901.8 System Voltage
The department standard is either 480-volt or 240-volt multiple circuit systems. The following guidelines are to be used to determine the system voltage:
- Basic Intersection Lighting (signal post systems are 120-volt).
- Basic Interchange Lighting at Diamond Interchanges.
- Basic Interchange Lighting at Complex Interchanges.
- Continuous Lighting of Interchanges or Roadways.
- High Mast Lighting.
Lighting system voltages cannot be mixed in a single interchange or system. Long cable runs may require an increase from 240 volts to 480 volts (see Voltage Drop and Wire Sizes). Both 240- and 480-volt secondary service are generally available at most locations. The availability of the desired system voltage is to be verified with the local utility company.
If a municipality will operate and maintain lighting in accordance with MoDOT's policy, other types of circuits may be used at the request of the municipality. State-owned equipment is mounted as shown on standard plans or approved special drawings. Utility-owned equipment is mounted in accordance with practices of the utility company.
901.8.2 Conduit and Cable
Circuiting from control station to lights is to follow the shortest possible route using current design standards. All circuits are underground, using cable-conduit with single conductor cables of the proper size so voltage drop will not exceed 5% of the rated voltage. #8 AWG (10 mm2) cable is the minimum size to be used. All rigid conduit carrying cable-conduit under pavement and shoulder is to have a 3 in. minimum diameter. Rigid conduit carrying cable-conduit located elsewhere is to have a 2 in. minimum diameter. Pushed conduits are 4 in. or larger. The amount of cable-conduit under pavement and shoulders is to be kept to a minimum. Where practical, cable-conduit runs parallel to the roadway are to be installed outside the shoulder.
Rigid conduit larger than 2 in. in diameter is used to carry lighting cables on structures or in median barriers. Design of conduit systems on structures is to be coordinated with Bridge. Where conditions require most of a system to be under pavement and shoulders or in median barriers or structures, the entire cable system is to be installed in rigid conduit.
901.8.3 Pull Boxes and Cable Splicing
Cable splicing is to be kept to a minimum and is only permissible in the base of a lighting pole, in a pull box or in a transformer base. Cable splicing in all poles is accomplished with a premolded fused splice. Cables are to be continuous and unspliced between the control station, pull boxes, and light poles.
If the distance between the control station, pull boxes, and light poles is greater than 500 ft. with cable-conduit or greater than 200 ft. with rigid conduit, pull box or junction box is to be provided to facilitate cable pulling. A pull box is installed on each end of any pavement crossing unless the pavement crossing ends in a light pole. The quantity and type of pull boxes are specified on the D-37A sheet.
Two pairs of cables may be spliced into one light pole. Pole and bracket cable are used from the base of the pole to the luminaire. Pole and bracket cable consist of two #10 AWG (6 mm2) single-conductor cables. Where three pairs of cable are spliced (three-way splices) a pull box is recommended. Three-way splices are to be avoided as much as possible. It is preferable to use an additional branch circuit from the control station, especially if the three-way splice is close to the control station. No more than three pairs of cables are spliced.
901.8.4 Poles, Bracket Arms and Foundations
Lighting poles may be either steel or aluminum. The type may be specified on the plans or left to the option of the contractor. The type of pole, as designated on the plans, shows the mounting height of the luminaire above the edge of travelway, not the actual length of pole. The proper length of pole and the method of attaining the mounting height of the luminaire above the edge of travelway are described in EPG 901.7.8 and EPG 901.7.9.
Bracket arms are available in 4, 6, 8, 10, 12 and 15 ft. lengths for 30 ft. mounting heights and 6 and 15 ft. bracket arms for 45 ft. mounting heights. Only bracket arms of these lengths are used. The bracket arm designs are shown in the standard plans. Mast arms for mounting traffic detectors or traffic signals are not used on lighting poles since the poles are not designed to carry the extra load of the mast arm and unit. Pole foundations will be in accordance with the standard plans. Poles, foundations, and appurtenances for lighting are found in Standard Plan 901.00 for 30 ft. mounting height and in [https://www.modot.org/media/16904 Standard Plan 901.01 for 45 ft. mounting height.
901.8.5 Trenching and Backfilling
Trenching and backfilling will be classified as Type I, Type II or Type III as described in the Missouri Standard Specifications for Highway Construction and shown on the plans. Great care is to be exercised in classifying trenching and backfilling for bidding purposes, especially as to the possibility of encountering Type III. The limits of each type of trenching and backfilling are shown on the plans. Standard plans show the trench locations.