Patent Publication Number: US-2023133170-A1

Title: Rigid bus ducts

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of U.S. Provisional Patent Application No. 63/203,187 filed Jul. 12, 2021, which is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to rigid bus ducts. Particular embodiments relate to rigid bus duct systems adapted for supplying electrical power to high rise buildings and other multi-floored structures. 
     BACKGROUND 
     In multi-story structures such as office towers, condominiums, apartments, and other buildings, electrical power is typically distributed with the use of bus ducts which run vertically through multiple floors of the building. At various points along a vertical bus duct, connection points such as bus plugs are needed to tie in cables that distribute power throughout each floor. Depending on power requirements, construction details, and other factors at play in a building a bus duct may have connection points on every floor, every other floor, or any other desired pattern. 
     Most types of existing bus ducts are typically expensive and time consuming to install and replace. A typical bus duct has a number of rigid conductors, and these conductors must be securely connected at a joint between adjacent bus duct sections, often with a blade-type connection with bolts used to squeeze the conductors together. Such joints can present relatively high resistance, and some jurisdictions require that the bolts in the joints of a bus duct be re-torqued every six months. Further, due to seismic considerations, rigid bus ducts must typically be installed with their longer lateral sides oriented perpendicularly to a structural wall, such that the area footprint required for the bus duct can be significantly larger than the cross-sectional area of the bus duct itself. 
     U.S. Pat. Nos. 10,305,263, 10,554,024, 10,693,282 and 10,903,630 disclose electrical power supply structures which alleviate drawbacks of many types of traditional bus ducts. Such structures are suitable for supplying electrical power among multiple floors of buildings. 
     The inventor has determined a need for further improvements to structures and systems for distribution of electrical power in multi-story buildings. 
     SUMMARY 
     One aspect of the present disclosure provides a rigid bus duct comprising a pair of side rails extending in a longitudinal direction, a plurality of support members extending between the side rails in a transverse direction, and a plurality of insulated conductors extending longitudinally, and held in a fixed relationship to one another by the plurality of support members. The plurality of insulated conductors are arranged in groups including a plurality of high current phase groups and a neutral group, and a plurality of shielding structures connected between the support members and positioned with at least one shielding structure located between the insulated conductors of each high current phase group. 
     In some embodiments the insulated conductors comprise hollow tubes. In some embodiments the insulated conductors comprise sixteen insulated conductors held within passages through the support members arranged in two rows of eight, with each high current phase group and the neutral group comprising two adjacent pairs of insulated conductors, and wherein each shielding structure has a cross-shaped cross-section. In some embodiments a center-to-center spacing between two adjacent passages in the same row for holding insulated conductors of the same high current phase group is greater than a center-to-center spacing between two adjacent passages in the same row for holding insulated conductors of different high phase groups. In some embodiments the shielding structures are held in place between the support members by engagement features formed on opposed faces of each support member. In some embodiments the bus duct comprises a pair of covers extending between the side rails to form an enclosure enclosing the support members, insulated conductors, and shielding structures, and each cover is held in place by fasteners received in the shielding structures. 
     Another aspect of the present disclosure provides a rigid bus duct system comprising a plurality of bus duct sections, each bus duct section comprising a pair of side rails extending in a longitudinal direction, a plurality of support members extending between the side rails in a transverse direction, and a plurality of insulated conductors extending longitudinally, and held in a fixed relationship to one another by the plurality of support members. The rigid bus duct system is characterized in that each insulated conductor comprises a hollow tube, and the plurality of insulated conductors are arranged in groups including a plurality of high current groups and a neutral group, and adjacent insulated conductors in different high current phase groups are separated by a smaller distance than adjacent insulated conductors in the same high current phase group. 
     Another aspect of the present disclosure provides apparatus for forming a sealed connection between two bus duct sections of a rigid bus duct system, wherein each bus duct section comprises a plurality of insulated conductors are arranged in groups including a plurality of high current groups and a neutral group, the apparatus comprising a pair of sealing boots for each group of insulated conductors. Each boot comprises a body constructed from a flexible material and having a closed first end and an open second end, with an opening through the closed end for each insulated conductor such that the open end of one boot of each pair is stretchable over the open end of the other boot of each pair to form a seal around a connection between each group of insulated conductors. 
     Further aspects of the present disclosure and details of example embodiments are set forth below. 
    
    
     
       DRAWINGS 
       The following figures set forth embodiments in which like reference numerals denote like parts. Embodiments are illustrated by way of example and not by way of limitation in the accompanying figures. 
         FIG.  1    shows a rigid bus duct system according to one embodiment of the present disclosure. 
         FIG.  1 A  shows an enlarged view of area A of  FIG.  1   . 
         FIG.  1 B  shows an enlarged view of area B of  FIG.  1   . 
         FIG.  1 C  is an exploded view of components of a floor seal assembly for mounting a rigid bus duct in a hole in a floor of a building. 
         FIG.  1 D  is a side view of a floor seal assembly with a flat backing plate according to one embodiment of the present disclosure. 
         FIGS.  1 E and  1 F  are side views of floor seal assemblies with offset backing plates according to other embodiments of the present disclosure. 
         FIG.  2    shows a portion of a rigid bus duct extending through a hole in a floor of a building. 
         FIG.  2 A  is a sectional view along line A-A of  FIG.  2   . 
         FIG.  2 B  is a view similar to  FIG.  2 A  with the insulated conductors removed, showing the phase arrangement and the relative spacing between passages formed by the transverse support members of an example rigid bus duct. 
         FIG.  2 C  shows sectional views through an example rigid bus duct with solid conductors showing current density (top graph) and flux density (bottom graph) when carrying 1500 A. 
         FIG.  2 D  shows sectional views through an example rigid bus duct with hollow conductors according to one embodiment the present disclosure showing current density (top graph) and flux density (bottom graph) when carrying 1500 A. 
         FIG.  2 E  shows sectional views through an example rigid bus duct with hollow conductors and shielding structures according to another embodiment of the present disclosure showing current density (top graph) and flux density (bottom graph) when carrying 1500 A. 
         FIG.  2 F  shows an example shielding structure according to one embodiment of the present disclosure. 
         FIG.  2 G  is an end view of the shielding structure of  FIG.  2 F . 
         FIG.  2 H  shows shielding structures coupled to a transverse support member of a rigid bus duct according to one embodiment of the present disclosure. 
         FIG.  2 I  is an enlarged view of area B of  FIG.  2 H . 
         FIGS.  2 J and  2 K  are end views of shielding structures with different thicknesses. 
         FIG.  3    is an exploded view of a transverse support member for a rigid bus duct according to one embodiment of the present disclosure. 
         FIG.  3 A  is a sectional view along line A-A of  FIG.  3   . 
         FIG.  3 B  is an enlarged view of area B of  FIG.  3   . 
         FIG.  3 C  is an enlarged view of area C of  FIG.  3   . 
         FIG.  3 D  is an enlarged view of area D of  FIG.  3   . 
         FIG.  3 E  shows a central block of the transverse support member of  FIG.  3    in isolation. 
         FIG.  3 F  shows a top/bottom block of the transverse support member of  FIG.  3    in isolation. 
         FIG.  3 G  shows the transverse support member of  FIG.  3    in an assembled state. 
         FIG.  3 H  is a side view of the transverse support member of  FIG.  3 G . 
         FIG.  3 I  shows a single-passage bushing of the transverse support member of  FIG.  3    in isolation. 
         FIG.  3 J  is a side view of the single-passage bushing of  FIG.  3 I . 
         FIG.  3 K  shows a double-passage bushing of the transverse support member of  FIG.  3    in isolation. 
         FIG.  4    shows an example insulated conductor of a rigid bus duct according to one embodiment of the present disclosure. 
         FIG.  4 A  is a transverse sectional view through the insulated conductor of  FIG.  4   . 
         FIG.  4 B  shows an example side rail of a rigid bus duct section according to one embodiment of the present disclosure. 
         FIG.  4 C  is a transverse sectional view through the side rail of  FIG.  4 B . 
         FIG.  5    shows details of an example sealed connection between bus duct sections of a rigid bus duct system according to one embodiment of the present disclosure. 
         FIG.  5 A  shows a pair of sealing boots and a retaining ring for sealing a group of conductors according to one embodiment of the present disclosure. 
         FIG.  5 B  shows an example sealing boot in isolation. 
         FIG.  5 C  shows an example retaining ring in isolation. 
         FIGS.  6 A- 6 H  show steps of an example method for assembling a rigid bus duct section according to one embodiment of the present disclosure. 
         FIG.  7    shows a rigid bus duct section configured for making a sealed connection to another rigid bus duct section according to one embodiment of the present disclosure. 
         FIGS.  7 A- 7 G  show steps of an example method of making a sealed connection between two rigid bus duct sections. 
         FIG.  8    shows two rigid bus duct sections coupled side by side according to one embodiment of the present disclosure. 
         FIG.  8 A  shows an end view of the connection between side rails of two adjacent rigid bus duct sections according to one embodiment of the present disclosure. 
         FIG.  8 B  shows three rigid bus duct sections coupled side by side according to one embodiment of the present disclosure. 
         FIG.  8 C  shows four rigid bus duct sections coupled side by side according to one embodiment of the present disclosure. 
         FIG.  8 D  shows an example floor seal and an example tap box configured to accommodate two rigid bus duct sections coupled side by side according to one embodiment of the present disclosure. 
         FIG.  9    shows an example rigid bus duct section having a smaller number of conductors according to one embodiment of the present disclosure. 
         FIG.  9 A  is an enlarged view of area A of  FIG.  9   . 
         FIG.  9 B  shows an example floor seal assembly for the rigid bus duct section of  FIG.  9   . 
         FIGS.  10 A- 10 E  show example junction boxes for connecting to bus duct sections according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes rigid bus duct systems for supplying electrical power to high-rise buildings or other structures where high current power needs to be distributed throughout a number of floors, or through walls or other barriers. Unlike many traditional bus ducts that must be de-rated if mounted in suboptimal positions, bus duct systems constructed according to certain embodiments of the present disclosure, and rigid bus duct sections thereof, are configured to be installed in any position without needing to be de-rated. As discussed below, the bus duct sections according to example embodiments provide an irregularly spaced array of hollow insulated conductors held in relative positions carefully selected to optimize performance, and an enclosure designed to reduce weight, allow airflow for cooling, and minimize hysteresis and eddy current losses. 
     For simplicity and clarity of illustration, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. Numerous details are set forth to provide an understanding of the examples described herein. The examples may be practiced without these details. In other instances, well-known methods, procedures, and components are not described in detail to avoid obscuring the examples described. The description is not to be considered as limited to the scope of the examples described herein. 
       FIG.  1    shows a rigid bus duct system  10  according to one embodiment of the present disclosure. The system  10  comprises a plurality of rigid bus duct sections  100  connected end to end, either directly by sealed connections  200  ( FIG.  1 B ) or through junction boxes  300  ( FIG.  1 A ), to supply electrical power to a structure, such as for example a high rise building or large industrial installation. The bus duct sections  100  of the example embodiments shown in the Figures are depicted as passing through floors F and are thus vertically oriented, but it is to be understood that the sections  100  could be differently oriented and pass through walls or other barriers. 
     Each bus duct section  100  comprises a pair of opposed side rails  102  extending longitudinally, with a pair of “front” and “back” (or “top” and “bottom”, depending on the orientation of the section  100 ) covers  104  extending between the rails  102  to form a longitudinal enclosure for a plurality of insulated conductors  110 .  FIG.  2    shows a portion of a rigid bus duct section  100  extending through a hole in a floor of a building, with a cover  104  removed to show the internal structure thereof. The conductors  110  are held in place by transverse support members  130 , and arranged in a plurality of high current phase groups and a neutral group with relative spacings therebetween selected to optimize performance, as discussed further below. In some embodiments a shielding structure  115  is located between the insulated conductors of each high current phase group. The shielding structures  115  are held in place between adjacent support members  130 , for example by grooves or other features on the support members  130 , as described below. The bus duct section  100  may also comprise ground cables  119  (see  FIG.  2 B ) running along the inside of each rail  102 . 
     In the illustrated examples, each insulated conductor  110  comprises a pipe or tube, for example an extruded aluminum pipe with a hollow center, covered with a layer of insulation, as seen in  FIGS.  4  and  4 A , and has a bent portion  111  and lug  112  formed at either end thereof. Each rail  102  may also comprise an extruded piece of aluminum of a desired length. Each rail  102  may have a plurality of holes drilled therein as shown in  FIG.  4 B  for facilitating attachment to the support members  130 , and optionally also to stubs  122  of a floor seal assembly  120  as described below.  FIG.  4 C  shows a cross-sectional view through an example rail  102 , which has a generally “I”-shaped cross section and comprises grooves  102 A formed on one side of the top and bottom flanges of the I for receiving edges of covers  104 . The rail  102  also has a thicker central portion  102 B where central blocks  131  of the support members  130  are to be attached, as described below, with an indent  102 C running along the inside thereof to accommodate a ground cable  119 . 
     In the example shown in  FIG.  1   , each bus duct section  100  comprises two pairs of rails  102  (each with their own pair of covers  104 ), with one pair of rails  102  extending upwardly from a floor seal assembly  120 , and one pair of rails  102  extending downwardly from the floor seal assembly  120 , with the conductors  110  running continuously through the entire length thereof. In other embodiments, depending on the building or other structure and the desired installation (e.g. where a section does not need to pass through a floor), some bus duct sections may not have a floor seal assembly, and only comprise a single pair of rails extending the entire length thereof. 
     Details of an example floor seal assembly  120  are shown in  FIG.  1 C , in relation to an opening through a floor, but it is to be understood that assembly  120  could also be used for passing a bus duct section  100  through an opening in a wall or other barrier. The floor seal assembly  120  comprises a floor seal plate  121  having a pair of stubs  122  extending from each side thereof. The stubs  122  are configured to engage with the side rails  102 , and may have pre-drilled holed for facilitating attachment of rails  102 . A seal block  123  extends across an opening through the plate  121  between the stubs  122 . The seal block  123  has a plurality of holes therethrough for receiving the plurality of insulated conductors  110 , each hole being fitted with a water seal  124  sized to fit snugly around an insulated conductor  110 . A plurality of firestop pillows  125  are packed around and between the conductors  110  within the opening through the floor, and a backing plate  126  is attached to the underside of the floor around the opening. In some embodiments, for example when the floor/wall is less than a predetermined thickness (e.g. 9 inches), an offset backing plate  126 A/ 126 B (See  FIGS.  1 E and  1 F ) is attached to the underside of the floor, to accommodate the firestop pillows  125 . A rubber gasket or butyl sealant may be used at the interface(s) between the floor and the floor seal plate  121  and/or backing plate  126 / 126 A/ 126 B to ensure a weatherproof seal. 
     As best seen in  FIGS.  2 A and  2 B , the insulated conductors  110  are arranged in a plurality of high current phase groups (three phases in the illustrated embodiment, with conductors  110 A in the first phase group, conductors  110 B in the second phase group, and conductors  110 C in the third phase group) and a neutral group (conductors  110 N). A shielding structure  115 A/ 115 B/ 115 C is held in place between the conductors  110 A/ 110 B/ 110 C of each high current phase group. The conductors  110  are not all evenly spaced, but instead adjacent insulated conductors in different high current phase groups are separated by a smaller distance than adjacent insulated conductors in the same high current phase group.  FIG.  2 B  shows the phase arrangement and the relative spacing between passages formed by the transverse support members of an example rigid bus duct, with each passage labelled with a letter (A, B, C, or N) indicating which group the conductor held within is to belong to. The passages are arranged in two rows, with the passages of the “top” and “bottom” rows aligned such that each passage has an adjacent paired passage in the other row. The spacing between adjacent pairs of passages in different rows is uniform (e.g. 3 inches in the embodiment shown in  FIG.  2 B ), but the spacing between adjacent passages in the same row differs depending on whether the passages hold conductors the same group or in different groups. As indicated by the example dimensions shown in  FIG.  2 B , the center-to-center spacing of between two adjacent passages in the same row for holding insulated conductors of the same high current phase group (e.g. 4 inches in the embodiment shown in  FIG.  2 B ) is twice that of the center-to-center spacing between two adjacent passages in the same row for holding insulated conductors of different high current phase groups (e.g. 2 inches in the embodiment shown in  FIG.  2 B ). Further, in some embodiments, each set of two adjacent passages for holding insulated conductors of different high current phase groups are each formed by a single feature formed in an interfacial surface of each of a central block and a top/bottom block of the support member  130 , as described further below with reference to  FIG.  3   . 
     The phase arrangement and spacing of the hollow insulated conductors  110  provides bus duct sections  100  according to certain embodiments of the present disclosure with increased thermal efficiency and reduced electromagnetic interference than traditional bus ducts using adjacent flat conductors. The shielding structures  115  can provide further reduction of interference, such that certain embodiments may provide, for example, up to 25% less voltage drop than a traditional bus duct. Bus duct systems according to certain embodiments of the present disclosure can have a length up to 70% longer than certain prior art busways carrying the same amount of current. 
       FIGS.  2 C,  2 D and  2 E  each show a finite element analysis (FEA) of sectional views through simulated bus ducts with a phase arrangement as shown in  FIG.  2 B  when carrying 1500 A. In each of  FIGS.  2 C,  2 D, and  2 E  the top graph illustrates current density and the bottom graph illustrates flux density.  FIG.  2 C  shows results for bus ducts with solid conductors,  FIG.  2 D  shows results for bus ducts with hollow conductors, and  FIG.  2 E  shows results for bus ducts with hollow conductors and a shielding structure located between the conductors in each high phase group. As shown by the bottom graphs, the magnetic flux density between and around adjacent high current phase groups of different groups (which are placed closest to one another) is relatively high for solid conductors ( FIG.  2 C ), but is reduced when using hollow conductors ( FIG.  2 D ) and further reduced when using the shielding structures ( FIG.  2 E ). Impedance (Z) is also reduced when using hollow conductors (Z=1.18 mΩ; R=0.68 mΩ; X=0.96 mΩ) as compared to the solid conductors (Z=1.51 mΩ; R=0.62 mΩ; X=1.37 mΩ), and is even further reduced when using hollow conductors with shielding structures positioned according to the present disclosure (Z=0.81 mΩ; R=0.6 mΩ; X=0.55 mΩ). 
       FIGS.  2 F and  2 G  show an example shielding structure  115  according to one embodiment of the present disclosure. The shielding structure  115  comprises an extruded elongated element of aluminum (or other suitable electrically conductive material) having a generally cross-shaped cross section, with four arms  116  comprising generally planar sheets or plates oriented at right angles. Two of the opposed arms  116  are formed with threaded connections  117  along the edges thereof for receiving bolts for securing covers  104  thereto. As the covers  104  and rails  102  are constructed from conductive material (aluminum in preferred embodiments), and grounded (e.g. by ground cables  119  running along the insides of the rails  102 ), the shielding structures  115  are also thus connected to ground. As shown in  FIGS.  2 H and  2 I , each shielding structure  115  is engaged by grooves  138  (see also  FIG.  3 E ) formed in opposed faces of the central blocks  131  of the support members  130  to stabilize the shielding structures  115 . (Only two shielding structures  115  are shown in  FIG.  2 H , but it is to be understood that another shielding structure  115  would be placed between the two shown.) In some embodiments, thinner or thicker shielding structures  115  may be provided, as shown in  FIGS.  2 J and  2 K , depending on the desired amount of shielding. 
       FIGS.  3  to  3 J  show features of an example transverse support member  130  for a rigid bus duct according to one embodiment of the present disclosure. The support member  130  comprises a central block  131  and two top/bottom blocks  132 , which are bolted together (see  FIG.  3 G ) to form the passages for receiving the insulated conductors. The blocks  131  and  132  are pressed together along interfacial surfaces thereof, and the passages are defined by semicircular “cutout” features (although blocks  131  and  132  are typically formed by injection molding, so the block material is not cut to form the features) along the interfacial surfaces. In the illustrated embodiments, each passage for holding the neutral conductors, and the conductors on the edges of the high current phase groups are formed by a pair of single-passage features  133 , and the passages for adjacent conductors of different high current phase groups are formed by a pair of double-passage features  134 . The single-passage and double-passage features  133  and  134  of the blocks  131 / 132  are configured to receive single-passage and double-passage inserts or bushings  135  and  136 . The sizes of the bushings  135  and  136  can be varied to accommodate insulated conductors of different diameters in some embodiments. The bushings  135  and  136  may each comprise a hard plastic body with a plurality of rubber sealing strips  137  along the inner surface thereof. 
       FIGS.  5 ,  5 A,  5 B and  5 C  show details of an example sealed connection  200  between bus duct sections of a rigid bus duct system according to one embodiment of the present disclosure comprising a pair of sealing boots  202  constructed from a flexible, insulating material, surrounding the connection between each group of insulated conductors  110 . The rails  102  of the sections  100  have covers (not shown in  FIG.  5   ) extending therebetween, and may be connected to each other using splice plates (see e.g.  FIG.  7 D ), as discussed below. One pair of boots  202  is removed in  FIG.  5    to show the connection between conductors within the same group, which is effected by bolting or otherwise securing the lugs  112  at the ends of the conductors  110  to a conducting strip  113 . The boots  202  cover the lugs  212  and any other uninsulated portions at the ends of insulated conductors  110 . Each boot  202  has a first end  204  with a generally rectangular cross section (with rounded corners) to conform to the pattern of the insulated conductors  110  in the group, and an open second end  206  with a more rounded cross section such that the second end  206  can snugly fit over the second end  206  of another boot  202  and form a seal therewith. The first end  204  is closed other than four openings  205  at the corners thereof, with each opening  205  sized to snugly fit over the insulated portion of an insulated conductor  110 . The second end  206  also comprises tabs  208  with holes for engaging a retaining ring  210 . The retaining ring  210  is constructed from a more rigid material than boots  202 , and comprises tabs  212  having protrusions  214  for engaging the holes in the tabs  208  of a boot  202 . One retaining ring  210  is used to secure each pair of boots  202 , with the ring placed inside a first one of the boots  202  and secured to the holes in the tabs  208 , and the second end  206  of the second boot  202  is stretched over the second end  206  of the first boot  202 . When installed vertically, the ring  210  is placed within the lower boot  202 , and the second end  206  of the upper boot  202  is stretched over the lower boot  202 . 
       FIGS.  6 A- 6 F  show steps of a method of assembling a rigid bus duct section according to one embodiment of the present disclosure. As shown in  FIG.  6 A , the rails  102  are secured to opposed transverse sides of central blocks  131  (with bushings  135  and  136  installed therein) of the support members, with three shielding structures  115  held in place between each adjacent pair of central blocks  131 . Ground cables  119  may be secured to the insides of the side rails, for example by ground clamps  119 A, prior to the rails  102  being connected to the central blocks  131 . The rails  102  may be secured to the central blocks  131 , for example, by bolts received in square nuts held in place in in a press fit manner in chambers on the transverse sides of the central blocks  131  (as seen in  FIG.  3 B ). After all of the shielding structures  115  and central blocks  131  are in place, the insulated conductors  110  are placed in position and secured in place by top/bottom blocks  132  (with bushings  135  and  136  installed therein), as shown in  FIG.  6 B . The top/bottom blocks  132  may be secured to the central blocks  131 , for example, by bolts received in square nuts held in place in in a snap fit manner in chambers in central portions of the central blocks  131  (as seen in  FIG.  3 C ). Next, the covers  104  extending across the space between rails  102 , are installed as shown in  FIG.  6 C . The covers  104  are installed by sliding the edges thereof into grooves  102 A formed in the flanges of the rails  102 , as best seen in  FIG.  6 D . As shown in  FIG.  6 E , the covers  104  may be secured by bolts  105  that are received in threaded connections  117  in the shielding structures  115 . In the illustrated example, the bus duct section is to be secured through a hole in the floor of a building, and a floor seal assembly  120  is connected to one end of the bus duct section by bolting the rails  102  to stubs  122 , as shown in  FIG.  6 F . In implementations where the bus duct section is to continue extending below the floor, the side rails and support blocks for the “bottom” portion (not shown in  FIG.  6 F ) would be attached in place around the conductors  110  after the floor seal assembly  120  is in place. The other end of the bus duct section may be connected to a junction box, or directly to another bus duct section by means of a sealed connection. As one of skill in the art will appreciate, the conductors  110  can be longer or shorter depending on whether the bottom portion is going to connect to directly another bus duct section (e.g. through a sealed connection), or to a junction box. In some embodiments, where an end of the bus duct section is to be connected to a junction box, a seal block  123  may be installed at the end thereof, as shown in  FIGS.  6 G and  6 H . In some embodiments, each bus duct section is pre-assembled off-site then shipped to a work site for installation (e.g. by dropping a pre-assembled bus duct section through a hole in a floor, securing the floor seal assembly  120  to the floor and connecting the conductors  110  to corresponding conductors other bus duct sections, to junctions boxes, or to other components as needed for the desired implementation). 
       FIG.  7    shows a rigid bus duct section  100 A configured for making a sealed connection to another rigid bus duct section according to one embodiment of the present disclosure. In the  FIG.  7    example, the side rails  102  and conductors  110  all have the same length, for facilitating sealed connections at both ends, but it is to be understood that in some embodiments the conductors  110  may extend past the rails at one end of the section where a direct sealed connection to another bus duct section is not desired (e.g. where one end is to be attached to a junction box).  FIG.  7 A  shows two bus duct sections  100 A of the type shown in  FIG.  7   , with a plurality of sealing boots  202  installed over the conductors  110 . The boots  202  for the neutral group have a slightly different shape to accommodate the spacing of the conductors  110  in the neutral group (which is different from that of the high current phase groups as discussed above), and in some embodiments the boots  202  for the neutral group may be a different color than the boots  202  for the high current phase groups. As best seen in  FIG.  7 B , retaining rings  210  are placed within the boots  202  of one of the sections  100 A (the lower section in vertical installations). As shown in  FIGS.  7 C and  7 D , with the boots in place over the conductors, the rails  102  of the two sections  100 A are connected to each other by splice plates  103 . The splice plates  103  also firmly clamp the ground cables  119  to the rails  102 . Next, as shown in  FIG.  7 E , the lugs  112  of the conductors  110  of each group are connected to a conducting strip  113 , then the lower boots are slid up and the upper boots are slid down overtop of the lower boots, as shown in  FIG.  7 F . Finally, a connection cover  104 A is placed overtop of the connection area, as shown in  FIG.  7 G  (another connection cover  104 A may be installed on the reverse side). 
     Bus duct sections according to the present disclosure may thus be connected end to end to form a straight run of any desired length. The bus duct sections disclosed herein are also designed to be modular, with sections (which may also sometimes be referred to as “cassettes”) stacked side by side to deliver any desired amount of electrical power. For example,  FIG.  8    shows a “two-cassette” bus duct system wherein two pairs of end-to-end connected sections are stacked side by side. The cassettes may be held together by a rail clamp  107  that engages the outside flanges of the rails  102 , as shown in  FIGS.  8 A and  8 D . In this manner any number of sections can be stacked together, as illustrated in  FIGS.  8 B and  8 C  which respectively show three-cassette and four-cassette but duct systems. When using a multi-cassette configuration, any floor seal assemblies or junction boxes would need to be corresponding adjusted to accommodate the stack of bus duct sections. For example,  FIG.  8 D  shows an example floor seal and an example tap box configured to accommodate two rigid bus duct sections coupled side by side according to one embodiment of the present disclosure. 
     In some embodiments, bus duct sections may be configured to have fewer conductors than the examples discussed above, such as for example when lower power requirements are needed. For example,  FIGS.  9  and  9 A  show an example rigid bus duct section having only eight conductors  110 , arranged in two rows of four conductors.  FIG.  9 B  shows an example floor seal assembly for the rigid bus duct section of  FIG.  9   . 
     In some embodiments, bus duct sections according to the present disclosure are configured to connect to junction boxes, as discussed above.  FIGS.  10 A- 10 E  show example junction boxes for connecting to bus duct sections according to embodiments of the present disclosure. The junction boxes may be configured to accommodate the desired electrical connections, for example by including bus bars for connecting the conductors to, and comprise lugs for connecting to the rails  102 , similar to lugs  122  of floor seal assembly  120 . The junction boxes may also comprise seal blocks for receiving the conductors. 
     It will be appreciated that numerous specific details are set forth in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered as limiting the scope of the embodiments described herein in any way, but rather as merely describing implementation of the various example embodiments described herein. 
     The description provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed. 
     As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible to the methods and systems described herein. While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as may reasonably be inferred by one skilled in the art. The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the foregoing disclosure. 
     The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.