8.3 Equipment Over 1000 Volts (Article 490 and Related Rules)

Why this section matters

Most journey level work is at 1000 V or less. The exam still expects you to find the high-voltage rules quickly. They are spread across Article 110 Part III, Article 300 Part II, Article 311 (medium-voltage cable, new in 2020), Article 250 Part X, and Article 490.

Article 490 at a glance

Key provisions:

• Circuit-interrupting devices (490.21): they must have an interrupting rating at least equal to the maximum available fault current and a voltage rating at least equal to the circuit's maximum voltage. Fuses that expel flame when they open must be arranged so they won't endanger people or property.
• Isolating means (490.22): there must be a way to completely isolate equipment from all ungrounded conductors. Draw-out switchgear and removable truck panels count as isolating means. Isolating switches that are not interlocked with a circuit-interrupting device need a sign warning against opening them under load.
• Metal-enclosed switchgear (Part III): energized parts are guarded, doors that could swing into energized parts need stops, and access to energized compartments is limited to qualified people.
• Mobile and portable equipment (Part IV): energized switching and control parts are enclosed in grounded metal enclosures that can be locked, and power cable connections to mobile machines are enclosed.

Article 110 Part III: installations over 1000 volts

Enclosure (110.31): electrical installations in vaults, rooms, closets, or areas surrounded by a wall, screen, or fence and controlled by lock and key are accessible only to qualified persons. An outdoor fence must be at least 7 ft (2.1 m) high, or 6 ft (1.8 m) of fence fabric plus a 1 ft (300 mm) or higher extension with three or more strands of barbed wire.

Entrance (110.33):

• At least one entrance at least 24 in. wide and 6-1/2 ft high to each working space.
• Where switchgear or control panels exceed 6 ft in width, an entrance is needed at each end, unless there is a continuous, unobstructed path of exit travel or the working space depth is doubled.

Working space depth (Table 110.34(A)):

The conditions mean the same thing as in 110.26 (Section 3.1):

• Condition 1: exposed live parts on one side and nothing live or grounded on the other.
• Condition 2: exposed live parts on one side and grounded parts (including concrete, brick, or tile) on the other.
• Condition 3: exposed live parts on both sides.

Locked rooms and warning signs (110.34(C)): entrances to rooms or enclosures containing exposed live parts over 1000 V must be kept locked unless under a qualified person's observation, with warning signs reading "DANGER - HIGH VOLTAGE - KEEP OUT."

Illumination (110.34(D)): working spaces need adequate lighting that can't be controlled only by automatic means, and the lighting may not be placed where people changing lamps are exposed to live parts.

Foreign piping (110.34(F)): pipes or ducts foreign to the electrical installation that need periodic maintenance, or could damage the equipment if they failed, may not be in the vicinity of service equipment, metal-enclosed power switchgear, or industrial control assemblies unless protection is provided.

Wiring methods and cable over 1000 volts

• Aboveground (300.37): permitted methods include RMC, IMC, EMT, RTRC, PVC, cable tray, busways, cablebus, other identified raceways, and exposed runs of metal-clad cable suitable for the use.
• Underground (Table 300.50), over 1000 V through 22 kV: at least 30 in. of cover for direct-buried cable; 18 in. for nonmetallic raceway listed for direct burial without concrete encasement; 6 in. for RMC or IMC.
• Medium-voltage cable (Article 311, 2001 to 35,000 V): in the 2020 NEC, medium-voltage conductors and cable moved out of Article 310 into Article 311, which covers shielding, insulation, and ampacity for Type MV cable.
• Grounding over 1 kV (250.180 to 250.194): covers solidly grounded neutral systems, multigrounded neutrals, and impedance-grounded systems.

Washington amendments that apply here

• Cable marking (WAC 296-46B-110): every customer-owned cable operating over 1,000 V must be legibly and permanently marked at each termination point and each point where it is accessible, showing phase, operating voltage, and circuit number if applicable.
• Service conductors over 1,000 V inside buildings (WAC 296-46B-235): limited to galvanized RMC, galvanized IMC, Schedule 80 PVC, metal-clad cable exposed for its entire length, cablebus, or busways, unless customer-owned overcurrent protection is provided outside the building.
• Concentric neutrals (WAC 296-46B-250): on existing multigrounded systems, a cable's concentric shield may continue to be used as the neutral for extensions and repairs. This requires testing and an engineer's certification. New installations need a separate listed neutral.
• Classified locations: only the AHJ or a Washington-registered PE may classify a hazardous location (WAC 296-46B-500), which also applies to high-voltage equipment rooms.