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Unusual design issues involved in service change, part 2.

A small multifamily dwelling service change uses some innovative construction techniques, some of which could be used in industrial applications.In addition to some unusual design requirements covered in the last issue, the service change involved some unique construction techniques to meet those requirements. Some of these techniques can be easily adapted for other, non-residential applications.

A small multifamily dwelling service change uses some innovative construction techniques, some of which could be used in industrial applications.

In addition to some unusual design requirements covered in the last issue, the service change involved some unique construction techniques to meet those requirements. Some of these techniques can be easily adapted for other, non-residential applications. This article looks at those construction aspects from the point of attachment to the branch-circuit raceway connections. Fortunately, the old service could be kept alive, which allowed an orderly progression of work.

* The point of attachment. Although normally the location of the point of attachment for a service drop reflects ground clearances, that wasn't the limiting factor here. The building is on a hill above both the driveway and the street, putting the drop about 22 ft over the driveway and almost 25 ft over the road. The problem was the porch roof. Although pitched, the pitch was very shallow [ILLUSTRATION FOR PHOTO 1 OMITTED]. This meant that the main rule in Sec. 230-24(a) would apply, calling for 8 ft. Fortunately, because the drop passed over the lowest part of the roof, that clearance could be obtained.

The concern was the location of the attic framing that would carry the force on the drop. This couldn't be raised, obviously. By arranging the drop location to meet the required roof clearances, no new framing was required in the attic. The point of attachment had to be very secure, to support a long, heavy service drop.

The contractor bent the end of a long piece of 5/8-in. heavily galvanized threaded rod with a 1/2-in EMT bender so it would point directly at the utility pole while its unbent portion passed through heavy framing members in the attic floor. The angled portion passed through a block of solid aluminum cut at the appropriate angle so one side seated squarely on the clapboards and the other faced the utility pole [ILLUSTRATION FOR PHOTO 2 OMITTED!. The threaded rod emerged from the block of aluminum just enough to engage all the threads of a female eye-nut.

* The conduit body. The riser was 3-in. aluminum rigid conduit, with a mogul LB at its base [ILLUSTRATION FOR PHOTO 3 OMITTED]. Although most manufacturers know that these conduit bodies have to be six times the conduit trade diameter (18 in. in this case) between the centers of the two conduit stops, not all of them also observe the other requirement [also in Sec. 370-28(a)(2)! that the inside depth between the short end and the cover must comply with Table 373-6(a), or 8 in. for the 600kcmil conductors involved. In this case, the contractor had to reject the first one that came up from the supply house, which only had about a 5-in. clearance.

* Indoor service work. The meter and disconnect layout was arranged to conserve space and allow a complete door swing, while preserving unobstructed workspace in front of the lineup [ILLUSTRATION FOR FIGURE 1 OMITTED]. The 18 x 18 x 12-in. pull box [ILLUSTRATION FOR PHOTO 4 OMITTED] had to be made in a local sheet metal shop from 16 gauge galvanized steel. This more than complied with the required thickness for such enclosures in Sec. 370-40(c). The size was dictated by Sec. 370-28(a)(2), requiring six times the raceway diameter in each direction in this case. The depth was dictated by another requirement in that section that the distance between conduit entries enclosing the same conductor must also observe the six times rule.

In this case, the service conduit entered about 6 1/2 in. from the back wall. The entire top of the service enclosure was fair game for a hole, but if that entry were also made 6 1/2 in. from the back wall, the diagonal distance would only have been about 17-in. By using the larger pull box, the entry could be moved forward, lengthening the diagonal to the required 18 in. Since the disconnect enclosure was about 15 in. deep, that wasn't any problem. In fact, the forward entry made it easier to work with the heavy 600kcmil conductors.

The next job was to properly bond the service enclosure as well as providing for the CATV and telephone systems. Both service entries to the pull box were bonded. This required drilling and tapping the edge of the chase nipple [ILLUSTRATION FOR PHOTO 5 OMITTED] so a bonding lug could be attached; the contractor couldn't find a bonding chase nipple anywhere, and a bonding wedge would have taken too many threads away from the inside of the service enclosure.

The bonding conductor was run through a lug on the right side of the box, which has a common brass bolt with the lug on the outside of the box. From there a full-sized bonding conductor passes through a bond bushing on the end of the communications raceway and over to a terminal bar above the telephone boxes and the future CATV location above the commercial metering [ILLUSTRATION FOR PHOTO 4 OMITTED]. There have been many problems with telephone and CATV technicians making improper grounding connections, and the contractor didn't want any excuses. The photo shows the telephone grounding conductors properly connected to the intersystem-bonding terminal bar.

* The service disconnect module. The manufacturer's 400A service module didn't meet code minimums for bending space where large conductors come in on the wall opposite the terminals, as in this case [ILLUSTRATION FOR PHOTO 6 OMITTED!. For example, the largest neutral conductor that could be terminated, per Table 373-6(b), was 300kcmil. The manufacturer was willing, however, to fit the 400A switch in its 600A enclosure for a relatively nominal marginal cost, and that solved the problem.

* The feeder wireway. The metered feeder conductors drop out of the meter stack to the left of the service module and run to a 42-circuit panel, with space available for two more panels to its left [ILLUSTRATION FOR PHOTO 7 OMITTED!. The raceway for the grounding electrode conductor (3/4-in. rigid conduit) leaves the bottom of this wireway as well. Since this portion of the wireway is an enclosure for the grounding electrode conductor, it must be bonded to the grounding electrode conductor using a conductor of equal (or larger) size. In accordance with Table 250-94, that means No. 1/0 minimum, which must terminate at each grounding electrode conduit point of attachment to the wire-way. The bonding lugs that were available for the 3/4-in. rigid conduit bushings wouldn't accept such a large conductor, so larger lugs had to be substituted.

In addition, the contractor had a longstanding suspicion of the fault-return capability of the little 8-32 screws and nuts that hold the wireway sections together. Therefore, and in accordance with Sec. 250-51, he insisted on adding bonding lugs to each individual wireway section, drilling and tapping each one for a 10-32 grounding screw. Therefore, the path to the grounding electrode is both over the grounding electrode conductor itself, and over each wireway section through lay-in lugs that receive that conductor or another conductor bonded to it. Note that the grounding electrode conductor was installed without sharp curves, which lowers its impedance, particularly under lightning conditions that approximate high-frequency AC.

* The branch-circuit wireways. The building had been rewired many years ago using EMT, a distinct rarity for residential construction. Perhaps anticipating this service location, the EMT home runs, some 25 of them, had been brought to a 18 x 12 x 8-in. pull box just outside the service location. The idea was to allow circuit repositioning depending on whether various rooms were part of different occupancies.

They had been fed, apparently temporarily, with Type NM cable, but the owner obviously wanted to retain the flexibility of raceway. One of the more challenging aspects of the job was allowing for (eventually) some 50 branch circuits and feeders to leave the electrical room without paying horrendous derating penalties, which could easily end up, per Note 8(a), robbing two-thirds of the ampacity of each conductor. For example, going from the electrical room to the pull box might otherwise involve No. 6 wire for 20A circuits.

The key to solving this was Sec. 362-5 Ex. 1, which allows up to 30 current-carrying conductors in a metal wireway to run without penalty. Two wireways 5-ft long were run in parallel [ILLUSTRATION FOR PHOTO 8 OMITTED]. If the first 30 conductors passed straight up from the panel furthest to the right and into the upper wireway, that would leave the lower wireway with plenty of room for any additional conductors from the right-hand panel, together with any from the next panel to the left (not installed at this time). The panel to the extreme left (also not installed at this time) could use the vertical wireway if the lower horizontal wireway approached the 30 conductor limit.

Although it might seem that even more conductors would be installed, that isn't likely. Remember that an additional panel would normally be installed when the large apartment is subdivided. When that happens, some circuits will move from one panel to the other, resulting in a smaller net increase in conductors.

The vertical wireway [ILLUSTRATION FOR PHOTO 8 OMITTED] connects to the horizontal feeder wireway [ILLUSTRATION FOR PHOTO 7 OMITTED], and includes apartment feeders that lead to remote panels. One such feeder had been installed and another was in process as the photos were taken.

* A wireway pull box. If the three wire-ways were chase-nippled into a conventional pull box, this box would need to be 20-in. on a side per Sec. 370-28, since conductors No. 4 or larger are involved. This would have obstructed the future location of one of the panels. The answer was to take a 12 x 12 x 6-in. pull box and make it into a wireway component [ILLUSTRATION FOR PHOTO 8 OMITTED].

The 6-in. wireway size easily allows for the deflection of the largest conductor likely to be installed, per Sec. 362-6. For example, the vertical wireway has some No. 1/0 conductors; the minimum bending space that would apply here per Table 373-6(a) is 3 1/2 in. Meanwhile, removing the side of the box preserves the lay-in characteristic of this wiring method, eliminating the need for using the larger Art. 370 dimensions.

As in the case with the feeder wireway, all wireway sections were bonded together including the pull box. In this case, however, the bonding conductor was sized based on the largest overcurrent device likely to be installed ahead of it, per Table 250-95.

The next problem was to get from the wireways out of this corner of the room without derating. The key here is the allowance in Note 8(a) Ex. 3 that waives derating in conduit nipples not exceeding 24 in. Given the 6-in depth of the wireway, that allowed for a 2 1/2-in. and a 1-in. nipple as shown in Photo 9. They only had to be long enough to reach the pull box on the other side of the wall, about 6 in.

* A very customized pull box. The last stage of the branch-circuit and feeder work was to provide a means to route the apartment feeders and also to provide a route for the branch-circuit conductors to reach the previously installed pull box, again, without derating penalties. Here again the sheet-metal shop proved invaluable.

The basic dimensions of this box had to meet the provisions of Sec. 370-28, and given the 2 1/2-in. conduit nipple, with the eight times multiplier for the feeder that will go straight across the box (but not yet installed as the photo was taken), the box needed to be at least 20 in. in that dimension. The other dimension could be somewhat smaller, but given the rule for six times the trade diameter between entries enclosing the same conductor, any reduction would be very slight due to the location of the 1 1/2-in. conduit. Therefore, the pull box would be 20-in. square.

Given the location of the brick column at the end of the old brick wall, there was no way to come out the left end of this pull box [ILLUSTRATION FOR PHOTO OMITTED! and offset into the old branch-circuit pull box 22 in. farther back. The solution was to in effect add a channel to the bottom of the pull box that would extend up between the joists, looking directly at the other pull box. This allowed a 2-in. conduit nipple to connect the two pull boxes, again avoiding derating penalties.

Again 16 gauge galvanized steel was used, except for the cover. The 20 x 20-in. cover, 400 [in.sup.2], needed to be thicker than 16 gauge to meet the other requirement in Sec. 370-40(c) (as referenced in Sec. 370-41 for equal thickness thereto) for "ample strength and rigidity." In referring to UL 50, Enclosures for Electrical Equipment, the cover (which exceeded the 360-sq.-in.-limit in the standard) needed a minimum thickness of 0.070 in. Although the standard recognizes reinforced 16 gauge covers, provided a 100-lb force exerted at the center won't cause a deflection greater than 1/4 in., it was far simpler to have a 14 gauge sheet cut.

Note that there is no NEC requirement for these boxes to be listed. The panel responsible for this material (CMP 9) rejected a proposal (in the 1993 cycle) that they be required to be listed. The panel, including its UL representative, recognized that custom pull boxes needed to be made in the field like this, and that a listing requirement would be unduly burdensome.

* The grounding electrode connections. In accordance with Sec. 250-92(b), the grounding electrode conductor and its enclosing raceway must be bonded together at the electrode as well as at the service. In addition, the interior water piping system must be bonded to the service ground with a conductor sized in the same way as the grounding electrode conductor. The water meter cannot be relied upon for continuity. Therefore, the simplest procedure is to leave the grounding electrode conductor long enough to pass over to the interior side of the meter without splice [ILLUSTRATION FOR PHOTO 10 OMITTED!.

In this case, the principal electrode is the copper water pipe, which in this case is buried up to 12 ft below grade for a distance over 100 ft to the street main. However, a water pipe cannot be the only electrode; if there are no others then a made electrode must be provided. The grounding electrode conductor was left long enough to continue past the water system connections and through the field stone foundation to reach a supplementary made electrode. Although such a connection need not be larger than No. 6, it was simpler and safer to avoid any splices by leaving the basic, full-sized grounding conductor long enough to fulfill all Code requirements.

There was, however, one final wrinkle. Like most water-pipe ground clamps used this way, the ground clamp technically violated Sec. 110-3(b) because it was used on copper water tubing and wasn't so marked, a violation of the UL Guide Card information on Grounding and Bonding Equipment. The manufacturer contacted UL and now has permission to revise its listing, adding the required marking to the attached card. Since there was no actual change in the product, the inspector accepted it after a discussion with the factory. Note that such a revised marking card cannot be sent through the mail and applied in the field, except in the presence of a UL employee.

As in the case of the service module as originally proposed by its manufacturer, installers need to stay on top of whether the equipment they are buying is really appropriate for the proposed use. Electrical professionals need to shift the market by insisting on appropriate product characteristics. That's not all bad, however. Where would the enjoyment be if we didn't keep asking questions and learning something new?

TAGS: Design
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