High Voltage Installations Over 1,000 Volts (NEC 2023)

High-voltage questions on the Connecticut journeyman exam cover systems operating above 1,000 volts ac — the 4,160-volt switchgear lineup, the 13.8 kV feeder, and the 12,470-volt service are the standard examples. The 2023 NEC reorganized this whole territory: old Article 490 (equipment over 600 volts) became Article 495, its overcurrent rules moved to new Article 245, service/feeder/branch-circuit rules above 1,000 volts were gathered into new Article 235, and Type MV cable got its own Article 315. Because many wrong answers cite numbers that no longer exist (490.21, the old 230.200 series), the first skill tested is knowing the 2023 structure; the second is applying the working-space, vault, grounding, and isolation rules that protect people near these systems.

Learning Objectives

Explain the 2023 NEC reorganization (Articles 235, 245, 495, 315) and recognize obsolete citations such as 490.21 and the former 230.200 series.

Apply Table 110.34(A) to working-space questions using nominal voltage to ground, not phase-to-phase voltage.

State the 110.34(C) locked-room and sign requirements, including the exact sign wording.

Recall 110.31(A) vault construction rules and the automatic fire-suppression exception.

Size grounded and equipment grounding conductors over 1,000 volts per 250.184 and 250.190, and identify large grounded conductors per 200.6(B).

Apply 495.22 isolating means and 495.24 minimum space separations.

Apply 245.21 breaker and fuse rules, especially interrupting ratings covering all connected fault sources.

Apply 235.408 service short-circuit protection (3x fuse / 6x breaker) and the 315.30 qualified-person rule for MV joints and terminations.

1.1 Where the Rules Live Now

The 2023 NEC moved the over-600-volt material to a new over-1,000-volt framework (1,000 V ac / 1,500 V dc is the dividing line) and consolidated it so one set of articles covers everything above 1,000 volts:

Subject	NEC 2023	Old home
Branch circuits, feeders, services over 1,000 V	Article 235 (services in Part V)	230.200 series
Overcurrent protection over 1,000 V	Article 245	490.21
Equipment over 1,000 V	Article 495	Article 490 (490.22 isolating means = now 495.22)
Type MV cable (2,001–35,000 V)	Article 315 (new)	—
General wiring methods over 1,000 V	Article 305	—

Two consequences follow. A citation to "490.21" is automatically wrong in 2023 — the content moved to 245.21. And "Articles 125/135/145" are invented numbers that have never existed; they are pure distractor bait. Before trusting any unfamiliar section number on a high-voltage question, check it against this map.

1.2 Voltage to Ground Versus Phase-to-Phase

Two voltages describe the same system, and nearly every spacing question turns on using the right one:

Phase-to-phase voltage — measured between any two ungrounded phase conductors.

Voltage to ground — on a solidly grounded wye, phase-to-phase voltage divided by 1.732 (√3).

A 4,160-volt grounded-wye system operates at 4,160 / 1.732 ≈ 2,400 volts to ground; a 13.8 kV wye at ~7,960 V to ground; a 12,470 V wye service at 7,200 V to ground. Table 110.34(A) is keyed to voltage to ground, so the 4,160-volt lineup sits in the "up to 2,500 volts" row. The same distinction explains why a solidly grounded wye has no high-leg conductor: a high leg exists only on a 4-wire delta system whose one phase winding is center-tapped to ground, and that conductor is orange-identified (110.15) because its voltage to ground exceeds the other phases'.

1.3 Working Space — 110.34(A) and Table 110.34(A)

Working space for equipment over 1,000 volts is governed by 110.34(A) and its table, entered on nominal voltage to ground. The three conditions mirror 110.26: Condition 1 — live parts on one side, no live or grounded parts opposite (or opposite parts guarded); Condition 2 — live parts on one side, grounded parts on the other (a concrete or grounded wall facing the gear); Condition 3 — live parts on both sides.

Nominal voltage to ground	Cond. 1	Cond. 2	Cond. 3
Up to 2,500 V	3 ft	4 ft	5 ft
2,501–9,000 V	4 ft	5 ft	6 ft
9,001–25,000 V	5 ft	6 ft	9 ft
25,001–75,000 V	6 ft	8 ft	10 ft
Above 75,000 V	8 ft	10 ft	12 ft

Worked example: a 4,160-volt grounded-wye lineup operates at 2,400 V to ground, with a grounded concrete wall opposite (Condition 2). Condition 2 × the "up to 2,500 V" row = 4 ft (1.2 m). Wrong answers come from entering the table at 4,160 V phase-to-phase (pushing you into a deeper row) or grabbing Condition 1's 3 ft. Given a wye voltage, divide by 1.732 first; given voltage to ground, use it as-is.

1.4 Locked Rooms and Signs — 110.34(C)

Any building, vault, room, or enclosure containing exposed live parts or exposed conductors over 1,000 volts must have its entrance kept locked unless under the observation of a qualified person at all times, and must display a permanent, conspicuous warning sign reading exactly "DANGER - HIGH VOLTAGE - KEEP OUT". Field-applied hazard markings must also satisfy 110.21(B). The sign wording is tested literally: "CAUTION - ELECTRICAL HAZARD" and "WARNING - QUALIFIED PERSONNEL ONLY" sound reasonable but are not the required legend, and caution-style wording belongs to potential hazards, not exposed 13.8 kV parts.

1.5 Electrical Vaults — 110.31(A)

Where high-voltage equipment is enclosed in a vault, 110.31(A) requires:

Vault element	Requirement
Walls and roof	Fire-resistant, minimum 3-hour rating
Exception	1-hour rating permitted where protected by an automatic fire-suppression system
Floor in contact with earth	At least 4 in. thick, fire-resistant
Doorways	Tight-fitting doors with rating equivalent to the walls

The exam asks this as a value question: 13.8 kV switchgear and transformers go in a vault with 3-hour walls and roof unless automatic suppression earns the 1-hour exception. "2 hours" is a trap borrowed from general fire-wall practice; "4 hours" inflates the rule. Memorize the whole row — floor thickness and door rating are the details follow-up questions test.

1.6 Services Over 1,000 Volts — Article 235, 235.408

Services exceeding 1,000 V ac are covered by Article 235, Part V (relocated from the former 230.200 series; old citations like 230.204 are obsolete). The key rule, 235.408, requires a short-circuit protective device on the load side of, or integral with, the service disconnect, protecting all ungrounded conductors. Sizing works on multiples of conductor ampacity:

Fuse rated at not more than 3× the conductor ampacity, or

Circuit breaker with trip setting not more than 6× the ampacity.

These devices must also meet Article 495, Part II, and the 80-percent continuous-load restriction for enclosed devices does not apply. Check: 400 A service conductors may be protected by a fuse ≤1,200 A (3 × 400) or a breaker trip ≤2,400 A (6 × 400). The reversed pairing (6× fuse, 3× breaker) is the classic trap — fuses get the tighter 3× limit, breakers the looser 6× limit.

1.7 Overcurrent Protection Over 1,000 Volts — Article 245, 245.21

Article 245 covers overcurrent protection for systems over 1,000 V ac / 1,500 V dc; 245.21 (former 490.21) carries the device rules — breakers in 245.21(A), power fuses in 245.21(B).

For a circuit breaker (245.21(A)):

(A)(2) Operating characteristics: a manual tripping mechanism independent of control power; capability of tripping whether the breaker is opened or closed while energized; and a mechanical indicator, visible at the operating point, showing whether the main contacts are open or closed. A breaker whose only trip path needs control power, or whose contact position is hidden, fails.

(A)(4)(1) Continuous-current rating: adequate for the maximum continuous current through the breaker.

(A)(4)(2) Interrupting rating: not less than the available fault current the breaker will be required to interrupt, including contributions from all connected sources of energy — the utility transformer is never the only source; in-plant generators and motors feed the fault too.

(A)(4)(3) Closing rating: capable of closing against the maximum asymmetrical fault current, because closing can land on an existing fault.

Power fuses carry the same interrupting rule at 245.21(B)(2). The exam separates the ratings: continuous (what the device carries), interrupting (what it must break, all sources included), closing (what it must survive closing into). Answers counting only the utility contribution, or swapping in a 125-percent continuous-current figure for the interrupting rating, mix the rules up.

1.8 Isolating Means and Spacing — Article 495

495.22 — Isolating means. Equipment must be capable of being completely isolated from all its ungrounded conductors for inspection and repair. A separate isolating switch is not required where the equipment can otherwise be de-energized — a draw-out (removable) circuit breaker or removable truck panel that de-energizes all energized parts when withdrawn satisfies the requirement. Where an isolating switch is used: it is not intended to interrupt load current; if not interlocked with an approved circuit-interrupting device it must carry a warning sign against operation under load per 110.21(B); and an identified fuseholder with fuses operable as a disconnecting means may serve as the isolating switch.

495.24 — Minimum space separation. For field-fabricated installations (not designed, manufactured, and tested to accepted national standards), Table 495.24 sets minimum air separations; the rule does not apply to interior portions or exterior terminals of nationally standard-tested equipment. For the 2.4–4.16 kV nominal range:

Separation	Indoor	Outdoor
Phase-to-ground	3.0 in. (80 mm)	6.0 in. (155 mm)
Phase-to-phase	4.5 in. (115 mm)	7.0 in. (180 mm)

The value asked most often is the 3.0 in. (80 mm) indoor phase-to-ground figure; 4.5 in. is the indoor phase-to-phase value, and 6.0/7.0 in. are outdoor. Do not quote outdoor values for an indoor bus.

1.9 Type MV Cable — Articles 305 and 315

Type MV medium-voltage cable, rated 2,001 to 35,000 volts ac, is governed by Article 315 (new in 2023): use, installation, construction, and ampacities of the conductors and cable plus their joints and terminations. General wiring-method requirements for systems over 1,000 volts live in Article 305. The exam rule is 315.30: MV cable joints and terminations must be made by qualified persons who have documented training and experience in installing MV cable joints and terminations. A general license alone is not enough — a poor stress cone or contaminated termination on 13.8 kV fails in minutes. "Any licensed electrician if inspected later," "only utility employees," and "only the manufacturer's technicians" all miss the point; the code requires documented training in exactly this task.

1.10 Grounding and Conductor Identification

Four rules cluster here, and mixing them causes most errors.

250.184(A)(2) — neutral on solidly grounded systems. The neutral must have an ampacity not less than the load imposed on it AND not less than 33-1/3 percent of the phase-conductor ampacity. Worked example: 500 kcmil copper phases at 400 A ampacity on a 4160Y/2400 V system need a neutral rated ≥ 400 × 1/3 = 133 A and ≥ the unbalanced load. An exception permits a 20-percent-based sizing only in industrial and commercial premises under engineering supervision — being an industrial plant does not, by itself, unlock it. This is not Table 250.102(C)(1) sizing (that applies to grounded service conductors of systems 1,000 V or less under 250.24(D)(1)), and a full-size neutral is never required by 250.184.

250.190(C)(1) — equipment grounding conductors. An EGC run with a feeder over 1,000 volts that is not an integral part of a listed cable assembly must be sized from Table 250.122 (based on the overcurrent device rating) but not smaller than 6 AWG copper or 4 AWG aluminum. A suitable concentric shield of a listed cable assembly may serve as the EGC; a ribbon or tape shield may not.

200.6(B) — identifying the grounded conductor. Grounded conductors 4 AWG and larger may be identified by a continuous white/gray finish, three continuous white/gray stripes, or — the method most tested — a distinctive white or gray marking applied at the terminations at the time of installation that encircles the conductor. Full-length color coding is not mandatory for these large conductors at any voltage.

No phase color code exists at any voltage. The NEC never requires a particular color sequence for phase conductors of a 12,470 V service. Mandatory identification covers only specific conductors: grounded (200.6), equipment grounding (250.119), and the high-leg of a 4-wire delta (110.15 / 230.56 at the service). Only where a premises has more than one nominal voltage system must ungrounded conductors be identified by phase and system — 210.5(C) for branch circuits, 215.12(C) for feeders. And 230.41, sometimes cited as a phase-marking rule, actually requires service-entrance conductors to be insulated (with limited exceptions such as a bare grounded conductor where permitted) — an insulation rule, not identification.

Code Navigation

Over-1,000-volt systems: services in Article 235 Part V, overcurrent protection in Article 245, equipment in Article 495, Type MV cable in Article 315, wiring methods in Article 305.

Working space, locked rooms, signs: 110.34. Vaults: 110.31.

Grounding: 250.184 (system grounding/neutral sizing), 250.190 (EGCs); identification: 200.6, 250.119, 110.15, 210.5(C), 215.12(C).

Translate old numbers before answering: 490 → 495, 490.21 → 245.21, former 230.200 series → Article 235 Part V. Anything else outside the 2023 structure (125/135/145) is invented.

Common Exam Traps

Entering Table 110.34(A) with phase-to-phase voltage. A 4,160 V wye is 2,400 V to ground — divide by 1.732 first.

Conflating conditions. Condition 2 (live one side, grounded other), ≤2,500 V = 4 ft — not Condition 1's 3 ft.

Accepting any sign wording. Only "DANGER - HIGH VOLTAGE - KEEP OUT" satisfies 110.34(C).

Remembering only the 3-hour vault rating. The 1-hour exception needs automatic suppression; earth-contact floors are 4 in. thick; doors match the wall rating.

Reversing 235.408. Fuse = 3× ampacity; breaker trip = 6× ampacity.

Understating interrupting ratings. All connected sources count — utility plus in-plant generators.

Demanding a separate isolating switch. Draw-out gear and truck panels satisfy 495.22; non-interlocked isolating switches are allowed with a load-operation warning.

Using outdoor spacing indoors. Table 495.24's 3.0 in. is indoor phase-to-ground at 2.4–4.16 kV.

Sizing the neutral like a low-voltage service. 250.184 wants load + 33-1/3% of phase ampacity; Table 250.102(C)(1) belongs to 1,000 V-or-less services.

Assuming a phase color code. None exists; 110.15's orange high leg applies to 4-wire deltas, and a wye has no high leg.

Practical Field Points

Before energizing an over-1,000-volt room, verify the door locks, the DANGER sign is posted, and working-space depths still meet Table 110.34(A) — added equipment and new walls can erase clearance.

Label drawings with voltage to ground (e.g., "4160Y/2400") so everyone enters the table with the right row.

If a vault is bid with a 1-hour rating, the automatic suppression system must actually be installed and maintained — the inspector verifies it.

On large grounded conductors without continuous markings, apply the encircling white/gray marking at every termination at installation time.

Keep documented MV-splicing training records on file; 315.30 is a paper-and-skill requirement.

When servicing draw-out gear, rack the breaker to the disconnected position and verify with the mechanical position indicator plus a voltage test before touching the compartment.

Summary

High-voltage questions test the 2023 NEC's structure and its numbers together. Working-space depth comes from Table 110.34(A) using voltage to ground under the right condition (4,160 V wye at 2,400 V to ground against a grounded wall: Condition 2, 4 ft). Vaults need 3-hour walls and roofs — 1 hour only with automatic suppression — 4-inch earth-contact floors, and matching doors. Locked entrances with the exact "DANGER - HIGH VOLTAGE - KEEP OUT" legend protect exposed parts. Services are protected under 235.408 with a fuse at 3× ampacity or a breaker trip at 6×. Article 245 (245.21) demands breakers with manual trip independent of control power, a visible contact-position indicator, and interrupting ratings covering every connected fault source. Article 495 allows draw-out isolation in place of isolating switches and sets 3.0 in. indoor phase-to-ground spacing for field-fabricated 2.4–4.16 kV bus. Neutrals need load-plus-33-1/3-percent ampacity (250.184), wire-type EGCs must be at least 6 AWG Cu / 4 AWG Al (250.190), large grounded conductors may be identified at terminations (200.6(B)), MV joints need documented-qualified splicers (315.30), and no phase color code exists at any voltage. Master the article map and these rule clusters and the questions become pattern recognition.

High Voltage