3.3 Over-1000-Volt Installations, Guarding & Manholes (Article 110, Parts III–V)
Key Takeaways
Outdoor over-1000 V installations accessible to unqualified persons need a fence at least 7 ft high, or a 6 ft fence topped with at least 1 ft of three or more strands of barbed wire (110.31).
Table 110.34(A) requires 4 ft, 5 ft, and 6 ft of working space depth for Conditions 1, 2, and 3 at 2501 to 9000 volts.
Rooms and enclosures containing exposed live parts over 1000 V must be kept locked unless under the observation of a qualified person, and must carry permanent DANGER warning signs (110.34(C)).
A round manhole access opening must be at least 26 in. in diameter, and a rectangular opening at least 26 in. by 22 in. (110.75(A)).
Manhole covers must weigh over 100 lb or require tools to open, and must carry an identifying mark or logo showing electrical use (110.75(D) and (E)).
Where the over-1000-volt rules live in the 2026 NEC
Journeyworkers install switchgear, transformers, and feeders at 4160 V, 13.2 kV, or higher on campus and industrial sites. Article 110 still contains the general rules:
- Part III: over 1000 V ac, 1500 V dc, nominal: enclosures, working space, guarding, and terminations.
- Part IV: tunnel installations over 1000 V.
- Part V: manholes and other enclosures intended for personnel entry, at any voltage.
The 2026 NEC moved the other over-1000-volt material into a dedicated group of articles:
| Article | Covers |
|---|---|
| 245 | Overcurrent protection for systems over 1000 V ac, 1500 V dc |
| 265, 266, 267 | Branch circuits, feeders, and outside branch circuits and feeders over 1000 V (formerly Article 235) |
| 268 | Services over 1000 V |
| 270 | Grounding and bonding of systems over 1000 V |
| 305 | General wiring methods and materials over 1000 V |
| 315 | Medium-voltage (Type MV) conductors, cable, cable joints, and terminations |
| 495 | Equipment over 1000 V ac, 1500 V dc |
Enclosing high-voltage installations (110.31)
Electrical installations in a vault, room, closet, or area surrounded by a wall, screen, or fence with access controlled by lock and key are considered accessible to qualified persons only.
| Situation | Requirement |
|---|---|
| Outdoor fence | At least 7 ft high, or a 6 ft fence plus 1 ft or more of three or more strands of barbed wire (or equivalent) |
| Fence clearance to live parts | Table 110.31: 10 ft for 1001 to 13,799 V; 15 ft for 13,800 V to 230 kV; 18 ft over 230 kV |
| Indoor, accessible to unqualified persons | Use metal-enclosed equipment; openings in ventilated dry-type transformers must deflect inserted objects away from energized parts |
| Outdoor, accessible to unqualified persons | Metal-enclosed equipment; doors or hinged covers locked, and bolts or screws on covers not readily removable |
Working space over 1000 volts (110.32 to 110.34)
Section 110.34(A) requires a working space at least 6½ ft high and 3 ft wide (or the equipment width) in front of equipment likely to need examination, adjustment, servicing, or maintenance while energized, with enough room for doors and hinged panels to open at least 90 degrees. Depth comes from Table 110.34(A), using the same three conditions as Table 110.26(A)(1):
| Nominal voltage to ground | Condition 1 | Condition 2 | Condition 3 |
|---|---|---|---|
| 1001 to 2500 V | 3 ft | 4 ft | 5 ft |
| 2501 to 9000 V | 4 ft | 5 ft | 6 ft |
| 9001 to 25,000 V | 5 ft | 6 ft | 9 ft |
| 25,001 V to 75 kV | 6 ft | 8 ft | 10 ft |
| Above 75 kV | 8 ft | 10 ft | 12 ft |
Condition 1 has no live or grounded parts across the space; Condition 2 has grounded parts (including concrete, brick, or tile walls) across the space; Condition 3 has exposed live parts on both sides. Working space is not required in back of equipment, such as dead-front switchgear, where there are no renewable or adjustable parts and all connections are accessible from other than the back.
Entrances (110.33)
At least one entrance 24 in. wide and 6½ ft high must give access to the working space. Where switchgear or control panels are more than 6 ft wide, an entrance is needed at each end, unless the location permits a continuous and unobstructed way of exit or the working space is doubled. Personnel doors less than 25 ft from the nearest edge of the working space must open in the direction of egress and have listed panic or fire exit hardware.
Locking, signs, and lighting (110.34(C) and (D))
- Buildings, vaults, rooms, or enclosures containing exposed live parts or exposed conductors over 1000 V must be kept locked unless under the observation of a qualified person.
- Permanent, conspicuous signs must read DANGER — HIGH VOLTAGE — KEEP OUT, following 110.21(B).
- Lighting outlets must be arranged so that people changing lamps or repairing the lighting system are not endangered by live parts, and the lighting controls must be located so people do not contact live parts when operating them.
Elevation of unguarded live parts (110.34(E))
| Nominal voltage between phases | Minimum elevation |
|---|---|
| 1001 to 7500 V | 9 ft |
| 7501 V to 35 kV | 9 ft 6 in |
| Over 35 kV | 9 ft 6 in plus 0.37 in. per kV above 35 kV |
Section 110.34(F) also keeps pipes and ducts foreign to the electrical installation away from service equipment, metal-enclosed power switchgear, and industrial control assemblies, or requires protection against leaks and condensation.
Pull and junction boxes over 1000 volts
The pull and junction box rules for systems over 1000 V (Part IV of Article 314 in the 2023 NEC; the 2026 NEC gathers over-1000 V wiring method and material rules in new Article 305) size boxes by cable diameter, not raceway trade size:
| Pull | Minimum box dimension |
|---|---|
| Straight pull | Length at least 48 times the outside diameter, over the sheath, of the largest shielded or lead-covered conductor or cable; 32 times for the largest nonshielded conductor or cable |
| Angle or U pull | Distance from each cable entry to the opposite wall at least 36 times the outside diameter of the largest cable, increased by the diameters of other entries through the same wall; distance between the entry and exit of the same cable at least 36 times its outside diameter |
Box covers must be permanently marked "DANGER — HIGH VOLTAGE — KEEP OUT" on the outside, in block letters at least ½ in. high. Example: the largest shielded cable entering a straight-pull box has an outside diameter of 1.5 in., so the box must be at least in. long.
Manholes and other enclosures for personnel entry (Part V)
Part V applies to manholes, vaults, and similar enclosures that people enter to install, operate, or maintain equipment and conductors, at any voltage.
| Rule | Requirement |
|---|---|
| Strength (110.71) | Designed under qualified engineering supervision to withstand all expected loads |
| Cabling work space (110.72) | At least 3 ft wide where cables are on both sides, or 2½ ft where cables are on one side only; at least 6 ft of vertical headroom unless the access opening is within 1 ft horizontally of the adjacent interior side wall |
| Equipment work space (110.73) | Where equipment with moving parts needs servicing, provide working space per Table 110.26(A)(1) or Table 110.34(A) |
| Access openings (110.75(A)) | Rectangular openings at least 26 in. × 22 in.; round openings at least 26 in. in diameter; an exception allows a 24 in. round opening in specific cases, such as a manhole with a fixed ladder that does not obstruct the opening |
| Obstructions and location (110.75(B), (C)) | Openings may not be obstructed by objects that would inhibit ready exit, and should not be directly above electrical equipment or conductors where possible |
| Covers (110.75(D), (E)) | Covers must weigh over 100 lb or require tools to open; they must be restrained so they cannot fall into the manhole or onto energized parts, and must carry a mark or logo showing electrical use |
| Guarding (110.78) | Conductors and equipment that could be contacted by objects falling or being moved through the opening must be protected |
| Fixed ladders (110.79) | Must be corrosion resistant |
Warning
A manhole is also a permit-required confined space under OSHA rules. Atmospheric testing, ventilation, attendants, and rescue planning come from OSHA and NFPA 70E, not the NEC.
Worked example
A 12.47 kV switchgear lineup faces a concrete block wall. The voltage to ground of a 12.47Y/7.2 kV system is 7200 V, which falls in the 2501 to 9000 V row. A concrete wall is grounded, so the space is Condition 2, and Table 110.34(A) requires a working space depth of 5 ft. If a second lineup with exposed live parts faced it instead, Condition 3 would require 6 ft.
Outdoor 13.2 kV equipment is enclosed by a fence to keep unqualified persons out. Which fence meets 110.31?
A 5 ft fence with 2 ft of barbed wire on top
A 6 ft fence topped with 1 ft of three barbed-wire strands
A 6 ft chain-link fence with no barbed wire on top
Any fence with a locked gate, regardless of its height or topping
Switchgear operating at 4160 V to ground has exposed live parts on one side of the working space and a concrete wall on the other. What working space depth does Table 110.34(A) require?
5 ft
3 ft
4 ft
6 ft
What is the minimum diameter of a round access opening for a manhole under 110.75(A), when the exception for a smaller opening does not apply?
22 in.
24 in.
26 in.
30 in.
Sections you finish are checked off in the contents.