Electrical Design of Commercial and Industrial Buildings – Chapter 2

Chapter 2: General Electrical Requirements

Overview

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Introduction

Almost every electrical design includes general electrical requirements, such as those for general purpose receptacle outlets (see FIGURE 2-1). Although these 120-volt receptacles (also known as convenience outlets) typically do not serve any specific type of equipment, they do serve a very important part in the overall design by providing power for general purpose devices and appliances.

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Figure 2-1: Typical office spaces require one general purpose receptacle outlet every 10 to 12 ft

Determining General Purpose Requirements

In commercial and industrial spaces, many 120-volt receptacles may be necessary to accommodate any devices that do not have special electrical requirements. The National Electrical Code (NEC) does not specify the quantity and location of these receptacles in commercial type occupancies, which ultimately depend on customer requirements.

If the end use customer is known during the design phase, the designer can customize the quantity and location of general purpose receptacles to meet the customer’s specific needs. The designer might meet directly with the customer to discuss any specific requirements and alter the design accordingly. If the end use customer is not known, the designer must strive to create a design that provides for the average user’s electrical needs and allows for flexibility for future additions and expansion. In this case, the design generally adheres to NEC requirements for general purpose receptacles in residential buildings [210.52(A)(1)] to provide a sufficient quantity of receptacles and meet any future needs. Though this section of the Code is not required for commercial and industrial buildings, the maximum advised distance between receptacles (12 linear feet) is a helpful guideline (see FIGURE 2-2).

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Figure 2-2: General purpose receptacles should be spaced approximately every 12 linear feet

Because general receptacle outlets are the least complex part of an electrical design, these receptacles are typically the first items designed. Referencing the floor plan, the designer adds general receptacles for general office spaces using the advised distance of 12 linear feet between receptacles. Additionally, the designer adds one receptacle for each restroom and one receptacle for approximately every 20 ft of wall space in hallways and corridors (to accommodate cleaning equipment such as vacuums). The NEC requires one 120-volt receptacle within 25 ft of any heating, air-conditioning, or refrigeration equipment, which may include any roof-mounted air-conditioning equipment [210.63], For any other areas within the facility, the designer should consider how frequently the space is occupied and what the primary usage of each space is to determine the proper quantity and placement of general receptacle outlets.

Determining Branch-Circuit Requirements

After determining the general receptacle requirements, the next step in the design is to determine the proper number of required branch circuits to serve these receptacle loads. The type of circuit used for this purpose is a general purpose branch circuit, a circuit that serves two or more receptacles or outlets for lighting and appliances. Because these receptacles have no specific electrical requirements, the same branch circuit can serve several receptacles without overload.

As per the NEC, each receptacle is valued at 180 VA [220.14(1)]. In general, every ten 120-volt receptacles require one branch circuit at 80 percent of rated capacity. Although 120-volt general purpose receptacles are not considered continuous loads, and the branch circuits that serve them could be rated at 100 percent of their capacity (see Chapter 6), it is good practice to design branch circuits to serve these receptacles at 80 percent of their rated capacity as is advised for overcurrent protective devices, electrical panels, and related equipment that serves continuous loads. This reduces the risk that the branch circuits that serve these receptacles will become overloaded. If the designer does not follow this advice and designs each branch circuit at 100 percent of the load value, then each branch can serve approximately 13 receptacles. For most commercial applications, the 120-volt general purpose receptacle circuits are wired with conductors rated for 20 A and are served by single-pole 20-A overcurrent devices.

Please note that for multioutlet assemblies, such as those permanently fastened in place in plug strips, each 5 continuous linear feet of the assembly shall be rated at 180 VA for nonappliance use; when utilized for appliance use, each 1 continuous linear foot shall be rated at a value of 180 VA [220.14(H)] (see FIGURE 2-3).

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Figure 2-3: The requirements of 220.14(h)(1) and (H)(2) as applied to fixed multioutlet assemblies. NFPA 70®, National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association, Quincy, MA

Balancing the Branch-Circuit Distribution

The branch-circuit layout must be designed for balanced distribution. Balanced distribution means that loads are designed with approximately equal volt-ampere values on each line or hot conductor that serves the panelboard serving the branch circuits. Balanced distribution allows for the best utilization of the equipment serving these loads. When loads are balanced equally among the lines (or phases) that serve them, the total load is distributed equally across the line wires (or phases), allowing the total load to be divided up with less load per line wire (or phase).

In commercial and industrial applications, balanced distribution reflects the daily usage for loads such as lighting that operate consistently during the day. It is more difficult to predict the usage of other loads such as devices operating on the general purpose 120-volt receptacles because it is highly unlikely that each 120-volt receptacle will be serving a load at one time. Even though the usage is inconsistent, these loads should be designed assuming a full-load condition with balanced distribution.

Most commercial and industrial office spaces are designed with a 120/208-volt. 3-phase. 4-wire wye system that allows for the use of multiwire branch circuits to serve the general receptacles. When properly designed, this system provides for balanced distribution and ensures that should any one branch circuit fail, the space(s) served by these multiwire branch circuits would not be left without power to all the receptacles. This 3-phase, 4-wire system uses three ungrounded conductors and one grounded conductor (also called a neutral). Because in this system the voltage from each ungrounded phase conductor to the ground equals 120 volts, it is an ideal system for balanced distribution of general purpose loads. When the ungrounded branch circuit conductors are derived from different phases, the grounded conductor carries only the imbalance of the currents in the phase conductors. This allows for three individual branch circuits—one from each phase—to supply the general purpose loads with one common grounded conductor. Each 120-volt branch circuit should be designed to the same approximate volt-amperage and assigned circuit numbers derived from alternate phases, which balances the loads (see FIGURE 2-4

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Figure 2-4: Multiwire branch circuit
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The design standard is for all loads to be balanced to within 10 percent across each of the line wires (or phases) supplying the loads.

The receptacles in FIGURE 2-4 are served by a multiwire branch circuit. The circuit numbers 2, 4, and 6 are derived from alternate phases that allow for the balancing of the loads. When circuits are designed and wired using this method, each branch circuit can serve 10 receptacles for a total of 30 receptacles in each branch-circuit wiring run (10 for each circuit).

Panel Schedules

To assign and balance the circuits as they will be served from the distribution system, designers use a panel schedule, which is a representation of the actual panelboard that will serve the loads (see FIGURE 2-5). In panel schedules—as with panelboards—all the odd-numbered circuits are on the left and all the even-numbered circuits are on the right. When circuits are assigned horizontally across the panel schedule (e.g., circuits 1 and 2) they are assigned to the same phase. Therefore, if the designer wishes to alternate phases to balance the system, the circuits must be assigned vertically in the panel schedule (e.g., circuits 1, 3, and 5). In a 120/208-volt, 3-phase, 4-wire system, this means assigning the circuits to each of the A, B, and C phases. When multiwire branch circuits are assigned to 120/208-volt systems, they are done so in groups of three odd circuit numbers and then in groups of three even circuit numbers. This grouping method also allows the circuit breakers to be located vertically adjacent to each other in the panelboard.

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Figure 2-5: A 120/208-volt, 3-phase, 4-wire panel schedule

In addition to the branch-circuit numbers, panel schedules can also provide information about the product itself, including:

  • Panel size

  • Panel voltage

  • Quantity of the branch-circuit overcurrent devices

  • Quantity of the devices served by the branch circuits

  • Panel identification

  • Location

  • Type of mounting

  • Voltage

  • Phase

  • Capacity in amperes

  • Type of load served

  • Load of the branch circuits, each phase, and the total load in both volt-amperes and amperage

  • Number of poles of the branch-circuit over-current device serving each of the individual branch circuits

Electrical contractors can submit copies of panel schedules to suppliers for pricing estimates to potentially eliminate the need for them to compile separate material lists of panelboards and their components.

Designating Branch Circuits in the Electrical Plan

To properly reference which branch circuit serves each receptacle on the design plan, the specified branch circuit for each receptacle must be shown on the print at each receptacle location. The designer may also indicate in the design plan the exact routing of the raceways that serve these loads. If the raceways are not shown, then the routing and installation of the raceways are left to the discretion of the installer (see FIGURE 2-6). When designers use a design method where they illustrate both the branch-circuit numbers and the raceway design on the plan, installers must not deviate from the assigned circuits and raceway routing in the field.

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Figure 2-6: Branch-circuit identification for general purpose receptacles
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Should the installer need to deviate from the design plan because of variations and/or changes in the field, a set of “as-built” drawings should be submitted that indicate any changes that were made to the original plan.

When the raceways are indicated in the design, designers identify the number of conductors in each raceway using slash marks (see FIGURE 2-7). This type of design includes how the raceways are installed for each receptacle and reference which raceway, known as the home run, will be installed back to the source. The home run raceway is identified with an arrow symbol and includes notation indicating which branch circuits will be installed in the raceway and which branch-circuit panelboard will serve these branch circuits. In the example in FIGURE 2-4, the home run indicates that four conductors assigned to circuit numbers 2, 4, and 6 are to be run back to panelboard P3.

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Figure 2-7: Branch-circuit identification including raceways, conductors, and home run detail
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Plans that show the raceway layout can be helpful to electrical technicians who need to make changes or repairs after the installation.

Unless the designer specifies the raceway and grounding methods, the installer can choose any raceway method approved by the NEC. Not all raceway methods are approved for use as a grounding method; therefore, an additional grounding conductor must be installed. Because the raceway method used may vary, the illustration of the number of conductors to be installed in a particular raceway typically does not include any references to an additional grounding conductor that may be necessary. The installer makes that determination for the raceway type based on knowledge of wiring practices and the requirements of the NEC.

As mentioned, depending on the raceway method used, installation of an additional equipment grounding conductor may be required. In this case, an additional equipment grounding conductor would not be required and is therefore not shown in the design. The equipment grounding conductor should be illustrated only when the raceway type chosen requires the additional conductor.

When designing with multiwire circuits that will share a common ungrounded conductor, it is good design practice to locate the circuit breakers side by side in a panelboard in a vertical sequence to create balanced distribution. In the 3-phase panelboard, this ensures that each branch circuit is assigned to an alternate phase; in the case of a single-phase distribution system, this method ensures that each branch circuit is assigned to alternate line wires. According to the NEC, at the point where a multiwire branch circuit receives its supply, it must have a means to simultaneously disconnect all ungrounded conductors [210.4(B)].

When more than one branch circuit serves two devices on a single mounting strap (yoke), as in the case with some split duplex receptacles, the NEC requires that circuit breakers serving the device must have their handles tied together [210.7(B)]. This method avoids the possibility of electrical shock during servicing by ensuring that if one branch circuit is disconnected, any other branch circuits serving the same device are also disconnected. Alternating the branch circuits in a panelboard for balanced distribution and aligning the circuit breakers in a vertical position enable multiwire branch circuits to conform to 210.(4)(B) and 210.7(B) and the handles of the circuit breakers to be tied together by an approved method. If the circuit breakers for the multiwire branch circuits were located across from each other in the panelboard, they cannot physically be tied together. Designers must consider conforming to these requirements when designing home runs.

Wrap Up: Master Concepts

  • All commercial and industrial offices have basic electrical equipment requirements that can be served with the general purpose 120-volt branch circuits through general purpose wall-mounted receptacle outlets.

  • General purpose electrical loads have no special requirements; therefore, several receptacle outlets may be served by the same branch circuit. All electrical loads in commercial or industrial buildings must be designed for balanced distribution.

  • Panel schedules (representations of the actual panelboard that provides distribution of the electrical supply) are used during the design phase to simulate how the loads will be distributed. A panel schedule is an excellent tool for achieving design goals to actual field conditions.

  • Each branch circuit serving general receptacle outlets or equipment is identified on an electrical plan at the receptacle or equipment location by its branch-circuit number. This information helps installers and is useful for servicing.

  • Proper identification of the circuit allows installers to properly wire the device to the correct circuit and service technicians to properly identify and de-energize the circuit, providing a greater degree of safety.

Charged Terms

Many of the definitions supplied here are from the NEC.

  • 120/208-volt. 3-phase. 4-wire. wye system A distribution system generated with three individual sine waves separated by 120 electrical degrees that are identified as phases A, B, and C. One leg of each of the three phase coils is electrically connected to the others at a common point, forming a wye, which when grounded, becomes the fourth wire (or neutral) in the system. This allows for each of the three individual phase voltages to supply 120 volts to the grounded point, while the line voltage across each of the phases produces 208 volts. The line-to-line voltages can supply both 208-volt 3-phase and 208-volt single-phase. Because the three individual phases each can supply 120 volts, this system is commonly used in commercial office applications where 120 volts is desired because the 120-volt loads can be balanced across each of the three phases.

  • balanced distribution An electrical distribution system in which the ungrounded conductors carry equal currents. In distribution systems that also include a grounded conductor, the grounded conductor will carry the imbalance of the currents in the ungrounded conductors.

  • branch circuit The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) [100].

  • continuous load A load where the maximum current is expected to continue for three hours or more [100].

  • general purpose branch circuit A branch circuit that supplies two or more receptacles or outlets for lighting and appliances [100].

  • home run The raceway designated on a plan as the one that carries branch-circuit conductors back to the serving source (such as a panelboard).

  • multiwire branch circuit A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system [100].

  • overload Operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, if it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload [100].

  • panel schedule An illustration of key panelboard information showing how branch circuitry is distributed, number of phases, voltage, and size in amperage; panel schedules are completed by hand calculation or by computer software.

  • raceway An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars [100].

  • receptacle outlet An outlet where one or more receptacles are installed [100].

Check Your Knowledge

1. 

In the basic layout for general purpose receptacles, the receptacles should be spaced at a linear distance of approximately_____from each receptacle.

  1. 6 ft

  2. 12 ft

  3. 4 ft

  4. 8 ft

2. 

As per the National Electrical Code, each 120-volt general purpose receptacle installed in a commercial application must be rated at:

  1. 360 VA.

  2. 120 VA.

  3. 180 VA.

  4. 540 VA.

3. 

A 120-volt 20-A branch circuit can serve_____general purpose receptacles.

  1. 2

  2. 10 to 13

  3. 15 to 17

  4. up to 20

4. 

A multioutlet assembly serving general loads has a load value based on _____ VA per five linear feet.

  1. 120

  2. 180

  3. 360

  4. 540

5. 

How many conductors are required for a raceway serving a general purpose, 120-volt multiwire branch circuit with three branch circuits served from a 120/208-volt 3-phase 4-wire system, assuming that the raceway is an approved equipment grounding conductor?

  1. 3

  2. 4

  3. 5

  4. 6

6. 

A 120-volt general purpose receptacle must have the branch circuit that serves the receptacle identified at what point on the design plan?

  1. At the panelboard that serves the branch circuit

  2. At the receptacle location, by circuit number

  3. It is left to the discretion of the designer.

  4. No identification is necessary.

7. 

When referencing an electrical plan, a home run is the:

  1. branch-circuit number that serves the devices.

  2. raceway installed from the serving panelboard to the first device.

  3. raceway connected to the last device in the run.

  4. conduit that contains the feeders that supply a panelboard.

8. 

General purpose 120-volt receptacles provided for roof-mounted equipment must be located within_____of the equipment.

  1. 5 ft

  2. 10 ft

  3. 25 ft

  4. 100 ft

9. 

As-built drawings are utilized to:

  1. differentiate between the original design plan and the completed structure.

  2. illustrate how the building is to be constructed.

  3. provide further details about a project.

  4. provide a reference when designing plans for a project requiring alterations.

10. 

When more than one branch circuit serves two devices on a single mounting strap:

  1. the circuit breakers serving the devices must be identified at the panelboard.

  2. the branch circuits that supply the devices must be identified at the location of the devices.

  3. the circuit breakers serving the devices must be provided with a method to physically join the circuit breaker handles together.

  4. Both A and B

You are the Designer