Electrical Design of Commercial and Industrial Buildings – Chapter 3

Chapter 3: Specialized Electrical Requirements

Overview

Image from book

Introduction

Specialized equipment loads (including manufacturing and specialized office equipment such as photocopiers) have more design requirements than do general purpose loads, including design of individual branch circuitry and specialized grounding methods that may be required. Therefore, greater emphasis is placed on the specific details, required calculations, and NEC regulations for these types of loads than for general electrical loads.

Equipment List

The owner or architect should give the electrical designer manufacturer electrical specification sheets for all the specialized equipment in a given facility along with a floor plan showing the location of all equipment in the facility so that the electrical designer can create an equipment list. The equipment list determines the electrical requirements of specialized equipment and machinery (see FIGURE 3-1). To create this list, the designer must gather the following information:

  1. A brief description of each piece of equipment

  2. Specific electrical requirements for each piece of equipment, including:

    1. Horsepower

    2. Voltage

    3. Number of phases

    4. Load (in amperes or kVA)

  3. Floor plan indicating the location of each piece of equipment

Click to collapse
Figure 3-1: The equipment list provides details about all specialized equipment and their electrical requirements

Once the designer compiles the equipment list, he or she should organize the equipment by type so that each piece can be easily referenced with its location on the floor plan. The designer then calculates the required number of branch circuits necessary to serve each piece of equipment. Later in the design process, the designer will use these totals to determine the quantity and operating voltages of panelboards to serve the equipment loads.

Tip

If the design is being completed for a preexisting building that is undergoing alterations and manufacturer electrical specification sheets are not available, the designer may need to visit the location of the existing equipment to obtain the electrical requirements for each piece.

Specialized Equipment Branch Circuits

As per the NEC, specialized equipment must be served by separate branch circuits to avoid overload [430]. The most basic type of separate branch circuit is a general branch circuit that can serve small office equipment such as photocopiers, computer equipment, and vending machines.

There are three common types of specialized equipment branch circuit wiring methods. In the first type, the equipment is served by its own ungrounded conductor and the grounded conductor is shared with other circuits such as a multiwire branch circuit (see FIGURE 3-2). In the second type, the grounded conductor will not be shared with any other branch circuits; this type of branch circuit provides better electrical isolation and is therefore often used for electrically sensitive equipment (see FIGURE 3-3). In the third type of separate circuit, all the branch-circuit conductors supplying an individual piece of equipment will not share any conductors with any other equipment or receptacles and, in some cases, will not share a raceway with any other circuit conductors. This type of circuit contains its own ungrounded, grounded, and, if applicable, equipment grounding conductors (see FIGURE 3-4).

Image from book
Figure 3-2: Individual branch circuits that share a common grounded conductor
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Figure 3-3: Individual branch circuits that do not share a common grounded conductor
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Figure 3-4: Individual branch circuits that are supplied with their own ungrounded, grounded, separate grounding conductor, and individual raceways

It is important to note that the NEC does not recognize flexible metallic conduit as an approved grounding method when installed lengths exceed 6 feet and are protected by overcurrent devices greater than 20 A [250.118(5)]; in such cases, a supplemental equipment grounding conductor is needed. For the third type of specialty circuit, the equipment must have its own grounding conductor; therefore, the raceway system will contain two equipment grounding conductors—one for the equipment and one for grounding the raceway. If the ground terminal of a receptacle is to serve equipment only and is separate from a grounding conductor utilized to ground a raceway, it must be listed and labeled as an approved isolated ground type. Designers identify these receptacles on a print as isolated ground (IG).

Tip

Electrical requirements for specialized equipment will be specified by the equipment manufacturer and should be noted on the plans in an equipment list.

Motors

Motors of 1 hp and greater (typically found in production machinery) must be provided with specialized branch circuits. As per the NEC, designers must determine the sizing of branch-circuit conductors, sizing of the devices protecting the motors, and wiring serving these devices [Article 430]. Designers can make these determinations using NEC guidelines, the equipment list, and the following steps:

  1. Determine the size of the motor equipment branch-circuit conductors based on the motor voltage and full-load amperage.

  2. Determine the size of the overcurrent protective devices for the motor and associated wiring.

  3. Determine equipment grounding methods.

  4. Select and size raceway method(s).

Tip

With all types of specialized equipment branch circuits, any special equipment grounding requirements must be noted on the plans either at the device being served or in a keynotes section.

Motor Equipment Branch-Circuit Conductors

In most applications, control devices start motors by applying full voltage to the terminals of the motor when the devices are energized. A motor’s starting current can be up to six times its running current. The NEC requires that the size of the branch-circuit conductors serving a single motor in a continuous duty application be based on a minimum size of 125 percent of the motor’s full-load current [430.22(A)]. By basing the conductor size on this calculated amperage designers size conductors larger than is needed for full-load amperage, which helps handle higher inrush currents associated with motors.

If the branch-circuit conductors serving the equipment will be more than 100 ft from the electrical source to the equipment, voltage drop (loss resulting from the length of the conductor, its resistance, and the amperage imposed on the conductor) may become a factor. Low voltage at the equipment can damage or greatly shorten the life of a motor. Designers must calculate voltage drop when a motor’s branch-circuit conductors are 100 ft or greater in length. These calculations ensure that conductors have adequate capacity to serve both the voltage and amperage necessary for the equipment to operate correctly and efficiently.

According to the NEC:

Conductors for branch circuits  [should be] sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation [210.19(1) FPN No. 4].

TABLE 3-1 outlines the maximum allowable voltage drop for branch circuits based on the NEC guidelines.

Table 3-1: Maximum Allowable Voltage Drop for Branch Circuits
 Open table as spreadsheet

Branch-Circuit Voltage

3% Voltage Drop

5% Voltage Drop

120 volts

3.6 volts

6 volts

208 volts

6.24 volts

10.4 volts

277 volts

9.23 volts

13.85 volts

480 volts

14.4 volts

20.4 volts

Tip

The following NEC tables are valuable references for sizing motor equipment branch-circuit conductors:

  • Table 310.16 Allowable Ampacities of Insulated Conductors

  • Table 430.247 Full Load Currents in Amperes, Direct Current Motors

  • Table 430.248 Full Load Currents in Amperes, Single Phase Alternating Current Motors

  • Table 430.250 Full Load Current, Three Phase Alternating Current Motors

To calculate the correct wire size to account for voltage drop in branch circuits, use the following equations.

For single-phase systems:

  • Image from book

For 3-phase systems:

  • Image from book

where

Cm

=

Circular mil area of conductor

K

=

DC wire constant

D

=

One-way distance

I

=

Load in amperes

Vd

=

Allowable voltage drop

The DC wire constant value (K) is a measure of a conductor’s DC ohmic value resistance for a conductor size of exactly 1 mil wide by 1 ft long. For calculating voltage drop, the DC wire constant value (K) is 12.9 for copper conductors and 21.2 for aluminum conductors. To determine the circular mil area of the conductor (Cm), refer to TABLE 3-2, which is NEC Table 8.

Tip

Voltage drop calculations should be performed whenever branch-circuit conductors will serve a load located 100 ft or more from the source.

Table 3-2: NEC Table 8 Conductor Properties
 Open table as spreadsheet

Size (AWG or kcmil)

Area

Conductors

Stranding

Overall

Quantity

Diameter

Diameter

Area

mm2

Circular mils

mm

in.

mm

in.

mm2

in.2

18

0.823

1620

1

1.02

0.040

0.823

0.001

18

0.823

1620

7

0.39

0.015

1.16

0.046

1.06

0.002

16

1.31

2580

1

1.29

0.051

1.31

0.002

16

1.31

2580

7

0.49

0.019

1.46

0.058

1.68

0.003

14

2.08

4110

1

1.63

0.064

2.08

0.003

14

2.08

4110

7

0.62

0.024

1.85

0.073

2.68

0.004

12

3.31

6530

1

2.05

0.081

3.31

0.005

12

3.31

6530

7

0.78

0.030

2.32

0.092

4.25

0.006

10

5.261

10380

1

2.588

0.102

5.26

0.008

10

5.261

10380

7

0.98

0.038

2.95

0.116

6.76

0.011

8

8.367

16510

1

3.264

0.128

8.37

0.013

8

8.367

16510

7

1.23

0.049

3.71

0.146

10.76

0.017

6

13.30

26240

7

1.56

0.061

4.67

0.184

17.09

0.027

4

21.15

41740

7

1.96

0.077

5.89

0.232

27.19

0.042

3

26.67

52620

7

2.20

0.087

6.60

0.260

34.28

0.053

2

33.62

66360

7

2.47

0.097

7.42

0.292

43.23

0.067

1

42.41

83690

19

1.69

0.066

8.43

0.332

55.80

0.087

1/0

53.49

105600

19

1.89

0.074

9.45

0.372

70.41

0.109

2/0

67.43

133100

19

2.13

0.084

10.62

0.418

88.74

0.137

3/0

85.01

167800

19

2.39

0.094

11.94

0.470

111.9

0.173

4/0

107.2

211600

19

2.68

0.106

13.41

0.528

141.1

0.219

250

127

37

2.09

0.082

14.61

0.575

168

0.260

300

152

37

2.29

0.090

16.00

0.630

201

0.312

350

177

37

2.47

0.097

17.30

0.681

235

0.364

400

203

37

2.64

0.104

18.49

0.728

268

0.416

500

253

37

2.95

0.116

20.65

0.813

336

0.519

600

304

61

2.52

0.099

22.68

0.893

404

0.626

700

355

61

2.72

0.107

24.49

0.964

471

0.730

750

380

61

2.82

0.111

25.35

0.998

505

0.782

800

405

61

2.91

0.114

26.16

1.030

538

0.834

900

456

61

3.09

0.122

27.79

1.094

606

0.940

1000

507

61

3.25

0.128

29.26

1.152

673

1.042

1250

633

91

2.98

0.117

32.74

1.289

842

1.305

1500

760

91

3.26

0.128

35.86

1.412

1011

1.566

1750

887

127

2.98

0.117

38.76

1.526

1180

1.829

2000

1013

127

3.19

0.126

41.45

1.632

1349

2.092

Notes:

  1. These resistance values are valid only for the parameters as given. Using conductors having coated strands, different stranding type, and, especially, other temperatures changes the resistance.

  2. Formula for temperature change: R2 = R1 [1 + α(T2  75)] where αcu = 0.00323, αAL = 0.00330 at 75°C.

 Open table as spreadsheet

Direct-Current Resistance at 75°C (167°F)

Copper

Aluminum

Uncoated

Coated

ohm/ km

ohm/ kFT

ohm/ km

ohm/ kFT

ohm/ km

ohm/ kFT

25.5

7.77

26.5

8.08

42.0

12.8

26.1

7.95

27.7

8.45

42.8

13.1

16.0

4.89

16.7

5.08

26.4

8.05

16.4

4.99

17.3

5.29

26.9

8.21

10.1

3.07

10.4

3.19

16.6

5.06

10.3

3.14

10.7

3.26

16.9

5.17

6.34

1.93

6.57

2.01

10.45

3.18

6.50

1.98

6.73

2.05

10.69

3.25

3.984

1.21

4.148

1.26

6.561

2.00

4.070

1.24

4.226

1.29

6.679

2.04

2.506

0.764

2.579

0.786

4.125

1.26

2.551

0.778

2.653

0.809

4.204

1.28

1.608

0.491

1.671

0.510

2.652

0.808

1.010

0.308

1.053

0.321

1.666

0.508

0.802

0.245

0.833

0.254

1.320

0.403

0.634

0.194

0.661

0.201

1.045

0.319

0.505

0.154

0.524

0.160

0.829

0.253

0.399

0.122

0.415

0.127

0.660

0.201

0.3170

0.0967

0.329

0.101

0.523

0.159

0.2512

0.0766

0.2610

0.0797

0.413

0.126

0.1996

0.0608

0.2050

0.0626

0.328

0.100

0.1687

0.0515

0.1753

0.0535

0.2778

0.0847

0.1409

0.0429

0.1463

0.0446

0.2318

0.0707

0.1205

0.0367

0.1252

0.0382

0.1984

0.0605

0.1053

0.0321

0.1084

0.0331

0.1737

0.0529

0.0845

0.0258

0.0869

0.0265

0.1391

0.0424

0.0704

0.0214

0.0732

0.0223

0.1159

0.0353

0.0603

0.0184

0.0622

0.0189

0.0994

0.0303

0.0563

0.0171

0.0579

0.0176

0.0927

0.0282

0.0528

0.0161

0.0544

0.0166

0.0868

0.0265

0.0470

0.0143

0.0481

0.0147

0.0770

0.0235

0.0423

0.0129

0.0434

0.0132

0.0695

0.0212

0.0338

0.0103

0.0347

0.0106

0.0554

0.0169

0.02814

0.00858

0.02814

0.00883

0.0464

0.0141

0.02410

0.00735

0.02410

0.00756

0.0397

0.0121

0.02109

0.00643

0.02109

0.00662

0.0348

0.0106

  1. Conductors with compact and compressed stranding have about 9 percent and 3 percent, respectively, smaller bare conductor diameters than those shown. See Table 5A for actual compact cable dimensions.

  2. The IACS conductivities used: bare copper = 100%, aluminum = 61%.

  3. Class B stranding is listed as well as solid for some sizes. Its overall diameter and area is that of its circumscribing circle.

Source: NEC® Handbook, NFPA, Quincy, MA, 2008, Table 8

Motor Branch-Circuit Short-Circuit and Ground-Fault Protection Devices

All conductors, including motor circuit conductors, must be protected from short circuits and ground faults. Short-circuit and ground-fault protective devices (e.g., fuses and circuit breakers) are placed to protect the motor and, more important, protect the conductors supplying the equipment. Motor types and the maximum size ratings allowed for the motor short-circuit and ground-fault protective devices are listed in TABLE 3-3, which is NEC Table 430.52. This table lists four types of protective devices (see FIGURE 3-5):

  1. Non-time delay fuse (single-element, fast-acting protective device)

  2. Dual-element fuse (time delay protective device)

  3. Instantaneous trip breaker (fast-acting protective device)

  4. Inverse time breaker (time delay protective device)

Table 3-3: NEC Table 430.52 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices
 Open table as spreadsheet

Type of Motor

Percentage of Full-Load Current

Nontime Delay Fuse[1]

Dual Element (Time-Delay) Fuse[1]

Instantaneous Trip Breaker

Inverse Time Breaker[2]

Single-phase motors

300

175

800

250

AC polyphase motors other than wound-rotor

300

175

800

250

Squirrel cage—other than Design B energy-efficient

300

175

800

250

Design B energy-efficient

300

175

1100

250

Synchronous[3]

300

175

800

250

Wound-rotor

150

150

800

150

Direct current (constant voltage)

150

150

250

150

Note

For certain exceptions to the values specified, see 430.54.

  1. The values in the Nontime Delay Fuse column apply to Time-Delay Class CC fuses.

  2. The values given in the last column also cover the ratings of nonadjustable inverse time types of circuit breakers that may be modified as in 430.52(C)(1), Exception No. 1 and No. 2.

  3. Synchronous motors of the low-torque, low-speed type (usually 450 rpm or lower), such as are used to drive reciprocating compressors, pumps, and so forth, that start unloaded, do not require a fuse rating or circuit-breaker setting in excess of 200 percent offull-load current.

Source: NEC® Handbook, NFPA, Quincy, MA, 2008, Table 430.52

[1]The values in the Nontime Delay Fuse column apply to Time-Delay Class CC fuses.

[2]The values given in the last column also cover the ratings of nonadjustable inverse time types of circuit breakers that may be modified as in 430.52(C)(1), Exception No. 1 and No. 2.

[3]Synchronous motors of the low-torque, low-speed type (usually 450 rpm or lower), such as are used to drive reciprocating compressors, pumps, and so forth, that start unloaded, do not require a fuse rating or circuit-breaker setting in excess of 200 percent of full-load current.

Image from book
Figure 3-5: Protective Devices A. Non-time delay fuse. B. Dual-element fuse. C. Instantaneous trip breaker D. Inverse time breaker

For each type of protective device, the maximum allowable size for the overcurrent device is based on a percentage of the motors full-load amperage. Also note that when determining the size of the protective device based on calculations, the calculated values most likely do not align with standard manufactured settings for the devices. As per the NEC, when calculated values do not correspond to standard manufactured settings, the next higher standard can be used [Table 430.52 Exception #1 and Article 240.6].

For most general motor applications that use fuse-based protection, a dual-element time delay fuse is used, and the ratings for this fuse are 175 percent of the motor’s full-load current. When circuit breakers are used to provide short-circuit and ground-fault protection (such as with panelboards), the allowable rating can be up to 250 percent of the motor’s full-load current. Circuit breakers are allowed to be set at a higher value than fuses are because they are more subject to tripping during motor starts. Some manufacturers recommend that for both fuse and circuit breaker protection, the maximum size of overcurrent protection should be set at 175 percent of the motor’s full-load current.

For motors that have starting currents greater than their full-load current, protective devices are allowed to be sized at values greater than the motor’s actual full-load current. This sizing allows the motor to pass through the starting process and eliminates the possibility of nuisance tripping. Non-time delay fuses may be rated at up to 300 percent of the motor’s full-load current.

Motor Branch-Circuit Overload Devices

Motor overload devices installed in the motor controller provide protection from overload (excessive motor running current) (see FIGURE 3-6). Motor overload devices operate on the principle of heat caused by the current in the circuit path and are not rated based on current alone as a method to open a circuit. Therefore, motor overload devices can be sized at a value much closer to the motor’s actual full-load current, which provides for greater protection of the motor. For most applications, the NEC requires that motor overloads be sized at 125 percent of the motor’s full-load current [430.32].

Image from book
Figure 3-6: Motor overload device in motor controller

Grounding of Motors and Equipment

Typical motor applications are installed using many different types of raceway systems that are approved as an equipment grounding conductor and all must be grounded based [250]. For example, when electrical metallic tubing (Type EMT) is used, no additional equipment grounding conductor is required because Type EMT is approved as a grounding conductor.

However, with motor circuits, almost all applications have maintenance and servicing needs that require that the final connection to the motor be a more flexible type of raceway. With a flexible conduit, motor vibration or servicing of the equipment can loosen the connection at the motor termination housing, possibly eliminating any equipment grounding accomplished solely by the raceway system. For this reason, most motor circuit applications with flexible connections require an additional equipment grounding conductor be installed. Some electrical designers install additional equipment grounding conductors the entire length of the raceway regardless of the type of raceway system used. Although not required, this design, when installed properly, provides a more reliable grounding system.

To size equipment grounding conductors, designers must determine the value of the short-circuit and ground-fault protection devices (outlined in the previous section), and then reference TABLE 3-4, which is NEC Table 250.122, to find the minimum size for equipment grounding conductors.

Table 3-4: NEC Table 250.122 Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment
 Open table as spreadsheet

Size (AWG or kcmil)

Rating or Setting of

Automatic Overcurrent

Device in Circuit Ahead of

Equipment, Conduit, etc.,

Not Exceeding (Amperes)

Copper

Aluminum or

Copper-Clad

Aluminum[*]

15

14

12

20

12

10

30

10

8

40

10

8

60

10

8

100

8

6

200

6

4

300

4

2

400

3

1

500

2

1/0

600

1

2/0

800

1/0

3/0

1000

2/0

4/0

1200

3/0

250

1600

4/0

350

2000

250

400

2500

350

600

3000

400

600

4000

500

800

5000

700

1200

6000

800

1200

Note

Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding conductor shall be sized larger than given in this table.

Source: NEC® Handbook, NFPA, Quincy, MA, 2008, Table 250.122

[*]See installation restrictions in 250.120.

When it is determined that branch-circuit conductors must be increased in size because of voltage drop concerns, the size of equipment grounding conductors must be increased in size as well. According to the NEC, the circular mil area of the equipment grounding conductors must be increased proportionally to the increase in the circular mil area of the ungrounded circuit conductors [250.122(B)]. An increase in the circular mil area of the equipment grounding conductors decreases resistance, thus allowing any short or fault currents to dissipate quickly from the system, avoiding damage to equipment and personnel (see “Adjusting the Size of Equipment Grounding Conductors for Voltage Drop” on page 39).

Tip

The added safety of an equipment grounding conductor along the entire length of the raceway often outweighs the additional material costs.

Tip

In any application where branch-circuit conductors are increased in size, the size of equipment grounding conductors must be increased proportionally.

Raceways for Branch-Circuit Distribution

Once all the branch circuit and equipment grounding conductor sizes have been determined, the next step is to determine the raceway type and sizes that will be used.

The NEC provides many approved raceway methods for branch circuits to supply loads and equipment. When the electrical wiring to be installed is in an existing facility the conduit and raceway system most likely will be an EMT raceway. EMT systems are easily installed and durable but may be more costly than other methods. In new buildings and where possible in other projects, installing conduits underground using rigid nonmetallic conduit (PVC) can help reduce costs.

Tip

The following NEC tables are valuable references for designing raceways for branch-circuit distribution:

  • Annex C Conduit and Tubing Fill Tables for Conductors and Fixture Wires of the Same Size

  • Table 4, Chapter 9 Dimensions and Percent Area of Conduit and Tubing

  • Table 5, Chapter 9 Dimensions of Insulated Conductors and Fixture Wires

In raceway systems, individual conduits can be installed from the distribution source to an individual piece of equipment, but often a single conduit of a larger size that contains multiple branch circuits for more than one piece of equipment is used. With this method, single, larger conduits are installed in close proximity to the equipment and terminated in properly sized junction or pull boxes. From these junction or pull boxes, smaller individual conduits are run to each individual piece of equipment (see FIGURE 3-7).

Click to collapse
Figure 3-7: Use of single, larger raceway containing multiple branch circuits for a home run

Regardless of the type of raceway system, the conduits must be properly sized and designed for the amount of conductors. To calculate the proper size of the raceway the designer must determine whether the conductors to be installed in a raceway are all of the same circular mil or AWG size and insulation type. If they are the same size and type, the designer can choose the raceway size using the tables in NEC Annex C, which lists raceway types by trade size in both metric and standard fractional sizes and the allowable number of conductors permitted. If the conductors are not of the same size and insulation type, the designer can determine the total area of the conductors and reference NEC Chapter 9 Table 4 and Table 5.

For conductors of different AWG/kcmil sizes and insulation types, NEC Chapter 9 Table 4 provides a great deal of information about each raceway type in both metric and standard measurements such as size, internal diameters, total area for its given size, and the allowable areas that may be used when multiple types of conductors are installed. The information in Table 4 is based on calculations of the area each conductor will fill in the raceway; therefore, all conductors, including any required equipment grounding conductors, must be counted as part of the total area of the conductors. Because conductors are available with many different insulation types of varying thicknesses, conductors have variable diameters and total square inch area. NEC Chapter 9 Table 5 provides information about conductor types, including insulation type, size in AWG or kcmil, approximate diameter, and approximate area. This table should be used to calculate the total area of conductors to be installed in any raceway type.

Tip

The appropriate conduit size in square inches must be more than the total calculated area for variable-sized conductors.

When determining the maximum conduit fill for an installation, it is permissible to increase a conduit size. Using a raceway larger than the minimum required size may lead to additional cost, but if the conduit run is long or contains several bends, larger conduit eases pulling of the conductors, thus saving labor costs and possibly preventing damage to the conductors’ insulation.

Determining the Number of Panelboards

All projects require a minimum number of panel-boards based on the minimum number of required branch circuits and the operating voltages of the loads to be served. Panelboards typically have a maximum number of 42 branch-circuit spaces available, but as per changes in the 2008 version of the NEC, the number of allowed branch circuits in any one panelboard is no longer limited to 42. Panelboards can contain as many branch circuits as listed for specific panelboards. According to the NEC:

  • A panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed, rated, and listed. For the purposes of this section, a 2-pole circuit breaker or fusible switch shall be considered two overcurrent devices; a 3-pole circuit breaker or fusible switch shall be considered three overcurrent devices [408.54].

With large quantities of single-phase loads requiring one or two poles and 3-phase loads requiring three poles, the 42 individual branch-circuit spaces in a single panelboard can quickly be used. If the required number of branch circuits for the equipment at their serving voltages exceeds 42, then an additional panelboard may be necessary.

To determine the minimum number of panel-boards, designers consider the following factors:

  • The operating voltage of the loads to be served

  • Number of branch-circuit requirements

  • Size of the facility

  • Location of the equipment in relationship to the panelboards

  • Horsepower rating of equipment to be served

  • Labor and material costs associated with each installed panelboard

A good design incorporates all the minimum requirements and provides an efficient system to serve the facility safely with an ability to expand for future needs.

Larger Motors

Motors 25 hp and greater require larger amounts of amperage than do motors of lower horsepower. If these loads are designed to be served from a branch circuit panelboard, the overall amperage capacity of the panelboard must be increased. To do so, designers can increase the amperage size of the panel-board, the feeder conductors, and conduit systems that serve these panelboards to accommodate the additional current and electrical stress. Labor and material costs also increase with the increase in amperage capacity of the panelboard. In such cases, it may be advisable to consider an alternative design method serving the loads directly from the main switchboard and eliminating a larger ampacity panelboard and associated costs.

Equipment Branch Circuits and Panel Schedules

Once a determination has been made as to which equipment branch circuits will be served from a particular panelboard, the designer should arrange the branch circuits within the panelboard. Just as with general purpose branch circuits (see Chapter 2), the designer enters the specialty branch circuits into a panel schedule. The process is very similar to the one used for general purpose equipment except that specialized equipment may contain both single- and 3-phase motors. (See “You Are the Designer” at the end of this chapter for an example of how to design a panel-board in this situation.)

For each specialty branch circuit, the designer must calculate a total volt-ampere rating using Ohm’s law. This calculation is based on the equipment’s voltage and amperage and determines the volt-ampere values to be entered into the panel schedule. The total volt-amperes of the equipment is divided by the appropriate number of ungrounded conductors required for the load.

Raceway Legends

Raceway legends in the design plan provide a great deal of information about individual raceways and conductors for special equipment loads (see FIGURE 3-8). Although no official industry-standard format exists, a raceway legend generally lists the following information:

  • Raceway I.D.

  • Quantity and size of conductors

  • Equipment grounding conductor size

  • Raceway size

  • Raceway type

  • Panelboard serving load

  • General remarks

Click to collapse
Figure 3-8: The raceway legend provides information about the raceways and conductors that serve specialized equipment loads

A raceway legend provides clear and concise information in a simple format and helps to avoid excess information on the electrical plan that might otherwise cause confusion, errors, or omissions. Although it can take time to develop a raceway legend and this legend is not a requirement, including a raceway legend provides a higher quality design and demonstrates a greater degree of professionalism.

Tip

When a raceway legend is included in a design plan, the individual raceways are identified on the design plan and referenced to the raceway legend using a common symbol developed by the designer.

Wrap Up: Master Concepts

  • Equipment lists outlining all the equipment in a facility are necessary to determine the number of branch circuits needed and the quantity and operating voltages of panelboards.

  • Because of their amperage requirements and manufacturer specifications, specialized equipment and motorized equipment must be served by their own individual branch circuits.

  • When loads are located farther than 100 ft from their source, because of the resistance of the conductors serving specialized and motorized equipment loads, the circular mil area of branch circuit conductors must be increased to provide for a maximum voltage drop of 3 percent of the source voltage.

  • Properly determining the size of the overcurrent protection for motors, equipment, and the branch circuit conductors that serve them is very important to protect these devices from short-circuit or ground-fault conditions.

  • When designing panelboards, designers should aim to meet all the minimum requirements and provide an efficient system to serve the facility safely with an ability to expand for future needs.

  • A raceway legend, though not a requirement, is an extremely useful tool to identify the raceways and the conductors contained within them for identification on a design plan.

Charged Terms

  • 277/480-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 277 volts to the grounded point, while the line voltage across each of the phases produces 480 volts. The line-to-line voltages can supply both 480-volt 3-phase and 480-volt single-phase. This system is commonly used in commercial applications where 480 volts is required for machinery loads and in applications to serve 277-volt lighting loads.

  • equipment grounding conductor The conductive path installed to connect normally non-current-carrying metal parts of equipment together and to the system grounded conductor, to the grounding electrode conductor, or to both [100].

  • equipment list A developed table that lists details about the specialized equipment that is to be incorporated into a design plan.

  • ground fault A condition in which high levels of current could flow when an ungrounded conductor accidentally comes in contact with a grounded reference.

  • inrush current A momentary high level of amperage flowing in a circuit such as those associated with motorized equipment loads.

  • isolated ground (IG) An additional equipment grounding conductor that, when installed, provides for the grounding of equipment separate from a grounding method that uses an approved raceway method; typically used for electrically sensitive equipment in computer applications and medical facilities. When isolated grounding is provided through the use of receptacles, the receptacle must be identified on the design plan as “IG.”

  • manufacturer electrical specification sheet Information provided by a manufacturer that lists specific details about the product; these specification sheets are often used to obtain information about motorized equipment and lighting fixtures.

  • raceway legend A table developed by an electrical designer that illustrates information about raceways installed for a project.

  • short circuit A dangerous condition in which circuit conductors contact each other and reduce the intended ohmic resistance of the circuit; often referred to as a line-to-line or line-to-neutral short. (See also ground fault.)

  • voltage drop A loss of voltage on a conductor resulting from the length of the conductor, its resistance, and the amperage imposed on the conductor.

Check Your Knowledge

1. 

The purpose of an equipment list is to:

  1. list the electrical requirements for any equipment to be installed on a project.

  2. provide manufacturer specification sheets to the owner.

  3. assign each equipment piece an identification number.

  4. assign each equipment piece a branch circuit number.

2. 

A raceway legend provides:

  1. the types and quantities of raceway material to be used on a project.

  2. a detailed layout for the routing of all the raceways for a project.

  3. a list of raceways by type, size, and the conductors to serve equipment loads.

  4. a listing of all the raceway types for a project.

3. 

A 10-hp, 208-volt, 3-phase motor located 50 ft from the source requires a size ____AWG copper Type THWN conductor.

  1. 12

  2. 10

  3. 8

  4. 6

4. 

A 10-hp, 208-volt, 3-phase motor located 350 ft from the source requires a size ____AWG copper Type THWN conductor.

  1. 20

  2. 8

  3. 6

  4. 4

5. 

What is the maximum allowable size for a time delay fuse that provides protection for a 10-hp, 208-volt, 3-phase motor?

  1. 30 A

  2. 40 A

  3. 50 A

  4. 60 A

6. 

What is the maximum allowable size for an inverse time circuit breaker that provides protection for a 10-hp, 208-volt, 3-phase motor?

  1. 60 A

  2. 70 A

  3. 80 A

  4. 90 A

7. 

A 10-hp, 208-volt, 3-phase motor located 75 ft from the source and protected by a time delay fuse requires a size_____copper Type THWN equipment grounding conductor.

  1. 12 AWG

  2. 10 AWG

  3. 8 AWG

  4. 6 AWG

8. 

A 10-hp, 208-volt, 3-phase motor protected by a time delay fuse and located 350 ft from the source requires a size_____copper Type THWN equipment grounding conductor.

  1. 12 AWG

  2. 10 AWG

  3. 8 AWG

  4. 6 AWG

9. 

What is the minimum raceway size for a schedule 40 PVC conduit serving a 10-hp, 208-volt, 3-phase motor located within 100 ft of the source?

  1. ½ in.

  2. ¾ in.

  3. 1 in.

  4. 1¼ in.

10. 

For the motor described in question 9, what is the total volt-amperage per phase?

  1. 6,406 VA

  2. 11,083 VA

  3. 14,784 VA

  4. 25,576 VA

11. 

For the motor described in question 9, what is the individual volt-amperage per phase?

  1. 2,135 VA

  2. 3,694 VA

  3. 4,928 VA

  4. 8,525 VA

You are the Designer