A faithful reader of ELECTRICAL CONTRACTOR asked how to design and install high-voltage feeders according to the 2008 National Electrical Code (NEC). He requested that I write an article pertaining to such an installation. He wanted to see Code validation of the requirements so that he could verify what the NEC had to say about these systems in cases where they are considered feeders.
Calculating the load
(B)(1) of 215.2 requires high-voltage feeder conductors supplying transformers to have at least the ampacities of the sum of the nameplate ratings of the transformers supplied by such feeders. (B)(2) of 215.2 requires the ampacity for combination of transformers and utilization equipment to be at least the sum of all transformer ratings plus 125 percent of the potential load of the utilization equipment that could operate simultaneously.
(B)(3) of 215.2 recognizes feeders designed under supervised conditions to have the ampacity of conductors determined by qualified people under engineering supervision. (B)(3)(1) recognizes and accepts conditions of design and installation under the supervision of sound engineering. (B)(3)(2) requires qualified people to have documented training and experience in systems of more than 600 volts before they can service, maintain and monitor such systems.
Permanently installed solid dielectric conductors that operate at more than 2,000 volts must be equipped with ozone-resistant insulation and be of the shielded type containing a grounded metallic shield that is properly earth-grounded.
High-voltage stresses are reduced, and corona-forming problems are prevented, by adding a metal tape or nonmetallic semiconducting tape around the conductor surface. For example, a corona appears as a glow adjacent to the surface of the conductor. Because of moisture, high-voltage stresses and charging current flow between the conductor and ground, creating ionized and ozone conditions. With the shield earth grounded properly, no voltage above ground will be present on the jacket outside the shield. Therefore, with no discharge from the jacket, dangerous ozone formations do not occur.
Exception 1 to 310.6 has been revised and limits the omission of shielding only up to the 2,400-volt level.
Grounding the shields
Ampacities outlined in Tables 310.69, 310.70, 310.81 and 310.82 are for cables with shields grounded at only one point. Such cables are usually grounded at the supply end.
Note that, where cable shields are grounded at more than one point, the ampacities must be adjusted to compensate for the heating that might be produced by the shield current. Short runs of cables usually are grounded at one end, if specifications are not violated. However, long runs of cables may need to be grounded at more than one or two points to reduce stress.
Preparing the insulation and shield
Always prepare high-voltage conductors by the manufacturer’s instructions. Note that for long operation, the treated braid covering must be stripped back a safe distance at the conductor terminals based on the voltage. Metallic and semiconducting insulation shielding components of shielded cables must be removed for a distance dependent on the circuit voltage and insulation. Stress reduction means must be provided at all terminations of factory-applied shielding. So, where it is practicable, the above distance must not be less than 1 inch for each kilovolt of the conductor-to-ground voltage of the circuit. Metallic shielding components, such as tapes, wires, braids or combinations thereof, must be connected to a grounding conductor, grounding busbar or a grounding electrode.
Bending radius of conductors
To prevent damaging the shielding and the insulation, the conductor must not be bent to a radius less than eight times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation procedures.
Consideration must be given for multiconductor or multiplexed single conductor cables that are equipped with individual shielded conductors. For example, the minimum bending radius must be 12 times the diameter of the individual shielded conductors or seven times the overall diameter, whichever calculates the greater for these cables.
Wiring methods above ground
Conductors installed above ground must be routed in RMC, IMC, EMT, RNC, cable trays, busways, cablebus, other identified raceways or exposed runs of metal-clad (MC) cable that is suitable for such use. Note that medium-voltage cable under specific conditions of use also can be used. When busbars are used, they may be either copper or aluminum.
Wiring methods underground
(A)(1) covers Type MC and moisture-impervious metal sheath cables, and such shields must be grounded per 250.4(A)(5) or (B)(4). (A)(2) requires other nonshielded cables outlined in 300.50(A)(1) must be installed in RMC, IMC or RNC encased in at least 3 inches of concrete.
Next month’s column will continue this discussion.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.