In outside-plant installations, conduit is usually installed underground to safeguard cables from damage and also to facilitate cable placement for immediate and future needs. You can also install Conduit Fittings inside buildings to facilitate pulling cable between two points such as through the telecommunications closet (TC) to operate-area outlets, or from an equipment room to some TC. To protect, isolate, and identify the cables, innerduct–also referred to as subduct–might be installed inside existing larger-diameter conduit.
Conduit is described as a rigid or flexible metal or nonmetallic raceway through which cables could be pulled. Furthermore, although conduit can be used to house various types of cable, the National Electrical Code (NEC) uses the word “optical fiber raceway” in Article 770 to clarify conduit, or raceways, for optical-fiber cable. Several kinds of conduit can be found, like electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit will not be recommended because of potential abrasion harm to the cable jacketing.
Metal conduit, which typically will come in 10-foot lengths, is rather rigid and requires special tooling and accessories to sign up with it. Nonmetallic conduit is accessible on reels in longer, continuous lengths which do not really need to be joined as much.
“The only problem with installing EMT conduit is that it demands a special skill set and training, as well as a lot of practice–or you find yourself making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit comes in 10-foot lengths so you must do any nonstandard bends by hand, and that`s where the technician`s special skill is necessary.”
Arnco Corp. (Elyria, OH) sells innerduct on the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “Within a building, several types of duct are used–for example, riser- and plenum-rated–but all of our products are produced from thermoplastic materials, like polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
There are actually three differing types (or ratings) of innerduct: outdoor, riser-rated, and plenum-rated. Robert Jensen, engineering manager at Endot Industries Inc. (Rockaway, NJ), explains: “Outdoor is often polyethylene and it`s certainly not rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which is generally a thermoplastic material like polyethylene or PVC with fire-retardant chemicals put into it. And also the third type of duct is UL plenum-rated, generally a pvdf product, which is fire-retardant and smoke-resistant,” says Jensen.
In accordance with Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most products that conduit and innerduct manufacturers make is perfect for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “frequently incorporating some sort of silicon,” he says. “For premises cabling, Pyramid supplies a plenum raceway (tested to UL-910) and a riser raceway (UL-1666) for installation in vertical shafts.” Moreover, the riser item is halogen-free and is also often used for military, shipboard, or tunnel applications, depending on the specifications.
Obviously contractors install conduit where building codes require it, but additionally where the cabling system needs physical protection or defense against unauthorized access.
“We use conduit in riser and backbone systems from your building entrance for the main distribution frame,” says Karl Clawson, senior vice president and partner, Clawson Communications (Greenwood, IN). “Therefore we also set it up for horizontal cabling, especially in university campuses. Within the living quarters, we install cable in conduit mainly because it allows the cable extra protection, and hopefully, keeps it of students` reach,” he says.
Some cabling contractors prefer to have other trades install conduit; for example, electricians who have more experience with performing this task. “Generally, really the only time we use Plastic Flexible Conduit happens when we`re constructing a riser or penetrating a fire wall,” says Smith. “Typically, we would not install conduit from your wiring closet towards the workstation outlet. For brief distances, as much as 100 feet, we would install conduit between buildings according to the existing infrastructure.
In addition to the traditional smooth-bore type, innerduct is available having a ribbed inner wall to lessen friction between the cable sheath along with the innerduct wall. “A wave-rib within the duct reduces surface contact in between the cable and the wall in the duct, thus lowering the coefficient of friction and enabling you to pull cable over longer distances,” says Stewart.
Another variation is the multicelled conduit system, that provides outerducts with pre-installed innerducts. Clawson says that, due to the cost, his company fails to use conduit with pre- installed innerduct. “We keep leftover conduit in stock to utilize on other jobs,” he says. “But pre-installed conduit can be a special application, so overages and underages are type of costly to handle.”
For premises applications, Dura-line (Knoxville, TN) has designed a conduit, generally known as Hex-line, for multiple-duct applications between buildings. “As you may pull the ducts away from the reel (two to each reel), they get into a collector, which Dura-line supplies free of cost,” says Ray McLeary, vice president of sales. “Each duct has a male and female part, that are snapped together, creating a multiple duct system. This saves time, space, and cash, but the most crucial savings is space.” He explains: “Normally, you are able to put three 1-inch innerducts right into a 4-inch conduit. With this system, you are able to fit four 11/4-inch or six 1-inch innerducts into the conduit.”
When choosing innerduct, you must also be concerned with its tensile strength and crush resistance. “The thicker the wall material, the higher the tensile rating,” says Stewart. “If you`re gonna pull it more than a cross country, choose a wall thickness that lets you pull the duct over that distance. The crush-resistance feature helps to ensure the innerduct won`t be damaged throughout the placing process–or maybe you can`t pull in the cable,” he explains.
Because of the limited volume of tensile pull you could exert around the cable, people look for strategies to lessen the coefficient of friction inside the conduit. “There are products out there including prelubricated conduit,” says Stewart. “And there`s also a different technology being used for placing cable, known as air-blown fiber (or ABF), where the fiber-optic cable is blown into the conduit. We manufacture whatever we call the `air-trak` system–a conduit system with chambers–to be used in ABF installations.” [Air-blown fiber is offered in america from Sumitomo Electric Lightwave Corp. (Research Triangle Park, NC).]
Conduit and innerduct have something in common: They facilitate pulling or replacing a cable for capacity in a premises cabling system. However, every contractor knows that as being an installation grows, the number of cables grows to fill each of the space within the conduit. Therefore, picking out the correct trade dimensions are important, simply because you must leave sufficient clearance in between the walls from the conduit as well as other cables (start to see the eia/tia-569 standard). Typically, conduit trade sizes vary from 1/2 to 6 inches in diameter. Minimum conduit size appropriate for backbone cables is 4 inches. Sufficient clearance should be offered to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the total amount (being a percentage) of various kinds of cable you can use within a conduit. “The NEC typically covers power cables,” says Stewart. “With higher-voltage cables, you need to consider temperature and impedance, which really don`t apply with regards to data cables in conduit. The genuine question for data cable is: Are you able to pull it into the dimensions of duct that you`ve selected?”
“The most crucial decision when installing conduit is the size of the conduit and clearance from your wall,” says Clawson. For external use, we use 4-inch PVC conduit, so we try to install as much conduit inside the trenches when we can for future use.”
Cables are continually included in conduit systems that are often filled to capacity with generations of older cable. When new cables are added, friction and pulling tension can harm existing cables inside the conduit. One method to look after future changes is always to subdivide larger conduits with innerducts, which can be smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“Within an existing structure, many installers will not want to pull new cable over the cable already within the conduit,” says Stewart, “simply because they risk damaging the present cable. To optimize a bigger conduit, they`ll install several smaller innerducts within it. They`ll pull a lesser fiber cable into one of many innerducts, and after that have additional ducts to use for future cable placement.”
Innerducts are classified by outside diameter (OD) whereas trade-size conduits use inside diameter (ID). One-inch innerduct is normally used within buildings; however, 11/4-, 11/2-, and two-inch innerducts are accessible for larger fiber cables. Although innerducts occupy space within a conduit, they supply additional protection and adaptability in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll wind up investing in three 1-inch innerducts: one for fiber, one for data, and something spare. What for you to do is pull just as much dexlpky51 you may at installation time.”
Typically constructed from thermoplastic materials, innerduct comes with a pull string already installed. It can be found in ribbed-, corrugated-, and smooth-wall styles. Some types have prelubricated inside walls. These special coatings and also the physical properties in the inner wall of the innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct is utilized in plenum and riser products,” says D`Errico. “And, when created from high-density polyethylene, it is typically useful for short–1000 feet or less–installations.” Smooth wall is used for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Metal Flexible Conduit is the fact that cable jacket is “lifted” away from and possesses a reduced region of contact with the pipe, lowering the coefficient of friction. However the general guideline is: the greater the hole, the simpler it`s going to be to tug the cable,” he says.
According to Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s simpler to handle. If we`re pulling using a directional boring machine and it`s a multiple pull, we use smooth innerduct. It really is simpler to pull smooth innerduct on the top of an even surface, plus it doesn`t kink as easily as ribbed innerduct.”
When using innerduct, it is very important verify whether it is a plenum or non-plenum area and to install the innerduct using the appropriate support. In case the innerduct is secured with tie wraps inside a plenum area, always use plenum-rated products.
Innerduct is often offered in a color–orange for your fiber-optic communications industry. Color can occasionally be installation-specific; as an example, one color for data cable, one for telephone, and so forth. “You will discover a movement afoot to try to use color designations for various applications,” says Stewart. “Orange is normally communications, red can be for electric power, and yellow for gas.”