What do you need to know about the new Part D of Art. 370, now that the '99 NEC covers manholes for the first time?
You may end up responsible for your facility's internal distribution system, including the manholes. That's because increasing financial pressure is forcing utilities to look for ways to concentrate on their core function: distributing electric power. This is the result of regulatory reform of utility practice, which ranks as one of the more significant developments of the past decade. Now the '99 National Electrical Code (NEC) steps into this picture, with its new coverage of manholes. Here's what you need to know.
New Part D of Art. 370 melds the requirements in Sec. 323 of the National Electrical Safety Code (NESC) with those in the NEC. The result is coverage of electric enclosures that function similarly to other enclosures within Art. 370, with the exception being we work inside them. Let's look at each new section in turn, so you won't miss anything.
370-50. General. Electric enclosures intended for personnel entry and specifically fabricated for this purpose shall be of sufficient size to provide safe work space about electric equipment with live parts that is likely to require examination, adjustment, servicing, or maintenance while energized. They 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.
This is the general statement of principle; the specific requirements follow. These enclosures need to allow for safe access and working conditions (work space) around equipment that must be worked hot. They must also be large enough to allow ready installation of conductors without damage to their insulation. There is precedent for this elsewhere in the article, such as Sec. 370-16, which says boxes must have sufficient size to provide free space for all enclosed conductors (then specific requirements follow).
At times, the general statement is important to the inspection community in unusual cases where you may meet the letter of the specific requirement, yet violate the spirit of the rules. For example, if you put three No. 12 conductors and a device in a 21/2-in. device box, and leave each of the three conductors 24 in. long, you'd comply with the rule while violating the intent. The general statement gives the inspector the tool he or she needs to address the specific case.
Exception: Where electric enclosures covered by Part D of this article are part of an industrial wiring system operating under conditions of maintenance and supervision that ensure only qualified persons will 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.
This exception allows you to design manholes, in accordance with "appropriate engineering practice," that are part of appropriately supervised industrial wiring systems. The inspector has the authority to receive a copy of the design documentation, similar to the allowance in Sec. 215-5 for seeing feeder diagrams. In this case, the design criteria are in the NESC. Its provisions, in some cases, may lead to smaller sizes. Some industrial users showed concern about getting too far away from their accustomed practice.
They may have overstated the differences. When the code-making panel reviewed the cited examples and did the relevant calculations, it turned out the NEC rules led, in at least one case, to a possible smaller manhole.
370-51. Strength. 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.
FPN: See NESC, ANSI C2-1997, for additional information on the loading expected to bear on underground enclosures.
This section covers mechanical strength, which is where you'll need engineering supervision; particularly under a roadway. The NESC covers this in-depth, including diagrams, live loads, frost heaving, vehicular loads, shear and bending moments, etc. There is ample precedent in the NEC for doing this; see Sec. 90-2 (a)(1) (FPN).
370-52. Cabling work space. A clear work space not less than 3 ft (914 mm) wide shall be provided where cables are located on both sides, and not less than 21/2 ft (762 mm) where cables are only on one side. The vertical headroom shall not be less than 6 ft (1.83m) unless the opening is within 1 ft (305 mm), 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 2 ft (608 mm) where the other horizontal clear work space is increased so the sum of the two dimensions is not less than 6 ft (1.83 m):
a. Optical fiber cables as covered in Article 770.
b. Power-limited fire alarm circuits supplied in accordance with Section 760-41.
c. Class 2 or Class 3 remote control and signaling circuits, or both, supplied in accordance with Section 725-41.
This "cabling work space" is a new concept. This is not a live parts work space, as covered in Art. 110. The dimensions (3 ft with cables on both sides, otherwise 21/2 ft, and 6-ft headroom) come from the NESC. An exception (also from the NESC) reduces the minimum space to 2 ft, if the clear space in the other horizontal direction is such that the sum of the two isn't less than 6 ft. The exception applies only if limited energy wiring is all that's in the enclosure.
Note that the exception makes no mention of Chapter 8 circuits. The original proposal included Chapter 8, but the communications utilities were adamantly opposed to being regulated in this way and persuaded CMP 16 to omit correlating language from Chapter 8. You can look forward to further developments along these lines in the next cycle.
370-53. Equipment work space. Where electric 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 Section 110-26 shall be met for installations operating at 600 volts or less. Where the installation is over 600 volts, the work space and associated requirements in Section 110-34 shall be met. A manhole access cover that weighs over 100 lb. (45.4 kg) shall be considered as meeting the requirements of Section 110-34(c).
This section covers the "equipment work space." This is a live parts work space, and its requirements follow Art. 110. In cases where manholes include equipment that would be worked hot, there isn't any reason to vary from the requirements of Art. 110. Besides, it would be beyond the authority and scope of Art. 370 anyway. This could turn out to be the area where most of the existing nonconformities will show up. The NESC does not have different clearance rules for this situation, reverting to the 3 ft by 6 ft high space described in the preceding section.
370-54. Bending space for conductors. Bending space for conductors operating at 600 volts or below shall be provided in accordance with the requirements of Section 370-28(a). Conductors operating over 600 volts shall be provided with bending space in accordance with Section 370-71(a) and 370-71(b) as applicable. Where any horizontal dimension exceeds 6 ft (1.83 m), all conductors shall be cabled or racked up in an approved manner.
Exception: Where Section 370-71(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.
You use manholes essentially as large pull boxes. That's why the NEC applies the requirements for similar enclosures being used for similar purposes, with one exception. In cases over 600V where Sec. 370-71(b) applies, you must use only the single row or column that gives the largest dimension. These enclosures allow for direct personnel access, and Sec. 370-52 will ensure plenty of room after pulling. Note that per Sec. 370-28(a)(2), the single row principle automatically applies to applications 600V and below. The drawing (on page 94) shows the scenario presented to the EC&M's Code Forum department in the February 1995 issue. That discussion led directly to the new set of rules.
370-55. Access to Manholes.
(a) Dimensions. Rectangular access openings shall not be less than 26 in. 2 22 in. (659 mm 2 557 mm). Round access openings in a manhole shall not be less than 26 in. (659 mm) 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 2 ft (608 mm):
a. Optical fiber cables as covered in Article 770
b. Power-limited fire alarm circuits supplied in accordance with Section 760-41
c. Class 2 or Class 3 remote control and signaling circuits, or both, supplied in accordance with Section 725-41.
The cover dimensions come from similar requirements in the NESC. The limited-energy exception addresses the same systems covered in Sec. 370-52 Ex., and in the same way. The exception also omits coverage of Chapter 8 wiring, for the same reason.
If you need to set a portable ladder into a manhole, it'll partially obstruct the opening, impeding egress. Thus, the exception also gives you a 2-in. credit (from 26 in. to 24 in.), if you have a fixed ladder.
(b) Obstructions. Manhole openings shall be free of protrusions that could injure personnel or prevent ready egress.
Note that special requirements in OSHA regulations apply to work in such underground confined spaces, which are beyond the scope of the NEC and this article. You may end up with monitored lift-out rigging, etc. Obviously, it would defeat the purpose of those requirements if an obstruction inhibited ready egress in an emergency.
(c) Location. Manhole openings for personnel shall be located where they are not directly above electric equipment or conductors in the enclosure. Where this is not practicable, either a protective barrier or a fixed ladder shall be provided.
Clearly you'd rather have a manhole you could get into without worrying about fighting with existing electrical equipment. If you have no alternative, you can put in a fixed ladder. Be careful. This doesn't waive work space rules in Sec. 370-53 for equipment that may need to be worked hot. A fixed ladder couldn't enter that type of work space.
(d) Covers. Covers shall be over 100 lb. (45.4 kg) 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.
The 100-lb. threshold comes from Sec. 110-34(c) and Sec. 370-72(e). A circular cover of wider diameter than the access opening would never fall in due to its geometry. Other covers would need a restraining mechanism. Be sure to keep cables and equipment far enough away from the access hole so the cover won't hit anything if it tips in. For a round over, this isn't too far. For other covers, you'll have to look at the limits of your restraint mechanism.
(e) Marking. Manhole covers shall have an identifying mark or logo that prominently indicates their function, such as "ELECTRIC."
One fundamental Code principle, repeated in many different ways and sections, is the duty to properly identify equipment with an electrical function. Circular manhole covers with the appropriate marking are a standard item for vendors dealing with excavators. You can adapt others if necessary.
370-56. Access to Vaults and Tunnels.
(a) Location. Access openings for personnel shall be located where they are not directly above electric 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 Section 110-34(c), if applicable, access openings for personnel shall be arranged so 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.
The new Part D also addresses electrical vaults and tunnels adjoining buildings, in a similar way. However, this section recognizes some unique issues in these areas. For top entries, your access opening cannot be above electric equipment, even if you do provide a fixed ladder. You can, however, provide additional access points that would allow you to lift out equipment for maintenance or repair.
This section also recognizes these vaults may have a doorway into an occupied space. For medium-voltage applications, Sec. 110-34(c) requires locked access unless the area is under continuous, qualified supervision. To meet this requirement in this case, however, you need to select and arrange the locking mechanism so someone can't be locked into the room.
Nothing in this section contradicts or circumvents the rules in Part C of Art. 450 on transformer vaults. If you have large transformers in these areas that otherwise require a vault per Art. 450, then you need to ensure compliance. Sec. 450-43 requires the communicating doorways be of a 3-hr rating (unless sprinklered). It also requires a 4-in (min.) curbing, and panic hardware on the locking mechanism to allow the door to open without using your hands.
370-57. Ventilation. Where manholes, tunnels, and vaults have communicating openings into enclosed areas used by the public, ventilation to open air shall be provided wherever practicable.
Here again, the Code recognizes unique issues relative to vaults that open to occupied space. If possible, arrange ventilation so that, for example, the products of combustion from a failure in the vault can vent to the outdoors. This is similar to the rule in Sec. 450-41 on transformer vaults. Again, in case your vault is additionally one required to meet Part C. of Art. 450, be sure to review additional ventilation rules in Sec. 450-45.
370-58. Guarding. 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 Section 110-27(a)(2) or 110-31(a)(1), depending on the voltage.
Product standards incorporate these rules as part of the listing. However, there are still many items of medium-voltage electrical equipment that aren't routinely listed. You may get involved in a field determination as to whether a "ventilating grating" is arranged so an inserted object would be [quoting Sec. 110-31(a)(1)] "deflected from energized parts."
370-59. Fixed Ladders. Fixed ladders shall be corrosion resistant.
Many times manholes are extremely wet and left unattended for years at a time. Some important safety aspects of these as well as OSHA requirements depend on a fixed ladder. A ladder that collapses due to corrosion could be life threatening. The panel considered requiring nonmetallic ladder construction for this reason. However, the NESC doesn't require this; only corrosion resistance. The panel decided in the first go-around to limit, as much as possible, any deviations from prior NESC practice.
The future. This new part of Art. 370 turned out to be one of the most controversial actions in the 1999 cycle. For example, CMP 1 tried to wrest control of the material to kill it, and only a single vote (5-4) on the Correlating Committee left it in Art. 370. Undoubtedly, this new part of Art. 370 will be modified as working electricians come to use it.
When we (August 1997 issue) called attention to this proposal, we got several letters from individuals who very clearly had extensive field expertise in this area. They wondered why other rules weren't included. The reason was simple: It became clear that any further muddying of the water the first time out could doom the entire initiative. Those proposals will be welcome in the next cycle.