 50,000 or less at Remainder over 50,000 at First 20,000 or less at From 20,001 to 100,000 at Remainder over 100,000 at First 12,500 or less at Remainder over 12,500 at Demand Factor (Percent) All others Total volt-amperes 100 * The demand factors of this table shall not apply to the calculated load of feeders or services supplying areas in hospitals, hotels, and motels where the entire lighting is likely to be used at one time, as in operating rooms, ballrooms, or dining rooms. Show window or track lighting (Article ): Show windows must use a calculated value of 660 voltamperes per linear meter (200 volt-amperes per linear foot), measured horizontally along its base. Track lighting in other than dwelling units must be calculated at an 150 volt-amperes per 660mm (2 ft.) of lighting track or fraction thereof. Receptacles in other than dwelling units (Article ): Demand factors for non-dwelling receptacle loads are given in table 3-3 (NEC table ). Table 3-3: Demand factors for non-dwelling receptacle loads (NEC [3] table ) Portion of Receptacle Load to Which Demand Factor Applies (Volt-Amperes) Demand Factor (Percent) First 10 kva or less at 100 Remainder over 10 kva 50 Motors (Article ): The feeder demands for these are calculated as follows: The load calculation for several motors, or a motor(s) and other loads, is 125% of the full load current rating of the highest rated motor per II.) B.) ii.) above plus the sum of the full-load current ratings of all the other motors in the group, plus the ampacity required for the other loads (Article ). The load calculation for factory-wired multimotor and combination-load equipment should be based upon the minimum circuit ampacity marked on the equipment (Article ) instead of the motor horsepower rating. Where allowed by the Authority Having Jurisdiction, feeder demand factors may be applied based upon the duty cycles of the motors. No demand factors are given in the NEC for this situation. 5

6 Fixed Electric Space Heating (Article ): The feeder loads for these must be calculated at 100% of the connected load. Noncoincident Loads (Article ): Where it is unlikely that two or more noncoincident loads will be in use simultaneously, it is permissible to use only the largest loads that will be used at one time to be used in calculating the feeder demand. Feeder neutral load (Article ): The feeder neutral load is defined as the maximum load imbalance on the feeder. The maximum load imbalance for three-phase four-wire systems is the maximum net calculated load between the neutral and any one ungrounded conductor. A demand factor of 70% may be applied to this calculated load imbalance. Refer to NEC article for neutral reductions in systems other than three-phase, four-wire systems. This demand factor does not apply to non-linear loads; in fact, it may be necessary to oversize the neutral due to current flow from non-linear load triplen harmonics. Continuous Loads (Article 215.3): The rating of the overcurrent protection for a feeder circuit must be at least the sum of the non-continuous load +125% of the continuous load, unless the overcurrent device is 100%-rated. Because the rating of the overcurrent protection determines the rating of the branch circuit (Article 210.3), the branch circuit must be sized for the non-continuous load +125% of the continuous load. In the final feeder circuit load calculation, the continuous portion of the load should therefore be multiplied by 1.25 unless the overcurrent device for the circuit is 100%-rated. Note that motor loads are not included in this calculation as the 125% factor is already included in the applicable sizing per above. Additional calculation data is given in NEC Article 220 for dwelling units, restaurants, schools, and farms. This data is not repeated here. Refer to NEC Article 220 for details. As this guide only presents the basic NEC requirements for load calculations, it is imperative to refer to the NEC itself when in doubt about a specific load sizing application. Computer programs are commercially available to automate the calculation of feeder and branch circuit loads per the NEC methodology described above. References Because the subject matter for this section is basic and general to the subject of electrical engineering, it is included in most undergraduate textbooks on basic circuit analysis and electric machines. Where material is considered so basic as to be axiomatic no attempt has been made to cite a particular source for it. For material not covered per the above, references specifically cited in this section are: [1] [2] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Standard , December Turan Gonen, Electric Power Distribution System Design, New York: McGraw-Hill, 1986, pp [3] The National Electrical Code, NFPA 70, The National Fire Protection Association, Inc., 2005 Edition. [4] IEEE Recommended Practice for Electric Power Systems in Commercial Buildings, IEEE Standard , December