The electrical industry is rapidly changing. Utility companies are selling high-voltage equipment, including transformers and distribution equipment, to end-users as part of divestiture agreements.
End-users are making deals with utility companies for purchase of higher voltage to be delivered to the property to lower their overall energy costs and provide better control over their power. These ownership changes require equally rapid electrical standards changes to keep abreast of the changing market and technology.
The new 2002 National Electrical Code (NEC) has more coverage of circuits over 600 volts than ever. Existing high-voltage installation requirements have been placed in appropriate articles and new requirements have been added within articles.
Even the definition in Article 100 of qualified person has been changed. A qualified person is one who has skills and knowledge related to the construction and operation of the electrical equipment and the electrical installation. Electrical personnel must also receive safety training related to any hazards involved in either the electrical equipment or its installation.
Section 110.31(A) has been added to cover the fire resistivity of electrical vaults containing conductors and equipment that exceed 600 volts. The walls, floors, roof, and doors in high-voltage electrical vaults must be constructed of materials that have adequate structural strength for the conditions and must have a minimum fire rating of three hours. The floors of a vault in contact with the earth (at grade level) must be 4-inch-thick concrete.
A vault constructed with a space located directly below it (other than at grade level) must have a floor constructed of materials and with methods to ensure adequate structural strength for any load imposed on the floor. The floor must have a fire resistance of three hours.
Studs and wallboard of any construction combination is not an acceptable means of constructing an electrical vault for compliance with this section. This information is also located in Section 450.42 and applies to transformers that are required to be installed in a transformer vault.
Section 210.6 has a new subsection (E) covering branch circuit voltage limitations for over 600 volts between conductors, where, previously, it only covered 600-volt and lower circuit limitations. Circuits that exceed 600 volts between conductors are permitted to supply electrical utilization equipment for installations where equipment maintenance and supervision ensures that only qualified personnel will service the installation.
Section 210.19 has been revised to cover the minimum ampacity and size of branch circuit conductors for both 600-volt and lower in 210.19(A) and for over 600-volt circuits in 210.19(B). It previously only covered branch circuits operating at 600 volts or less. The ampacity of over 600-volt conductors will be based upon either Section 310.15 for conductors operating up to 2,000 volts and Section 310.60 for those operating at 2001 volts up to 35,000 volts. Section 215.2(B)(1) has a similar requirement for feeders operating at over 600 volts.
Section 210-19(B)(1) requires the ampacity of high-voltage branch-circuit conductors to be sized at not less than 125 percent of the designed potential load of all electrical that is operated simultaneously.
For supervised installations, Section 210.19(B)(2) permits branch-circuit conductor sizing to be determined by qualified persons under engineering supervision.
This section further defines a supervised installation as those portions of a facility where conditions of design and installation are provided under engineering supervision. The supervised installation must also have qualified persons with documented training and specific experience in over 600-volt systems to provide maintenance, monitoring, and servicing of the system.
The key words in the last sentence are “documented” training and “experience” in high-voltage equipment. A similar requirement for supervised installations has been made for feeder circuits operating at over 600 volts in Section 215.2(B)(3).
For feeders supplying transformers only, at over 600 volts, Section 215.2(B)(1) requires the ampacity of the feeder conductors to be not less than the sum of the nameplate ratings of the transformers. This is a major difference between those transformers at less than 600 volts and those at over 600 volts. For transformers operating under 600 volts, the conductors must only be sized based upon the load on the transformer, not the nameplate ratings.
Section 215.2(B)(2) covers the ampacity of feeder conductors of over 600 volts supplying a combination of transformers and electrical utilization equipment. Conductor ampacity must not be less than the sum of the nameplate ratings of the transformers plus 125 percent of the designed potential load of the utilization equipment that may be operated simultaneously.
Only a few 2002 NEC changes involve circuits of over 600 volts. If you are involved in installing or maintaining circuits and equipment operating at over 600 volts, look for the marginal lines adjacent to these appropriate high-voltage sections.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at email@example.com.