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Section 3: Circuit Design

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Introduction

Once the service type is determined, the circuits can be designed. This section covers the major considerations in the designing of roadway lighting circuits. Designers should refer to the National Electrical Code (NEC) Handbook for additional information.

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Voltage Drop

The primary limitation on the length of circuits is the voltage drop. Calculating Voltage Drop, Section 4 of this chapter explains how to calculate voltage drop.

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Conductors and Conduit Size

Another consideration is the number of conductors that can be installed in the various sizes of conduits. Annex C of the 2002 NEC Handbook shows the maximum number of conductors allowed in each conduit based on the capability of the wire to dissipate heat. However, with conduit runs of any substantial length, it is usually not practical to install the maximum number of conductors allowed in the conduit. A good rule of thumb is to limit the number of conductors to approximately one-half of the number shown in Annex C of the 2002 NEC Handbook.

The following tables provide dimensions for electrical metal tubing (EMT), Schedule 40 rigid metal conduit (RMC), and rigid non-metallic conduit (PVC).

Anchor: #i1003588Dimensions for EMT

Size

I.D.

O.D.

Radius of Bends*

½

0.622

0.706

4

¾

0.824

0.922

1

1.049

1.163

1.380

1.510

1.610

1.740

2

2.067

2.197

2.731

2.875

10½

3

3.356

3.500

13

3.834

4.000

15

4

4.334

4.500

16

* Radius to centerline of conduit for standard bends. Larger radius bends are available and are required for duct cable casings.



Anchor: #i1003652Dimensions for Schedule 40 RMC & Schedule 40 PVC

Size

I.D.

O.D.

Thickness

Radius of Bends*

½

0.622

0.840

0.109

4

¾

0.824

1.050

0.113

1

1.049

1.315

0.133

1.380

1.660

0.140

1.610

1.900

0.145

2

2.067

2.375

0.154

2.469

2.875

0.203

10½

3

3.068

3.500

0.216

13

3.548

4.000

0.226

15

4

4.026

4.500

0.237

16

5

5.047

5.563

0.258

24

6

6.065

6.625

0.280

30

* Radius to centerline of conduit for standard bends. Larger radius bends are available and are required for duct cable casings.



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Conduit Run Length

As conduit run length and number of bends increases, the effort required to pull conductors also increases. Generally continuous conduit runs in excess of 700 feet are not recommended. Continuous conduit runs in excess 500 feet are not recommended when bends total 180 degrees. The NEC limits total conduit bends between pulling points to 360 degrees (no matter what the length of the run).

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Overcurrent Protection

The ampacity of conductors is shown in Table 310-16 and accompanying notes of the National Electrical Code Handbook. This ampacity should be observed in sizing overload protection for the circuit.

When sizing overcurrent protection, the device closest to the load should be the smallest size. Devices should increase in size toward the service disconnect. For example, a 10 amp fuse might be used in the pole base where the branch circuit breaker is 20 amps, and the main disconnect is a 30 amp circuit breaker.

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Electrical Service Equipment Sizes

Electrical service equipment sizes are based on the branch circuit load and the number of branch circuits. Note that all electrical equipment on the standard electrical service, except the lighting contactor, must be reduced to 80 percent of its rating (For example, only 40 design amperes is allowed on a 50 amp breaker.) The lighting contactors are rated for operation at full current rating.

The following tables show standard industry sizes of breakers.

Anchor: #i1003740Standard Circuit Breaker Sizes by Voltage

 

Standard Circuit Breaker Sizes (Amps*)

Voltage

15

20

30

40

50

60

70

100

125

150

175

225

120

X

X

X

X

X

X

X

X

X

-

-

-

240

X

X

X

X

X

X

X

X

X

-

-

-

480

-

X

X

X

X

X

X

X

X

-

-

-

600

-

X

X

X

X

X

X

X

X

X

X

X

* Remember, total load should not exceed 80 percent of circuit breaker rating.



The standard lighting contactor sizes for all voltages are 30, 60, 100, and 200 amps.

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Grounding Conductor Size

The Following table shows recommended equipment grounding conductor wire sizes for various size circuit breakers and branch circuit wire sizes. (The NEC allows smaller ground wire in a few cases.)

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Branch Circuit

— Branch Circuit Breaker Size* —

Wire Size

15

20

30

40

50

60

70

100

12

12

12

10

10

10

10

8

8

8

8

8

8

6

6

6

6

8

8

8

4

4

4

4

6

6

8

8

2

2

2

2

4

4

4

4

8

* Remember, total load should not exceed 80 percent of circuit breaker rating. Therefore, size the branch circuit breaker at 125 percent of the branch circuit current (or the next higher standard rating.)



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