Section 2: Electrical Service
Anchor: #i1005904Introduction
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.
Anchor: #i1005914Electrical Service Data
Each electrical service in a plan set should be defined in an electrical service data chart. The data in the chart, combined with Item 628, DMS-11080, and the ED standards gives the contractor enough information to build the exact service equipment required for the project.
Figure 7-1. Electrical Service Data. The main sections of the chart are defined below.
Anchor: #i1020810Electrical Service ID
Each service in the plans should have a unique identifying ID. To aid in identification, the ID should describe the location rather than just using a number or letter.
Anchor: #i1020901Electrical Service Description
The electrical service description describes the attributes of the service and should match that of the bid code. For information on service description codes see Chapter 5 Section 5 - Electrical Services and standard sheet ED(5).
Anchor: #i1021036Service Conduit Size
Service conduit size refers to the size of either the overhead or underground service conduit that feeds the service from the utility pole. For underground service conduit, the size is typically 2 inch PVC conduit. For overhead service conduit, the size is determined by the size of the conductors and by the requirements of the utility. Typical sizes for overhead service conduit are 1¼", 1½", and 2" RMC.
Anchor: #i1021052Service Conductors
Service conductor sizes are based on the size of the load as computed according to the NEC. The smallest service conductor size is #6 AWG. Since TxDOT standard services are always 3 wire single phase, the number of service conductors is always three, with two ungrounded (hot) conductors and one grounded (neutral).
Anchor: #i1021067Safety Switch
A safety switch is only needed if required by the local electric utility. Some utilities require a safety switch to be placed before the meter to allow their employees to turn off power to the meter for servicing. Most utilities that require a safety switch only require it for 480V services. The safety switch is always unfused. The safety switch amp rating is the same size or larger than the main breaker. Standard sizes are 60, 100, and 200 amps.
Anchor: #i1021236Main Circuit Breaker
A main circuit breaker is used on Type A, C, and D services. Type T services do not have a main breaker. The main breaker size is based on the size of the load as computed according to the NEC. Since TxDOT standard services are 3 wire single phase, the main breaker always has 2 poles.
It is recommended to use the smallest size main breaker allowed by the load and the NEC, with a minimum of 60 amps. For the types of breakers specified by DMS-11080, the smaller sizes can reduce the intensity of arc flash in a service cabinet in case of accidental ground faults or short circuits.
Anchor: #i1021256Two-Pole Contactor
Two-pole contactors are used on services that require lighting controls. On Type A and C services, the contactor switches all circuits on and off, and is the same size or larger than the main breaker. On Type D services, the contactor switches only the lighting circuit and is the same size or larger than the lighting breaker, typically 30 amps. Standard contactor sizes are 30, 60, 100, and 200 amps.
Anchor: #i1021268Panelboard/Load Center
A panelboard or load center contains an electrical bus with spaces for plug-in breakers. They are used in Type C, D, and T services only. The standard sizes are 70 (Type T only), 100, 125, and 225 amps. Type C and D services use panelboards with a minimum size of 100 amps.
Anchor: #i1021280Branch Circuit ID
Each breaker in the service should have a unique identifying ID. To aid in identification, the ID should describe the purpose of the circuit rather than just using a number or letter.
Anchor: #i1021292Branch Circuit Breakers
Double pole breakers use the line to line voltage on a single phase 3-wire service, and single pole breakers use the line to neutral voltage. Standard sizes for branch circuits are 15, 20, 30, 40, and 50 amps. Larger breakers may be used for feeder circuits to multiple high mast poles.
Anchor: #i1021634Branch Circuit Amps
The branch circuit amps column contains the actual expected load in amps for each branch circuit. For non-lighting circuits, the breaker size is typically the smallest standard size larger than the computed load. For lighting circuits, the breaker size is typically the smallest standard size larger than 125% of the computed load.
Anchor: #i1021654KVA Load
The KVA load is the sum of the computed loads for each branch circuit on the service. The formula for the load of each branch circuit in KVA is:
Circuit voltage (Volts) * Circuit load (Amps) / 1000
Anchor: #i1005967Separate Electrical Service for Signs
Separately metered electrical services 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 5.)
Anchor: #i1028395Electrical Service Calculator
An electrical service spreadsheet calculator is available from the TxDOT Traffic Operations Division (TRF). The calculator takes the load and design information as inputs, and creates the service descriptive code and electrical service data chart.
Anchor: #i1005979Where 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.
Anchor: #i1028662Coordination with Electric Utilities
Electric utilities may have requirements for service that conflict with TxDOT standard sheets or specifications. TxDOT typically has to follow utility requirements in order to get electrical service connected. If the designer is aware of any conflicts between standards and electric utility requirements, a detail or note should be added to the plans to help the Contractor correctly install the service equipment and avoid connection delays.