The Electrical Code

Manholes and Other Electrical Enclosures Intended for Personnel Entry Sorry about the missed Thursday. It was thanksgiving and I celebrate it like a champ. But let’s go.

110.70 General. In part 4 the code made reference to tunnels. This part we talk about manholes and the like. OSHA makes reference to this and has a certificate to the effect.

“Electrical enclosures intended for personnel entry and specifically fabricated for this purpose shall be of sufficient size to provide safe work space about electrical equipment with live parts that is likely to require examination, adjustment, servicing, or maintenance while energized. Such enclosures shall have sufficient size to permit ready installation or withdrawal of the conductors employed without damage to the conductors or to their insulation. They shall comply with the provisions of this part.

Exception: Where electrical enclosures covered by Part V of this article are part of an industrial wiring system operating under conditions of maintenance and supervision that ensure that only qualified persons monitor and supervise the system, they shall be permitted to be designed and installed in accordance with appropriate engineering practice. If required by the authority having jurisdiction, design documentation shall be provided.“

110.71 Strength.We have seen accidents televised on the news from people getting hurt or even killed by getting buried alive because the earth caves in. This makes sure that we can go into those places and safely get out with out incident.

“Manholes, vaults, and their means of access shall be designed under qualified engineering supervision and shall withstand all loads likely to be imposed on the structures.

Informational Note: See ANSI C2-2007, National Electrical Safety Code, for additional information on the loading that can be expected to bear on underground enclosures.“

110.72 Cabling Work Space. So not only do we want to install wiring and equipment in these spaces but we want to make improvements and add to these spaces as much as we can. This is were this comes into play.

“A clear work space not less than 900 mm (3 ft) wide shall be provided where cables are located on both sides, and not less than 750 mm (21∕2 ft) where cables are only on one side. The vertical headroom shall be not less than 1.8 m (6 ft) unless the opening is within 300 mm (1 ft), measured horizontally, of the adjacent interior side wall of the enclosure.

Exception: A manhole containing only one or more of the following shall be permitted to have one of the horizontal work space dimensions reduced to 600 mm (2 ft) where the other horizontal clear work space is increased so the sum of the two dimensions is not less than 1.8 m (6 ft):

(1) Optical fiber cables as covered in Article 770

(2) Power-limited fire alarm circuits supplied in accordance with

760.121

(3) Class 2 or Class 3 remote-control and signaling circuits, or both,

supplied in accordance with 725.121”

110.73 Equipment Work Space. This gives us the space need to work on the equipment installing in this space and if voltage exceeds 1000 volts it allows for the space needed for the equipment to be services in the future.

“Where electrical equipment with live parts that is likely to require examination, adjustment, servicing, or maintenance while energized is installed in a manhole, vault, or other enclosure designed for personnel access, the work space and associated requirements in 110.26 shall be met for installations operating at 1000 volts or less. Where the installation is over 1000 volts, the work space and associated requirements in 110.34 shall be met. A manhole access cover that weighs over 45.4 kg (100 lbs) shall be considered as meeting the requirements of 110.34(C).“

110.74 Conductor Installation. Pretty much the same as the last two.

“Conductors installed in manholes and other enclosures intended for personnel entry shall be cabled, racked up, or arranged in an approved manner that provides ready and safe access for persons to enter for installation and maintenance. The installation shall comply with 110.74(A) or 110.74(B), as applicable.

(A) 1000 Volts, Nominal, or Less. Wire bending space for conductors operating at 1000 volts or less shall be provided in accordance with the requirements of 314.28.

(B) Over 1000 Volts, Nominal. Conductors operating at over 1000 volts shall be provided with bending space in accordance with 314.71(A) and (B), as applicable.

Exception: Where 314.71(B) applies, each row or column of ducts on one wall of the enclosure shall be calculated individually, and the single row or column that provides the maximum distance shall be used.“

110.75 Access to Manholes. Here we need to remember anything that we install in these boxes will need to be services and at some point needs replacement. So if the manhole is installed in the middle of a busy street there will be a point where traffic will be interrupted while service is being done. Now some towns require additional service like a police escort and that is not cheap. Then you have the added expense when manholes are installed where vehicle traffic is the weight of the vehicles going over. At a minimum they need to be rated to hold 80,000 lbs. which is the weight of a commercial truck. There is an engineering code to this that like parking decks need to be built to a certain standard so do manhole cover.

“(A) Dimensions. Rectangular access openings shall not be less than 650 mm × 550 mm (26 in. × 22 in.). Round access openings in a manhole shall be not less than 650 mm (26 in.) in diameter.

Exception: A manhole that has a fixed ladder that does not obstruct the opening or that contains only one or more of the following shall be permitted to reduce the minimum cover diameter to 600 mm (2 ft):

(1) Optical fiber cables as covered in Article 770

(2) Power-limited fire alarm circuits supplied in accordance with

760.121

(3) Class 2 or Class 3 remote-control and signaling circuits, or both,

supplied in accordance with 725.121

(B) Obstructions. Manhole openings shall be free of protrusions that could injure personnel or prevent ready egress.

(C) Location. Manhole openings for personnel shall be loca‐ ted where they are not directly above electrical equipment or conductors in the enclosure. Where this is not practicable, either a protective barrier or a fixed ladder shall be provided.

(D) Covers. Covers shall be over 45 kg (100 lb) or otherwise designed to require the use of tools to open. They shall be designed or restrained so they cannot fall into the manhole or protrude sufficiently to contact electrical conductors or equipment within the manhole.

(E) Marking. Manhole covers shall have an identifying mark or logo that prominently indicates their function, such as “electric.”

110.76 Access to Vaults and Tunnels. Same as the previous description where these are larger structures and they are usually in heavy traffic area. Government wants to implement infrastructure weather it be communication, mass transit or what ever comes in the future in these area there is no discussion about that. We see this in New York city, Los Angeles, or where ever a lot of people live and work.

“(A) Location. Access openings for personnel shall be located where they are not directly above electrical equipment or conductors in the enclosure. Other openings shall be permitted over equipment to facilitate installation, maintenance, or replacement of equipment.

(B) Locks. In addition to compliance with the requirements of 110.34, if applicable, access openings for personnel shall be arranged such that a person on the inside can exit when the access door is locked from the outside, or in the case of normally locking by padlock, the locking arrangement shall be such that the padlock can be closed on the locking system to prevent locking from the outside.“

110.77 Ventilation. There are dangerous fumes that collect in these spaces. And for that reason the code feels that when ever possible there should be a ventilation system installed.

“Where manholes, tunnels, and vaults have communicating openings into enclosed areas used by the public, ventilation to open air shall be provided wherever practicable.”

110.78 Guarding. Common sense says you don’t want stuff falling into the manhole when ever possible.

“Where conductors or equipment, or both, could be contacted by objects falling or being pushed through a ventilating grating, both conductors and live parts shall be protected in accordance with the requirements of 110.27(A)(2) or 110.31(B)(1), depending on the voltage.

110.79 Fixed Ladders. This became code because yes contractors would install fixed ladders that would corrode over time. Yes not doing that right thing. Saving a dollar should not be something that we do when it can cause someone to get hurt or even die from a fall.

“Fixed ladders shall be corrosion resistant.”