Power distribution options: Comparing feeder methods
Experienced designers and contractors will choose power distribution feeders to meet the specific needs of a project
- Understand the main types of distribution feeders and different versions of each.
- Discover the basic design considerations for three common power distribution feeder methods.
- Learn the important advantages and disadvantages for each feeder method.
Power distribution system insights:
- NFPA 70 dictates requirements for wire and conduit.
- Variations in distribution feeder methods can affect many things, including design and cost.
- The process of designing, installing and maintaining could be a challenge without a skilled team.
Power distribution systems are made up of three main types of distribution feeders to transport power between pieces of equipment within a facility:
- The first type is traditional building wire routed in conduit, with parallel sets routed alongside each other as needed. This is the most common type of installation for commercial and industrial systems and the most familiar to engineers and contractors.
- The second type is single or multiconductor power cable routed in cable tray or hangers.
- The third main type is busway or bus duct.
Choosing one of these methods over the others can have a significant impact on the design, installation and future of a project. It is important to consider them carefully for each situation.
Wire and conduit
Variations of wire and conduit mainly consist of material choices. Wire is typically provided in either copper or aluminum alloy, with varying types of insulations that are suitable for different types of installations and uses. Conduit is typically provided in steel, polyvinyl chloride, high-density polyethylene, aluminum and reinforced thermosetting resin conduit, but galvanized steel is the most applicable to compare to installations that could use power cables or busway.
When designing feeders, conductor ampacity and voltage drop are the characteristics that drive the size and number of conductors and conduit. Consideration must be given to the location of the feeders, their proximity to each other, the installation media (air, concrete, ground) and their length.
Ultimately, heat will reduce the ampacity of each single conductor and drive the feeders to be larger and more costly. Heat is generated by resistance in the conductor, cross inductance from conductor to conductor and outside influences such as installations on a roof. Heat is dissipated from the conductors at different rates dictated by the installation media.
Careful consideration to all of these must be given when designing appropriate feeder sizes using this traditional method. Most experienced designers and contractors are familiar with sections of NFPA 70: National Electrical Code dictating these and other requirements relating to wire and conduit.
Installation challenges for wire and conduit feeders include routing the individual conduits, locating appropriate junction/pull boxes and pulling the wire through the conduit, once installed. Although most electrical contractors are most familiar with this method, it is typically more labor intensive than the other two.
Power cable and tray
Power cabling also comes in different material types and insulation types. However, cables can be provided in premanufactured assemblies with different numbers of conductors, different types of insulations and armored or not. Methods to support these cables vary more than traditional conduit types as well, including ladder tray, center rail trays, through-type trays, solid bottom trays, channel type trays and individual support methods. For the purposes of this article, we are including cable bus systems in cable tray methods.
Designing power cable feeders is very similar to the first method, although the wider range of conductor assembly types dictate more specific ampacity calculations for specific products. Cable tray options also tend to steer designs toward a basis of design that is unique within the industry. Keeping these designs generic enough to facilitate competitive bidding while still planning for all appropriate accessories and options is more challenging than traditional designs.
Tray system designs must account for the specific tray dimensions, support requirements and bend radius, especially when transitioning through walls or to equipment. NEC Article 392 has code requirements for cable tray systems that designers and installers must follow. These requirements include sizing, supporting, ampacity calculations and other important items.
Installation of the cable tray must follow manufacturer-specific instructions and can have similar challenges to conduits. Power cable installation in the tray is one of the main advantages of this method. The cables are pulled in the tray, secured by an approved means and terminated appropriately. This allows the installation contractor to avoid setting up the labor-intensive wire-pulling processes.
The main forms of bus duct include sandwich style, air insulated and track. With all of these types, there is a fundamental difference between bus duct and the first two methods. Bus duct brings a decentralized approach to feeder loads and allows the ability to tap loads directly to the duct, although bus duct can also be used in a centralized approach as well. Rather than routing individual feeders from a distribution panel, bus duct typically runs along the service area with loads tapping onto the bus along the way.
Even more than cable tray, each model of bus duct will have design-specific requirements to plan around. Again, it can be challenging to facilitate a competitive bidding process with these unique designs. Bus duct components are less able to be field modified, so designs must be coordinated in relatively high detail.
However, designers only need to account for this in the bus duct, rather than the cables and the tray mentioned in the second method. Additionally, Article 368 of the NEC has specific design and installation requirements that need to be accounted for. Most of the key points of Article 368 include support requirements, tapping the bus duct and overcurrent requirements.
Distribution feeder pros and cons
Variations of each of these distribution feeder methods can have a significant impact on their design, installation, performance/use and cost. However, they can be discussed with several general advantages and disadvantages to each. Owners, contractors and engineers must be careful when researching these because biased opinions tend to contradict each other, whether from manufacturer publications or other sources.
Physical space: With larger feeders or larger quantities of feeders, space can become an important consideration. Traditional conduit and wire require at least 60% open space inside the conduits and appropriate spacing of the conduits between each other for fasteners/mounts. When sized appropriately, cable in tray can reduce the overall cross-sectional area required for feeder installations due to routing the cables relatively close to each other.
However, for significant space savings, bus duct can be used. The construction of bus duct allows the conductors to be routed relatively close to one another with minimal insulation and raceway/support requirements.
Installation time: Bus duct is generally the fastest of the three methods to install, followed by cable in tray and then wire in conduit. There are some trade-offs with equipment needed to lift the bus duct in place, but the time savings generally outweighs these. Of course, you can always find an installation where the opposite is true, but this is generally true.
Field changes: Although every designer strives to create the perfect design, field changes do happen. When they do, wire and cable methods offer easier modifications than bus duct because bus duct must be ordered with specific components suited for the installation. Routing conduits around obstructions or to a different location can be done in the field without ordering costly new components. This is a good reason for designers to take extra measures to ensure a complete detailed design for bus duct and cable tray. Even small shifts in electrical equipment locations could require new bus duct components, which would mean more time and money for the installation.
Design effort: As mentioned above, each method will have different design requirements. Although the differences in design effort can vary significantly per specific project, overall design times generally increase as you go from traditional wire and conduit to cable tray and then bus duct. However, assumptions on field installations and conditions leave more room for error with the more traditional methods. Designers must consider each method carefully, but the design effort differences are relatively insignificant compared to the other factors involved in choosing one method over the other.
Maintenance: This is one category where bus duct can be less advantageous than the other two methods. Maintenance of wire or cable assemblies is mainly limited to inspecting and torquing the terminations, usually with the aid of thermal imaging equipment. Bus duct components have more torque locations (at each section/component end) and can require more frequent inspections. Bus duct may also be susceptible to condensation in certain environments where wire and cable assemblies are less so.
Service life: The main components to break down over time are cable insulation, metal corrosion of raceways and enclosures and mechanical connections. Power distribution systems are often pushed to the limits of their expected useful lives in aging facilities. Electrical components are mostly stationary and will function until a specific event happens that forces them to fail. This is typically a mechanical failure with a component such as a breaker or switch. As bus duct has mechanical connections and typically have bus plugs that are added and removed over time, it is most likely to fail first.
However, most designers will consider service life relatively equal over the three methods because other components of the system are likely to fail first. The main caveat to this is future part replacement. Commodity items such as conduit and wire are relatively easy to replace in the future, while bus duct will have specific parts that may be needed to repair or maintain the duct at some point in the future. These specific parts may not be available and may be expensive.
Future flexibility: This may or may not be a priority on your project. Flexibility varies greatly from the first method to the third. Traditional wire and conduit are inherently inflexible when compared to the other two. Any changes to the feeder could require complete demolition and reinstallation. Cable in tray offers a bit more flexibility because the feeders can be easily moved and reterminated, assuming they are long enough.
However, it is unlikely that the cables will be the appropriate size for the new application. The designs of plug-in style bus duct products are mostly driven with flexibility in mind. Bus duct offers the most flexibility as new taps can be added and removed relatively easily.
Voltage drop, short circuit, coordination, arc flash: Bus duct is inherently less resistive than wire/conduit and cable/tray options. While this is a positive feature for voltage drop, it also allows more fault current through to downstream equipment, increasing the potential arc flash hazard and affecting overcurrent device coordination. Many facility owners and managers run into challenges with reducing arc flash hazard to levels low enough for safe operations on electrical equipment, while still maintaining proper coordination of the system’s overcurrent protective devices. Wire and cable type feeders are centralized and offer greater flexibility with arc flash and coordination of downstream equipment than the decentralized bus duct approach.
Installation quality: Some argue that factory connections offered by bus duct products offer less room for installation issues. Terminating cables with appropriate field terminations is a significant point of potential failure for this to occur, especially with complex cable assemblies. Additionally, pulling wire through conduit can damage conductor insulation if not done properly. Field quality tests can check for these issues, but they are not 100% guaranteed to catch them. Factory connections are bolted together, often times using torque-to-yield type fasteners, with little room for error.
Installed cost: The most contradicting information from various sources has to do with installed cost. It appears any manufacturer can make an argument for why their system has the lowest installed cost and what they’re saying may be true for a specific situation.
However, the system must be looked at for each situation individually. Speaking in general terms, material costs are lowest in wire/conduit systems and highest in bus duct systems, with cable/tray systems somewhere in the middle. Labor costs are the opposite, with wire/conduit systems being the highest.
Overall, labor costs do not offset the material costs and the method with the lowest installed cost is traditional wire in conduit, followed by the cable/tray option and finally bus duct. The high labor costs in major metropolitan areas, such as New York City and Chicago, can skew these costs though. Make sure to consider local labor rates for your specific project.
Additional considerations: In today’s market, material costs and lead times can be volatile and drive the choice of product used. Many owners are opting for products and methods that may not be ideal for their situation, simply to get their project built. Designers, contractors and owners should research lead times early in the design process to plan for the right solution to meet the schedule and budget.
With the less traditional methods, all parties involved must be familiar with the system that they’re designing, purchasing or installing. Inexperience with the requirements of each system is a likely cause for problems, whether they are design issues, installation issues or maintenance issues. Designers experienced with sizing, specifying and routing wire and conduit feeders may find power cables and cable tray challenging. The same is true for contractors and maintenance personnel.