EET 323 – Electrical System Design Lecture 1: Introduction to Electrical System Design Radian Belu, PhD

Lecture Objectives Determine the estimated demand load for a specific branch, feeder, or service Apply the demand factors to account for load diversity Understand and comply to the minimum NEC load requirements Determine the demand load for 1-phas and 3-phase motors Determine the demand load for household electric ranges, cook-tops, and ovens Determine the demand load for household electric dryers Determine service entrance ampacity requirements

Minimum Estimated Demand Load Accurate estimates of the demand load on a particular branch circuit, feeder, or service are necessary to determine the required rate of these circuits. Actual number of branch circuits and feeders is also based on the determination of the estimated demand load. The required rules for determining the minimum estimated demand load for branch circuits, feeders, and services are discussed in the Article 200 of the NEC; The basic rule is that is that the branch circuit, feeder, or service must have a rating sufficient to supply the demand load to be served. The demand load on a circuit is equal to the minimum estimated load as determined by applying the NEC minimums, or the actual load, whichever is greater.

General Lighting Load

Area (sq ft) = 50 ft x 100 ft = 5000 sq ft The total lightning load from the NEC is: The total lightning load based on the connected equipment is:

Notes: 1) Table 220.3(A) applies to general lightning loads. 2) The load from general-purpose receptacle outlets in habitable areas of dwelling units, motels is permitted to be included with general lighting load for the purpose of load estimate. 3) For office building and banks, an additional load of 1 VA per square foot shall be included if the actual number of general-purpose receptacles is unknown. The designer may not know at the time of design what the final layout will be, so this extra provision for load must be included.

1) Section 220.3(B) of the NEC requires a minimum of 180 VA be assigned to other general-use receptacle outlets that are not located in habitable areas or connected to a dedicated branch circuit. The 180 VA applies to a device located on a single yoke or mounting strap. 2) The estimated load is usually greater than the required NEC minimum of 180 VA. 3) Use common sense and judgment when estimating loads on circuits where you suspect large loads might be connected.

1) Tables 430-147, 430-188, 430-149, and 430-150 (see Chapter 13, for details) can be used for load estimated purpose of the electric motors (the full load current). 2) In addition, for feeders or branch circuits supplying multiple motor loads, a 25% of the load of the largest motor in the group must be added to the total motor load.

Load Estimation (cnt.) For the purpose of load estimation, the electric ranges are considered individually. A counter-mounted cook-top and no more than two –wall-mounted ovens (all supplied from the same branch circuit) may be considered as a single unit for the purpose of load estimate (the ratings of units are added to determine the equivalent rating). The demand load for household electric cooking appliances is taken from Table 220.19 of the NEC. For household electric ranges (or equivalent cook-top/oven) individually rated at and not over 12 kW, the estimated demand may be determined form column A of Table 220.19. If the range (or equivalent cook-top/oven) has a rating over 12 kW, but not exceeding 27 kW, the demand from column A must be increased by 5% for each kW in excess of 12 kW.

Electric Clothes Dryers Section 220.18 of the NEC requires that the load demand for electric household clothes dryers be computed as being = 5000 VA or the nameplate rating, whichever larger. The branch circuit supplying the dryer must have a rating sufficient to supply the demand load of the dryer. Demand factors for more than one dryer connected to a given feeder or service are in Table 220.18 of the NEC.

The concept of load diversity: Adding up the individual demand loads from the riser diagram of a facility will result in a total connected load apparent power (VA). However, the actual demand load will be less than the total connected load due to the fact that not all loads will be ON at the same time. For example considering the simple Riser diagram in Figure 3-1

Total connected load is: 47.673 kVA Figure 3-1: Let assume that this is a riser diagram for a 2000-sq ft residence. There are 4 circuits for lighting and receptacles, 4 circuits for dedicated small-appliance branch circuits, and one dedicated laundry circuit. Dedicated circuits supply the microwave, overhead door operator, electric range, footer sump pump, laundry sump pump, dishwasher, air-conditioning unit, electric dryer, refrigerator, garbage disposal, deep freeze, and electric hot water heater. The ratings of these large appliances are shown on the riser diagram. Lighting is estimated fro table 2203(A); Small-appliance branch circuit load (1500 VA each); Laundry circuit load (1500 VA each); Miscellaneous general-use receptacles estimated at 180 VA each. Total connected load is: 47.673 kVA

a) Lighting load determined from Table 220.3(A) Notice: 1) Demand factors may be applied to portions of the total load due to the diversity of use that typically occurs in a particular occupancy. 2) Demand factors for general lighting loads are given in NEC Tables 220.11 and 220.13 (shown in Table 3-3). Note that the use of these tables is not permitted when determining the required number of branch circuits for general lighting. These demand factors are only used to determine the total load, that typical apply only to services and feeders. 3) The lighting load that may be subject to the demand factors of Table 220.11 is comprised of the following individual loads: a) Lighting load determined from Table 220.3(A) b) Small-appliance branch circuit load (1500 VA each) c) Laundry circuit load (1500 VA each) d) Miscellaneous general-use receptacles that were calculated at 180 VA each

1. Utility rates schedules vary considerably among utilities nationwide, depending on type of generation, geographic, location, size and service voltage of the load being served, customer class, and other factors. 2. All rate schedules include a cost per kWh of energy consumption 3. Other rate schedules may incorporate charges relating to peak demand, power factor, time-of-day usage, interruptible service riders, and so forth.

Rates and Billing Special rates are sometimes offered by utilities for street lighting, water heating, private outdoor lighting, traffic signals, and other types of specialized services. The cost of generation power is directly related to the type of energy source used to drive the prime mover in generating plant. Geographic location of the generation facilities will have a direct impact on the type of prime mover used. Most electric utilities separate the customer into three major billing classes: residential, commercial, and industrial. The ampacity and service voltage rating of the load generally determine in which class the customer would be billed.

Rates and Billing The determination of customer classification is prescribed by rules and regulations usually approved by the state utilities commissions. Within each billing class there may be several rare structures and schedules. A rate schedule is simply the method by which the electric bill is calculated. Customers within the billing class will sometimes have a choice among several different schedules. Rate schedule of your area can be obtained from the local utility company.

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