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Sizing manholes.

Feb 1, 1995 12:00 PM

We were asked to comment on an installation similar to the one pictured in the drawing. Half of the conductors coming in from the24-duct bank turn out toward the reader, and the other half go the other way. In other words, the manhole is being used as a verylarge tee. Note that with the voltage over 600V, traditionally this type of work has belonged to the electric utilities. In this case,however, the service point is on the line side of this installation and the work appeared to be all premises wiring covered by theNEC.

The EC&M Panel's analysis

We think that the installation is squarely within the scope of the NEC as covered in Sec. 90-2(a)(1) and (3). The fact that thevoltage is over 600V is irrelevant to whether or not it is within the scope of the NEC. Nevertheless, the NEC does not include all theanswers. A manhole is unique in that, quite clearly, personnel must be able to enter and work within its confines. This isn't true ofany other pull box, and the NEC does not include provisions that directly address this point, at least in terms of dimensional size.For example, Sec. 110-12(b) requires accessibility to be maintained, but without giving any minimum dimensions. As with the

racking requirement in Sec. 370-28(b) (generalized to medium voltage by Sec. 370-70), this requirement applies at all voltages. Ifthere were terminations that must be worked hot, however, then the installation would need to comply with the clearances given inTable 110-16(a) or Table 110-34(a) as applicable.

If, as in the case at issue, there is nothing that would trigger workspace requirements, then there is no minimum dimension limit inthe NEC that directly applies to workspace access. There is, however, a workspace rule in the National Electrical Safety Code(NESC) that does apply. Sec. 323 of that code covers, in great detail, the required strength, dimensions, access dimensions andlocations, cover supports, ladder requirements, drainage, ventilation, mechanical protection, and identification of manholes. Wethink that the requirements in this section should be applied in concert with those in the NEC, as indicated in Sec. 90-2(a)(1)(FPN):

(FPN): For additional information concerning such installations in an industrial or multibuilding complex, see the National ElectricalSafety Code, ANSI C-2-1990.

Minimum workspace

The NESC, in Part B of Sec. 323, reads in part as follows:

B. Dimensions

Manholes shall meet the following requirements: A clear working space sufficient for performing the necessary work shall bemaintained. The horizontal dimension of the clear working space shall be not less than 3 ft (900 mm). The vertical dimensions shallbe not less than 6 ft (1.80 m) except in manholes where the opening is less than 1 ft (300 mm), horizontally, of the adjacent interiorside wall of the manhole. EXCEPTION 1: Where one boundary of the working space is an unoccupied wall and the oppositeboundary consists of cables only, the horizontal working space between these boundaries may be reduced to 30 in. (750mm).

Remember, the idea is to apply the NESC in concert with the NEC, not to replace the NEC. For example, in cases where NECclearance rules apply, they should be applied because those rules are generally more strict. This is appropriate. The NESC isdesigned to be used by highly qualified and supervised utility personnel. Its provisions are generally less strict than the NEC.We think, however, that in instances such as this, where the NEC is silent as to required work spaces, the NESC should beconsulted. This particular NESC rule fits quite well with NEC provisions and we think it can and should be enforced. As we willshow, however, in this case there probably will be plenty of room at the unoccupied end of the manhole.

Cable bends

With regard to required spaces for routing conductors, it is the NESC that is silent. For example, Sec. 341-A-1 of the NESCmerely requires "Bending of the supply cable during handling, installation, and operation shall be controlled to avoid damage."There is no specific dimension rule.

On the other hand, Sec. 370-71(b) of the NEC is very specific. A box used for an angle pull, such as the, one shown here,must meet the following dimensions:

(b) For Angle or U Pulls. The distance between each cable or conductor entry inside the box and the opposite wall of the box shall

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not be less than thirty-six times the outside diameter, over sheath, of the largest cable or conductor. This distance shall beincreased for additional entries by the amount of the sum of the outside diameters, over sheath, of all other cables or conductorentries through the same wall of the box.

The distance between a cable or conductor entry and its exit from the box shall be not less than thirty-six times the outsidediameter, over sheath, of that cable or conductor.We think that if this type of requirement is appropriate for bending allowances in the traditional sheet-metal pull box, there is nojustification for treating a manhole differently, where it is used for the same purpose. The only difference would be to recognize

personnel access and, in doing so, to additionally check that the NESC minimums were observed.

In this case, however, the strict application of this rule leads to a grotesque result that we find difficult to support. Using the formulain the rule based on 2 in. (1/6 ft) diameter cable we end up (rounding up 2-in. to the nearest ft) with a 18-ft by 12-ft manhole (36 x1/6 ft +71 x 1/6 ft [approximately equal to] 18 ft; 36 x 1/6 ft +35 x 1/6 ft [approximately equal to] 12 ft). We note that this sectiondoes not correlate with Sec. 370-28(a)(2) for 600V installations and below, which was changed to require considering only thelargest single calculated row instead of all the entries on one wall of the box.

If that principle were carried over to this installation, the result would be large but more manageable. If the concept of the largestrow also includes the largest column, we would end up (again after rounding up 2 in. to the nearest ft) with a 9-ft by 8-ft manhole(36 x 1/6 ft +17 x 1/6 ft [approximately equal to] 9 ft; 36 x 1/6 ft +11 x 1/6 ft [approximately equal to] 8 ft). We think the AuthorityHaving J urisdiction should be consulted to see whether a waiver in this direction might be entertained. Certainly one could pointout the inconsistency with Sec. 370-28(a)(2). Otherwise, there is no alternative to arranging a custom, palatially sized manhole.

Note also, that for medium voltage installations such as this one, the warning-label requirement in Sec. 370-72(e) will apply. The"DANGER HIGH VOLTAGE - KEEP OUT" marking must be added to the manhole cover, in addition or in place of the familiar"ELECTRICAL" marking (Sec. 323-J of the NESC requires this identification but not the warning) on such covers. We are awarethat this is seldom done; however, this is in the same subsection that deals with typical manhole covers, recognizing that theirweight alone secures them to their enclosures.