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Section 2: Electrical Service

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Introduction

Before designing the electrical circuit, the designer must first determine the service type. This section explains the types of service available and other concerns related to electrical service.

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Typical Service Types

The first consideration in electrical system design is determining the type of electrical service. The Electrical Detail standard sheets show four standard electrical service arrangements. These service types are shown in the following table.

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Service Type

Description

Notes

Type A or C

480 volts, 3-wire with center ground

Three-wire branch circuits consisting of two line conductors and a ground.

Type T

120/240 volts, 3-wire with center ground (main lug only — no main breaker)

Used for traffic signals, traffic management systems, and irrigation systems. Not typically used for illumination (except at lighted intersections with traffic signals).

Type D

120/240, 3-wire with center ground (main breaker)

Used for traffic signals, traffic management systems, and irrigation systems. Not typically used for illumination (except at lighted intersections with traffic signals).



Service Types A and C are recommended for roadway lighting.

If 480 volts is not available, 120/240 volt, 3-wire service is recommended with luminaires served at 240 volts.

shows schematics of Types A and C service types and an alternate of Type A sometimes used.

 Typical electrical service types. (click in image to see full-size image) Anchor: #i999106grtop

Figure 7-1. Typical electrical service types.

NOTE: Type A may also be used for 120/240 volt service. 480 volt to ground services must not be used.

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Circuit Length Limits

At 480 volts, a 4,000 foot circuit of twin-arm poles can be served without unduly large conductors. At 240 volts, the circuit is limited to about 2,000 feet. (Conductor size and voltage drop are discussed in Circuit Design and Calculating Voltage Drop of this chapter.)

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Separate Electrical Service for Signs

Separately metered electrical service should be installed for sign lighting and roadway lighting. This is necessary because different entities usually maintain the two systems. TxDOT usually maintains the electrical service supplying sign lighting, while cities or others may maintain illumination. This separation of service also eliminates conflicts between maintenance organizations of the two entities. (For further information, see Maintenance Responsibilities of Cities in Chapter 9, Section 6.)

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Where Service Does Not Exist

If there are no distribution lines near a necessary electrical service location, the utility company can provide cost estimates for extending service to these points. This cost should be charged to the project under a force account function code.

The contractor must consult with the appropriate utility company to determine cost and requirements, and must coordinate the utility work as approved by the engineer. The contractor will be reimbursed only the amount billed by the utility; no additional amount for supervision of the utility’s work will be paid.

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