This article is a continuation of designing and installing electrical systems rated over 600 volts. Note that specific equipment as well as wiring methods and other items that form a complete high-voltage system are discussed.
Selecting switchgear—490.30 through 490.47
Metal-enclosed power switchgear used for feeder circuits and equipment must be determined by the requirements outlined in Part III to Article 490 of the National Electrical Code (NEC).
Make note of 490.40 that requires an inspection window so that maintainers can actually see that the switching devices and blades are open and disconnection of the supply voltage is accomplished.
The Exception to 225.51 for feeders permits the use of indicating lights or a computer screen to verify that the supplying voltage has been disconnected and the equipment is safe to work on. Electrical equipment housing high-voltage components must be firmly secured to the surface on which they are mounted and all openings closed per 110.12(A) and 13(A). Section 408.17 prohibits open-bottom switchgear to be mounted over combustible material. Conductors connecting to the terminals of high-voltage switchgear are 90°C as permitted by 110.40.
Selecting ampacities—Table 310.77
Consider six 90°C, 500 kcmil underground electrical ducts rated at 12.8 kV and run from a substation to a metal-enclosed power switchgear located in a small industrial facility. Referring to Figure 310.60, Detail 3 illustrates such a duct system, and Table 310.77 can be used to determine the ampacity of the conductors. For example, six 500 kcmil, 90°C, conductors connected in parallel are listed in Table 310.77 as having a total ampacity of 1,740 amps (290A × 6 = 1,740A).
Means must be provided to completely isolate high-voltage components that require servicing or repairing. Isolation switches are not necessarily required where there are other methods of de-energizing the items or equipment for inspection and repairs. Such items, for example, could be a draw-out-type metal-enclosed switchgear unit and removable truck panel. Consider conditions where isolating switches are not interlocked with an approved circuit-interrupting device; for safety, field electricians must post a sign warning against opening under loaded situations. Installers applying these requirements may use fuseholders and fuses specifically designed as an isolation switch.
Sizing of pull and junction boxes—314.71(A) and (B)
There are times when pull or junction boxes must be installed because of long runs and 90-degree bends in raceway systems. Such installations require adequate space to prevent overfilling and damaging the conductor’s insulation.
The box length must be at least 48 times the outside diameter of the over sheath and have the largest shield or lead-covered conductor or cable entering the box. For nonshielded conductors or cables, the box length must be at least 32 times the outside diameter of the larger nonshielded conductor or cable entering the enclosure.
Angle or U pulls—314.71(B)(1)
For angle or U pulls, different requirements must be applied to size the pull or junction box. For example, the distance between each cable or conductor entry inside the box and opposite wall must be at least 36 times the outside diameter, over sheath, of the largest cable or conductor plus the sum of any additional cables or conductors on the same wall of the box.
The Ex. 2 to 314.71(B)(1) permits the 36 times length of nonshielded and nonlead covered type to be reduced to 24 times as outlined in 314.71. Designers and installers must also review the distance between entry and exit requirement per 314.71(B)(2).
Warning labels—110.34(C) and 314.72(E)
Covers on boxes enclosing high-voltage cables or conductors must be securely fastened in place. Additionally, they must be permanently marked on the outside cover with a label that reads, “Danger—High Voltage—Keep Out.” This marking must be visible with letters of the block type at least ½ inch in height. The high-voltage workspace and guarding requirements of 110.34 requires this type of label at locked rooms or equipment enclosures. Section 110.16 requires the field electrician to provide a label on the equipment to warn of an electrical arc that could be dangerous to maintenance personnel.
Grounding requirements—250.184 and 490.36
Metal frames of switchgear must be grounded with an equipment-grounding conductor or where Code-permitted, the grounded conductor can be used for this purpose. If a single-point grounded neutral system is used, apply 250.184 (A)(2) and (B) of the 2008 NEC.
Hopefully, this information, as well as the information in last’s month’s column, will be beneficial to anyone designing or installing a high-voltage electrical system, using the NEC as a guide.
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