How to specify metal-clad cable and busways
Wiring methods like metal-clad cable and busways, using premanufactured components, can offer significant labor savings, but their limitations should be understood before specifying them
- Review basic criteria for evaluating and selecting wiring methods.
- Examine industry standards and physical characteristics for metal-clad cable and busways that are commonly used in commercial construction.
- Understand relative advantages and disadvantages associated with conductors, conduit raceways and busways.
While traditional pipe and wire is still the dominate wiring method for electrical distribution systems in many parts of the country, alternate wiring methods often offer significant advantages. These advantages can include reduced installation labor cost, simplified installation methods and speed of installation. That potential for cost savings is often the determining factor for many clients, but the use of certain methods can dramatically impact the overall electrical distribution system’s performance and reliability.
To that end, all electrical engineers need a working understanding of commonly used wiring methods along with their relative advantages and disadvantages to help ensure that the most appropriate method is selected. The focus of this article will be two specific contemporary wiring methods: metal-clad cable and busways.
Metal-clad cable is the prevailing commercial construction branch circuit wiring method in many areas of the country. An MC cable assembly consist of three primary parts: the electrical conductors, a binder tape (typically mylar or polypropylene) around those conductors and an outer metal armored jacket.
Conductors are usually standard THHN/THWN. The metal armor is typically steel or aluminum. Because of this metal armor, use of conduit is not necessary with MC cable. Copper is used as the armored jacket in some special use fire-rated MC cables. Installation in wet locations or direct burial applications typically requires an impervious polyvinyl chloride or similar nonmetallic over-jacket on the cable.
MC cable is distinguished from armor-clad cable in that a green insulated grounding conductor is required within the cable assembly. The outer metallic armored jacket is not acceptable by itself as an equi
pment grounding conductor. The armor is typically one of three types: an interlocked metal tape, corrugated metal tube or a smooth metal tube. The metal can be either steel or aluminum. Steel provides superior electromagnetic interference shield, but aluminum is lighter in weight. From an installation standpoint, the lighter weight associated with aluminum is more attractive to most electrical contractors.
Standard MC cable cannot be used in health care facilities. NFPA 70: National Electrical Code Section 517.13 requires a two-part redundant ground fault return path configuration where the raceway system or cable armor qualify must also function as an equipment grounding conductor. Since a standard MC cable’s metallic jacket is not acceptable by itself as an equipment grounding conductor, special health care facility MC cable must be used in those applications. Health care facility MC cable has an additional bonding strip below the cable armor to meet this requirement. Health care facility MC cable can be identified by its green colored armor. While this rated MC cable is available, not all jurisdictions may accept its use.
MC cable is dramatically easier to install than traditional pipe and wire methods. However, it is also more likely to be installed in a less than a neat and workmanlike manner. The flexibility of MC cables often results in less than straight runs, with sagging and haphazard cables that leave a poor impression (see Figure 1). While NEC requires that MC cable be supported at intervals not exceeding 6 feet and within 12 inches of each termination point, it is not unusual to encounter installations that deviate from that requirement. Poor installation practices can also cause kinks in the metal jacket that cannot be repaired. Improper configuration of pulling sheaves can damage the armor jacket. The bending radius for MC cable cannot be less than 7 times its external diameter.
While traditionally used for branch circuit wiring, MC cable is also available in larger sizes for use in feeders. However, special care must be used when specified paralleled feeders to ensure that the equipment grounding conductor is the proper size per NEC Table 250.122.
The applicable code and standard: NEC Article 330 and UL 1569: Metal-Clad Cables.
Busways are insulated conductors (bus bars) enclosed within a metallic housing that provides both physical protection for the bus bars and usually act an integral equipment grounding path. Most manufacturers offer an option for a dedicated equipment ground bus bar in addition to the integral housing ground.
Busways are inherently modular in nature. They are available in standard lengths (typically 10 inches) that are simply bolted together — often using torque-to-yield connectors to simplify installation. The busbar conductor material can be either aluminum or copper. Enclosures are either painted steel or aluminum. Except for track busway, most modern busways are typically totally enclosed with no ventilation openings in the housing. This minimizes dirt/dust accumulation and/or accidental contact with the busbars within. Older busway designs can often be identified by the presence of ventilation slots in the housing.
Where the path is reasonably straight and accessible, the modular nature of busway can provide installation labor savings compared pipe and wire feeders of comparable ampacity. For larger ampacity feeders that would require multiple paralleled sets of traditional conductors, this savings can be dramatic. Busway manufacturers quote potential labor savings from 30% to 70%, depending on busway ampacity.
However, routing busway around obstructions, while possible, typically requires the use of custom offsets that can significantly increase cost and complexity. In all instances, extensive field verifications of routing and associated dimensions before fabrication are required for a successful installation. Even with careful measurement of the field conditions, the need to order custom offsets at the last minute to accommodate unforeseen conflicts is not unusual. Most busway manufacturers have special order programs to facilitate these last-minute changes.
Busways are available as feeder or plug-in type. Feeder busways are a direct replacement for traditional pipe and wire feeders. However, plug-in style is special in that it has regularly spaced tap-off locations along the length of the busway. This effectively allow plug-in busway it to operate like an oversized plug strip. Bus plugs (fused switches or circuit breakers) can be plugged into any of these openings to provide power to downstream loads. The ability to interchange bus plug sizes and locations provide considerable flexibility in accommodating future electrical distribution modifications. In industrial applications where the manufacturing equipment may be changed on a regular basis, busways are commonplace because of this flexibility.
Busway typically use significantly less space than traditional pipe and wire methods. Because the housing is usually engineered by the manufacturer to help dissipate heat from the bus bars, the housing on a busway typically tightly conforms to the shape of the bus bars. Compare this to the 40% fill requirement associated with traditional pipe and wire methods. To further reduce the size of the busway, UL 857 also allows for ampacity ratings based on temperature rise rather than more traditional current density calculations associated with pipe and wire.
Even with a reduction in conductor material cross-sectional area, sandwich type busways typically have extremely low reactance resulting in excellent voltage drop characteristics. However, these same electrical characteristics can dramatically increase available fault current to individual loads plugged in along the length of the busway.
While busways are generally smaller in cross-sectional area than pipe and wire feeders, careful consideration must be given to how that busway is interconnected with a source of power. Often, the interconnections between the busway a switchboard are via pipe and wire, especially if the route between the two is convoluted.
The final point of interface between the busway and that conduit is a large cable tap box, typically located at one end of the busway (see Figure 2). The cable tap box contains the lugs that are required to terminate the individual cables. These boxes are necessarily larger than the conduit that is connected to it. So, if the busway is being specified to save space, such as in a riser closet application, space required by the large cable tap boxes at the base of a busduct riser can come as unpleasant surprise. If multiple busways are routed side by side, the tap boxes often must be offset at the base of the riser to keep the busways from being spread apart and negating the busway’s space saving potential.
Busways do have one notable disadvantage compared to pipe and wire. Busways typically are not weatherproof. In wet locations, gasketed busway must be specified to prevent the entry of moisture (see Figure 4). Where routed vertically instead of horizontally, water exposure can become a significant hazard.
For example, a common application in high-rise buildings is to use plug-in busduct risers, routed vertically, to provide power for individual floors. The need for weatherproof busduct would not normally be expected in this type of application in interior dry location. However, once water is inside the housing, busduct typically has no way to prevent water from flowing all the way down the inside of a vertically mounted busway. A simple spilled mop buck or damaged sprinkler head, when considered in conjunction with the elevated available fault current levels associated with busways could mean disaster.
A 4-inch-high waterproof curb around the penetration through each floor for vertical busduct is required by NEC 368.10(C)(2) to prevent spilled water from running down the busduct (see Figure 3). Some municipalities, such as Chicago, require additional precautions, like mandating “weatherproof” busduct for this reason.
However, the UL 857 standard does not define what exactly weatherproof is. Busways are not assigned National Electrical Manufacturers Association ratings, such as NEMA 3R. As such, weatherproof busways typically meet either IP54 (protection against entry of dust and splashing water) or IP65 (dust tight and protects against water jets) requirements as defined by the European standard IEC 529: Degrees of Protection Provided by Enclosures (see Figure 4). Plug in busways are typically only available up to a IP54 rating. Feeder busways that do not have plug-in openings along its length area available up to an IP65 rating.
The applicable code and standard: NEC Article 368, UL 857: Standard for Busway and Associated Fittings.
The three primary types of busways
There are three primary types of low-voltage busways: track, air-insulated and sandwiched.
Track busway have a basic configuration like track lighting systems where the conductive busbars are accessible via a continuous open slot along the entire length of the busway housing. Power can be tapped off nearly anywhere along its length. This type of busway is typically found in data centers and light industrial applications.
However, they are only available in ratings up to 800 amperes and have a lower short-circuit withstand capacity than sandwiched type busway. They also are not suitable for use in dusty or wet environments due to the open slot in the housing.
Air-insulated busways (see Figure 5) differ from track busway in that the busbars are fully enclosed within a metallic enclosure. The busbars are attached to the enclosure with insulated blocks that provide physical separation between them, effectively creating an insulating air gap. Unlike track type busway, power can only be tapped off at regularly spaced tap-off locations along the length of the bus. While traditionally found in light industrial applications, low voltage air-insulated busways have been displaced in the marketplace by track busway due to costs. These types of busways are typically available up to 600 amperes.
Air-insulated busways can be identified by the fact that bus plugs typically plug into the long axis of the busduct cross-section. For sandwiched busways, bus plugs typically plug into the short axis of the busduct cross-section. Older, obsolete versions of air-insulated busways often featured slotted ventilation openings along the full length of the busway housing. Ventilation openings are not used in modern designs.
Sandwiched busways contain tightly stacked busbars, separated by insulation (typically mylar and/or epoxy) and enclosed by a metallic housing. The close spacing of the busbar within the housing provide low reactance and improved voltage drop characteristics. Sandwiched busways typically have the most compact cross-sectional area of any busway type. They also have ampacity ratings up to 5,000 amperes, making them the preferred method to deliver large amount of power across/up a building. High-rise building risers are a typical application for this type of busway.
While MC cable and busways can offer significant benefits over traditional wiring methods, their application may not be the most appropriate solution in every case. When designing electrical distribution systems, the engineer should the limitations of each before specifying them.
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