Direct –earth burial conductors must be laid in the trench with uniform spacing (about 150mm); without crossovers; slightly ”snaked” to provide slack for normal earth settlement or frost heave; and have a cushion and cover of sand or screened fill as protection against sharp objects in the trench and backfill. Packing of the conductors with a soft fill such as sand will also improve the heat dissipation, which helps prolong the service life of a conductor. Where future digging might occur, mechanical protection in the form of treated planks or concrete slabs over the soft fill should be added. Plastic warning marker strips are effective in alerting a digging crew of the presence of the underground electrical services. Raceway must be used to protect conductors that are run under buildings and is often specified for installations under roadways, runways, and railroad tracks. When direct buried conductors or cables exit the ground and rise up a p[ole or building, they must be protected with the way extending down to the minimum cover distance below grade(but do not have to extend lower than 450mm below grade), up to no less than 2400mm above grade. If the raceway is exposed to physical damage, the rigid conduit , IMC, PVC, Schedule 80 or equivalent must be used. A seal or bushing must be used where the cables leaves the conduit. Direct-buried conductors are permitted to be spliced using listed materials and approved methods without being in a box. Limitations. Conductors for circuits of different voltages up to 600V may be run in a common raceway provided all conductors have insulation rated not less than the maximum voltage of any circuit in the enclosure, cable, or raceway. However, mixing of control and power conductors for different motors in a common raceway is limited to instances where the motors are functionally associated as integral parts of a machine or process. If the motors in question are serving separate, individual loads ( with no interconnection of their control circuits and no mechanical interlocking), than a separate raceway must be used. The code prohibits installation of any wiring method in ventilating ducts that are used for removing dust, particles, flammable vapors, or cooking odors and any shaft that is used exclusively for this type of duct work. Installation of wiring and of any equipment in prefabricated ducts and plenums that are used for the supply and return of environmental air is limited to that which is “necessary” for operation of that system. Hung ceilings used for the supply and return of environmental air may contain “other” electrical equipment, such as switches, starters, motors, etc.’ provided wiring materials and equipment construction are suitable for the ambient temperature to which they are exposed. Cables installed in hung ceilings must be supported as if they were installed in the open. They may not be treated as they were run through closed-in building spaces or fished through hollow spaces in masonry block. Support requirements given in the NEC for the particular cable must be followed. In circuits made up of paralleled single conductors, all conductors that make up a given phase must be the same length, conductor material, size and insulation, and use the same terminating device. Additionally, if the conductors are not run together in the same raceway or cable, then the raceway or cable must have the same physical characteristics. When run in metallic conduit, the code requires that all the phase conductors, and neutral if any, of a circuit be run in the same enclosure or raceway to avoid induction heating of the metal. Because sheet steel is a magnetic metal, passing single conductors of an AC circuit individually through the wall of such a box must be avoided to reduce hysteresis. When it is impossible to pass all cables of the circuit through one opening, slots should be cut between the individual openings or the cable should exit through a nonmetallic insulating plate that is large enough to accommodate all cables. Where more than three conductors occupy the same trench or raceway, ensure that conductor ampacities are properly derated in accordance with Note 8 to the NEC ampacity tables. Where large currents are involved, it is especially important that the phase conductors be located close together to avoid excessive voltage drop and ensure equal division of current. Also, the neutral and grounding wires, if any must be run with the same conductors, even if a nonmetallic raceway is used. Splicing and terminating a cable or conductor involves three basic operations: removing sheath and insulation to prepare the bare conductor(s); applying the connector or lug to provide a mechanically solid and electrically continuous connection; and covering with insulating material that will match the dielectric, moisture-resistant, heat resistant, and mechanical-protection characteristics of the original insulation. Splice kits tailored to a specific conductor size, insulation, voltage cable outer sheath construction, and type of splice or termination are available. Generally, they provide all items required to make a finished splice or termination plus detailed instructions. Ensure that conductor and lugs or terminals are suitable for use together. Terminals on 15A and 20A receptacles that are unmarked are for use with copper. Those marked “CO/ALR” may be used with copper and aluminum conductors. Lugs without any markings are for use only with copper conductors. Those marked “AL-CU” or “CU-AL” may be used with copper or aluminum conductors. One of the major causes of termination failure is improperly torqued connections. Always use recently calibrated torque-wrench or screw driver and tighten lugs and connectors to the value given in the manufacturer’s instructions. Torque –wrenches are intended to be pulled, not pushed. Pushing instead of pulling can give a false reading of actual torque value, leaving the lug improperly torqued.
Posted on: Sun, 24 Aug 2014 10:50:50 +0000
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