3.5 Installations Over 600 Volts: Article 110 Part III, Article 490 & Medium-Voltage Cable

3.5 Installations Over 600 Volts: Article 110 Part III, Article 490 & Medium-Voltage Cable

The PSI content outline lists two sub-topics that use legacy voltage language: approved methods of installation of electrical equipment in excess of 600 volts under Electrical Installation Requirements, and services in excess of 600 volts under Services, Feeders, and Branch Circuits. The 2023 NEC no longer uses 600 volts as its dividing line — it uses 1000 volts. When an exam item says "in excess of 600 volts," find the over-1000-volt rules in your codebook.

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1. Working Space Over 1000 Volts (110.34)

The three "conditions" mean the same thing they do at 1000 volts and below, and the depths simply get larger:

• Condition 1 — exposed live parts on one side of the working space and no live or grounded parts on the other side, or exposed live parts on both sides effectively guarded by suitable insulating materials.
• Condition 2 — exposed live parts on one side and grounded parts on the other. Concrete, brick, and tile walls count as grounded.
• Condition 3 — exposed live parts on both sides of the working space, not guarded.

Two supporting rules travel with the table:

• 110.32 requires sufficient space around electrical equipment to permit ready and safe operation and maintenance, with the work space not less than 6½ feet (1.9 m) high, measured vertically from the floor or platform, or not less than the height of the equipment, whichever is greater.
• 110.34(F) repeats the 6½-foot headroom requirement for the working space about the equipment.

2. Entrances, Guarding, Illumination, and Signage

• 110.33(A) — Entrance. At least one entrance not less than 24 inches (610 mm) wide and 6½ feet (1.9 m) high must give access to the working space about electrical equipment. Where the equipment is rated 1200 amperes or more and is over 6 feet wide, an entrance is required at each end of the working space, unless an unobstructed continuous exit path or a doubled working space depth is provided.
• 110.33(B) — Access. Permanent ladders or stairways must be provided to give safe access to the working space around equipment installed on platforms, balconies, or mezzanines.
• 110.34(A) and (B) cover the working-space depth and the separation from low-voltage equipment.
• 110.34(C) — Locked rooms or enclosures. Entrances to rooms and other guarded locations containing exposed energized parts must be marked with conspicuous warning signs forbidding unqualified persons to enter — traditionally the "DANGER — HIGH VOLTAGE — KEEP OUT" placard. Under 110.21(B), such labels must be permanently affixed, of sufficient durability for the environment, and not handwritten.
• 110.34(D) — Illumination. Adequate illumination must be provided for all working spaces about electrical equipment; the Code specifically prohibits controlling that lighting by automatic means only.
• 110.34(E) — Elevation of unguarded live parts. Where live parts over 1000 volts are not guarded, they must be elevated by the distances given in Table 110.34(E), which scale with voltage.
• 110.31 requires the entire installation to be in a vault, room, closet, or enclosure accessible only to qualified persons, or surrounded by a wall, screen, or fence — with metal-enclosed switchgear treated as an approved enclosure.

[!WARNING] The over-1000-volt working space is not an academic topic for a journeyman. Approach boundaries, arc-flash energy, and the class of insulating rubber goods all change: a Class 2 glove is rated 17,000 volts AC and a Class 4 glove 36,000 volts AC. Medium-voltage work is qualified-person work under NFPA 70E, and the only lawful de-energized state is one established by a documented switching order, grounding of the isolated conductors, and a test with a properly rated instrument.

3. Article 490 — Equipment Over 1000 Volts

Article 490 applies to equipment operating at more than 1000 volts nominal. The provisions a journeyman is most likely to see on an exam:

• Circuit-interrupting devices — the article distinguishes circuit breakers, power fuses, distribution cutouts, oil-filled cutouts, load interrupters, and disconnecting means, and requires that a device be capable of interrupting the fault current available at its location.
• Isolating means with a visible break. Where a switch or circuit breaker is not capable of interrupting load, an isolating switch must be interlocked or be plainly marked to warn that it is not to be operated under load.
• Metal-enclosed switchgear and substations must be provided with barriers, interlocks, and grounding provisions so that maintenance can be performed without exposure to adjacent energized compartments.
• Mobile and portable equipment (Part IV) has its own rules for supply cables, grounding, and ground-fault detection.

4. Article 315 — Medium-Voltage Conductors and Cable

The 2023 NEC deleted Article 311 and created Article 315, Medium Voltage Conductors, Cable, Cable Joints, and Cable Terminations, covering 2,001 volts through 35,000 volts. What changed matters for the exam:

• The scope was widened from cable alone to include joints, terminations, and connectors — the field-installed accessories where medium-voltage cable most often fails.
• New listing requirements were added for MV cable joints, terminations, and connectors, phasing in on January 1, 2026.
• New shielding provisions govern the installation and the termination point, because a shielded MV cable that is terminated with the shield improperly handled concentrates electrical stress at the cutback and fails.
• Type MV cable is the wiring method itself: single- or multiconductor, rated 2001 volts and higher, permitted in raceway, direct burial where identified for the use, in cable tray where identified for the use, and in messenger-supported wiring.

5. Services Over 1000 Volts — Now Article 235

The 2023 NEC created Article 235, Branch Circuits, Feeders, and Services Over 1000 Volts ac, 1500 Volts dc, Nominal, and pulled the medium-voltage material out of Articles 210, 215, 225, and 230 into it. If you learned this material on the 2020 Code, the page you memorized has moved:

Everything in Article 230 that applies at 1000 volts and below still applies to the service; Article 235 Part V is what you add on top of it. The concepts tested at the journeyman level are:

• Warning signs and guarding. Service conductors and equipment over 1000 volts must be installed so that unqualified persons cannot contact energized parts, with the enclosure and signage rules of 110.31 and 110.34(C) applying.
• Isolating switches. Where oil switches or air, oil, vacuum, or sulfur hexafluoride circuit breakers constitute the service disconnecting means, an air-break isolating switch is required on the supply side so the disconnect can be worked on with a visible break established. The isolating switch must be interlocked with the disconnect or carry a sign warning against opening it under load.
• Disconnecting means. The service disconnecting means must simultaneously open all ungrounded service conductors and must indicate whether it is open or closed; where the disconnect is remote, a means of manual operation and an indication at the equipment are required.
• Overcurrent protection. Service-entrance conductors over 1000 volts are protected by a short-circuit protective device in each ungrounded conductor, coordinated with the supply transformer protection.
• Surge arresters. Surge protection is required at the service where the supply is exposed, and the arrester must be connected on the supply side of the disconnecting means.

For the exam, remember the boundary rather than the sub-paragraph number: Article 230 is the general service article, and Article 235 Part V is what you add when the service exceeds 1000 volts ac. Article 235 did not change the technical requirements — it relocated them, which is exactly the kind of change that separates a candidate studying the 2023 Code from one studying a 2020-based course.

6. Grounding Over 1000 Volts (Article 250 Part X)

Part X of Article 250 applies to systems and circuits over 1000 volts. Its most tested concepts are that a grounding electrode system is still required and is built from the same electrodes as a low-voltage system; that where a neutral is derived from a grounding transformer, that neutral becomes the system grounded conductor for the purpose of the Part X rules; and that portable and mobile over-1000-volt equipment must be supplied from a system with its neutral grounded through an impedance, with the frame of the equipment bonded to the ground conductor of the supply cable and ground-fault detection provided.