Section 126.35 - Direct Buried Cable(1)General.(a) Cables operating above 600 volts to ground shall have a continuous shield, sheath or concentric neutral which is effectively grounded.(b) Cables of the same circuit operating below 600 volts to ground and without an effectively grounded shield or sheath shall be placed in close proximity (no intentional separation) to each other.(c) Communications cables containing special circuits supplying power solely to communications equipment shall comply with the requirements of 220 CMR 126.34(5)(a) through 126.34(5)(e).(2) Location and Routing. (a)General. 1. Cables should be located so as to be subject to the least disturbance practical. Cables to be installed parallel to other subsurface structures should not be located directly over or under other subsurface structures, but if this is not practical, the rules on clearances in 220 CMR 126.35(3) should be followed.2. Cables should be installed in as straight and direct a line as practical. Where bends are required, the minimum radius shall be sufficiently large to prevent damage to the cable being installed.3. Cable systems should be routed so as to allow safe access for construction, inspection and maintenance.4. The location of structures in the path of the projected cable route shall, as far as practical, be determined prior to the trenching, plowing or boring operation.(b)Natural Hazards. Routes through unstable soil such as mud, shifting soils, corrosive soils or other natural hazards, should be avoided. If burying is required through areas with natural hazards, the cables shall be constructed and installed in such a manner as to protect them from damage. Such protective measures should be compatible with other installations in the area.(c)Other Conditions. 1.Swimming Pools. Supply cable shall not be installed within five feet of a swimming pool or its auxiliary equipment.2.Buildings and Other Structures. Cable should not be installed directly under building or storage tank foundations. Where a cable must be installed under such a structure, the structure shall be suitably supported to prevent transfer of a harmful load onto the cable.3.Railroad Tracks. The installation of cable longitudinally under the ballast section for railroad tracks should be avoided. Where cable must be installed longitudinally under the ballast section of a railroad, it should be located at a depth of not less than 60 inches below the top, of the rail. Where a cable crosses under railroad tracks, the same clearances indicated in 220 CMR 126.32(1)(e) shall apply.
4.Highways and Streets. The installation of cable longitudinally under traveled surfaces of highways and streets should be avoided. When cable must be installed longitudinally under the roadway, it should be installed in the shoulder or within the limits of one lane of traffic if practical.5.Submarine Crossings. Submarine crossings should be routed and/or installed so they will be protected from erosion by tidal action or currents. They should not be located where ships normally anchor.(3)Clearance from Other Underground Structures (sewers, water lines, fuel lines, building foundations, steam lines, other supply or communication conductors not in random separation, etc.). (a)Horizontal Clearance. The horizontal clearance between direct buried cable and other underground structures should be controlled at a minimum of 12 inches or larger as necessary to permit access to and maintenance of either facility without damage to the other. Installations with less than 12 inches horizontal separation shall conform with the requirements of 220 CMR 126.35(3)(c) and/or 126.35(5).(b)Crossings. 1. Where a cable crosses under another underground structure, the structure shall be suitably supported to prevent transfer of a harmful load onto the cable system.2. Where a cable crosses over another underground structure, the cable shall be suitably supported to prevent transfer of a harmful load onto the structure.3. Adequate support may be provided by installing the facilities with sufficient vertical separation.4. Adequate vertical clearance shall be maintained to permit access to and maintenance of either facility without damage to the other. A vertical clearance of 12 inches is, in general, considered adequate but the parties involved may agree to a lesser separation.(c)Parallel Facilities. If conditions require a cable system to be installed with less than 12 inches horizontal separation or directly over and parallel to another underground structure (or another underground structure installed directly over and parallel to a cable), it may be done providing all parties are in agreement as to the method. Vertical clearance shall be at least 12 inches except where the parties mutually agree that lesser separation will be adequate.(d) Thermal Protection. Cable should be installed with sufficient clearance from other underground structures, such as steam or cryogenic lines, to avoid thermal damage to the cable. Where it is not practical to provide adequate clearance, a suitable thermal barrier shall be placed between the two facilities.(4)Installation. (a)Trenching. The bottom of the trench receiving direct buried cable should be relatively smooth, undisturbed earth, well-tamped earth or sand. When excavation is in rock or rocky soils, the cable shall be laid on a protective layer of well-tamped backfill. Backfill within four inches of the cable shall be free of materials that may damage the cable. Backfill should be adequately compacted. Machine compaction should not be used within four inches of the cable.(b)Plowing. 1. Plowing in of cable in soil containing rock or other solid material should be done in such a manner that the solid material will not damage the cable, either during the plowing operation or afterward.2. The design of cable plowing equipment and the plowing-in operation should be such that the cable will not be damaged by bending, side-wall pressure or excessive cable tension.(c)Boring. Where a cable system is to be installed by boring and the soil and surface loading conditions are such that solid material in the region may damage the cable, the cable shall be adequately protected.(d)Depth of Burial. The distance between the top of a cable and the surface under which it is installed (depth of burial) shall be sufficient to protect the cable from injury or damage imposed by expected surface usage. 1. Minimum burial depths shall be as indicated in 220 CMR 126.35(4)(d)1. Table 1: Table 1
CONDUCTORS OR CABLES
Voltage Phase to Ground | Depth of Burial (Inches) |
600 and below | 24 |
601 to 22,000 | 30 |
22,001 to 40,000 | 36 |
40,001 and above | 42 |
2. In areas where frost conditions could damage cables, greater burial depths than indicated above may be desirable.3. When lesser depths than indicated above cannot be avoided, adequate supplemental protection shall be provided.4. Where the surface under which a cable is to be installed is not to designated final grade, the cable shall be placed so as to meet or exceed the requirements indicated above, both at the time of installation and subsequent thereto.(5)Random Separation -- Additional Requirements. The following rules apply to conductors or cables when the radial separation between them will be less than 12 inches. (a)Supply Conductors or Cables. The conductors or cables of a supply circuit and those of another supply circuit may be buried together at the same depth with no deliberate separation between facilities, provided all parties involved are in agreement.(b)Communication Conductors or Cables. The conductors or cables of a communication circuit and those of another communication circuit may be buried together and at the same depth with no deliberate separation between facilities, provided all parties involved are in agreement.(c)Supply and Communication Conductors or Cables. Supply cables or conductors and communication cables or conductors may be buried together at the same depth with no deliberate separation between facilities, provided all parties involved are in agreement and the following requirements are met: 1.Voltage. Grounded supply systems shall not be operated in excess of 22,000 volts to ground. Ungrounded supply systems shall not be operated in excess of 5300 volts phase to phase.2.Bare Grounded Conductor. A supply facility operating above 300 volts to ground must include a bare grounded conductor in continuous contact with the earth. This conductor, adequate for the expected magnitude and duration of the fault current which may be imposed, shall be, one of the following: a sheath and/or shield; multiple concentric conductors closely spaced circumferentially; a separate bare conductor in contact with the earth and in close proximity to the cable where such cable or cables also have a grounded sheath or shield not necessarily in contact with the earth. The sheath and/or shield as well as the bare conductor shall be adequate for the expected magnitude and duration of the fault currents which may be imposed. Note: This is applicable when cable in non-metallic duct is considered as a direct buried cable installation and random separation is desired.
Exception: Where a buried cable passes through a short section of conduit, such as under a roadway, the contact with earth of the grounded conductor can be omitted, provided the grounded conductor is continuous through the conduit. The bare conductor(s) in contact with the earth shall be of suitable corrosion-resistant material.
3.Ungrounded Supply Systems. Cables of an ungrounded supply system operating above 300 volts shall be of effectively grounded concentric shield construction in continuous contact with the earth. Such cables shall be maintained in close proximity to each other.(d)Multiple Cable Systems. More than one cable system buried in random separation may be treated as one system when considering clearance from other underground structures or facilities.(e)Protection. 1. Supply circuits operating above 300 volts to ground or 600 volts between conductors shall be so constructed, operated and maintained that when faulted, they shall be promptly de-energized initially or following subsequent protective device operation. (Phase-to-ground faults for grounded circuits, phase-to-phase faults for ungrounded circuits.)2. Ungrounded supply circuits operating above 300 volts shall be equipped with a ground fault indication system.3. Communication protective devices shall be adequate for the voltage and currents expected to be impressed on them in the event of contact with the supply conductors.4. Adequate bonding shall be provided between the effectively grounded supply conductor(s) and the communications cable shield or sheath. (Preferably at intervals not to exceed 1000 feet.)