Section 2: Electrical Service
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.
Electrical 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.
Electrical 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.
Electrical 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
- Electrical Services and standard sheet ED(5).
Service 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.
Service 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).
Safety 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.
Main 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
.Two-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.
Panelboard/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.
Branch 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.
Branch 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.
Branch 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.
KVA 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
Separate 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
.)
Electrical 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.
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.
Coordination 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.