In outside-plant installations, conduit is usually installed underground to shield cables from damage as well as facilitate cable placement for immediate and future needs. Also you can install Conduit Fittings inside buildings to facilitate pulling cable between two points for example through the telecommunications closet (TC) to operate-area outlets, or from an equipment room to some TC. To safeguard, isolate, and identify the cables, innerduct–also called subduct–may be installed inside existing larger-diameter conduit.
Conduit is described as a rigid or flexible metal or nonmetallic raceway in which cables can be pulled. Furthermore, although conduit can be used to house many types of cable, the National Electrical Code (NEC) uses the word “optical fiber raceway” in Article 770 to describe conduit, or raceways, for optical-fiber cable. Various kinds conduit can be found, including electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit is not recommended because of potential abrasion problems for the cable jacketing.
Metal conduit, which typically is available in 10-foot lengths, is fairly rigid and needs special tooling and accessories to join it. Nonmetallic conduit is accessible on reels in longer, continuous lengths which do not must be joined as much.
“The only issue with installing EMT conduit is it needs a special skill set and training, together with a great deal of practice–or you wind up making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit is available 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 kinds of duct are used–for instance, riser- and plenum-rated–but all of our products are manufactured from thermoplastic materials, for example polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are simpler to install than metal.”
You can find three different types (or ratings) of innerduct: outdoor, riser-rated, and plenum-rated. Robert Jensen, engineering manager at Endot Industries Inc. (Rockaway, NJ), explains: “Outdoor is generally polyethylene and it`s not really rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], that is generally a thermoplastic material for example polyethylene or PVC with fire-retardant chemicals added to it. And the third form of duct is UL plenum-rated, generally a pvdf product, which can be fire-retardant and smoke-resistant,” says Jensen.
As outlined by Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most products that conduit and innerduct manufacturers make is for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “very often incorporating some sort of silicon,” he says. “For premises cabling, Pyramid offers a plenum raceway (tested to UL-910) plus a riser raceway (UL-1666) for installation in vertical shafts.” Additionally, the riser product is halogen-free and is often employed for military, shipboard, or tunnel applications, based upon the specifications.
Of course contractors install conduit where building codes require it, and also where cabling system needs physical protection or protection from unauthorized access.
“We use conduit in riser and backbone systems from the building entrance for the main distribution frame,” says Karl Clawson, senior v . p . and partner, Clawson Communications (Greenwood, IN). “And we also install it for horizontal cabling, particularly in university campuses. Within the living quarters, we install cable in conduit as it affords the cable extra protection, and hopefully, keeps it all out of students` reach,” he says.
Some cabling contractors would rather have other trades install conduit; as an example, electricians who have more experience in performing this task. “Generally, really the only time we use Flexible Plastic Conduit for Cables is when we`re constructing a riser or penetrating a fire wall,” says Smith. “Typically, we would not install conduit in the wiring closet for the workstation outlet. In short distances, just as much as 100 feet, we will install conduit between buildings depending on the existing infrastructure.
Besides the traditional smooth-bore type, innerduct is available with a ribbed inner wall to minimize friction between your cable sheath as well as the innerduct wall. “A wave-rib within the duct reduces surface contact in between the cable and the wall of the duct, thus decreasing the coefficient of friction and letting you pull cable over longer distances,” says Stewart.
Another variation will be the multicelled conduit system, that provides outerducts with pre-installed innerducts. Clawson says that, due to its cost, his company is not going to use conduit with pre- installed innerduct. “We keep leftover conduit in store to make use of on other jobs,” he says. “But pre-installed conduit can be a special application, so overages and underages are type of costly to cope with.”
For premises applications, Dura-line (Knoxville, TN) has created a conduit, generally known as Hex-line, for multiple-duct applications between buildings. “When you pull the ducts away from the reel (two to each and every reel), they enter into a collector, which Dura-line supplies free of charge,” says Ray McLeary, v . p . of sales. “Each duct includes a male and female part, that happen to be snapped together, setting up a multiple duct system. This saves time, space, and funds, but the most significant savings is space.” He explains: “Normally, you are able to put three 1-inch innerducts in to a 4-inch conduit. With this system, you are able to fit four 11/4-inch or six 1-inch innerducts in the conduit.”
When selecting innerduct, you also need to be concerned with its tensile strength and crush resistance. “The thicker the wall material, the greater the tensile rating,” says Stewart. “If you`re likely to pull it across a long distance, pick a wall thickness that lets you pull the duct over that distance. The crush-resistance feature helps to make certain that the innerduct won`t be damaged through the placing process–or else you can`t pull from the cable,” he explains.
As a result of limited quantity of tensile pull you could exert around the cable, people seek out ways to minimize the coefficient of friction within the conduit. “There are products in the marketplace like prelubricated conduit,” says Stewart. “And there`s a good different technology used for placing cable, called air-blown fiber (or ABF), where fiber-optic cable is blown to the conduit. We manufacture what we call the `air-trak` system–a conduit system with chambers–to use in ABF installations.” [Air-blown fiber is accessible in the United States from Sumitomo Electric Lightwave Corp. (Research Triangle Park, NC).]
Conduit and innerduct have one thing in common: They facilitate pulling or replacing a cable for capacity within a premises cabling system. However, every contractor understands that for an installation grows, the volume of cables grows to fill all of the space from the conduit. Therefore, selecting the correct trade size is important, simply because you must leave sufficient clearance between the walls from the conduit as well as other cables (begin to see the eia/tia-569 standard). Typically, conduit trade sizes range from 1/2 to 6 inches in diameter. Minimum conduit size suitable 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 exact amount (as a percentage) of different kinds of cable you can utilize in a conduit. “The NEC typically covers power cables,” says Stewart. “Rich in-voltage cables, you need to consider temperature and impedance, which really don`t apply in the case of data cables in conduit. The actual question for data cable is: Is it possible to pull it into how big duct that you`ve selected?”
“The main decision when installing conduit is the actual size of the conduit and clearance from your wall,” says Clawson. For external use, we use 4-inch PVC conduit, and that we try and install as much conduit within the trenches as we can for future use.”
Cables are continually included with conduit systems that are often filled to capacity with generations of older cable. When new cables are added, friction and pulling tension can damage existing cables in the conduit. A good way to offer future changes is to subdivide larger conduits with innerducts, that happen to be smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“Inside an existing structure, many installers tend not to wish to pull new cable over the cable already within the conduit,” says Stewart, “simply because they risk damaging the existing cable. To optimize a greater conduit, they`ll install several smaller innerducts within it. They`ll pull a reduced fiber cable into one of the innerducts, and then have additional ducts to be utilized for future cable placement.”
Innerducts are classified by outside diameter (OD) whereas trade-size conduits use inside diameter (ID). One-inch innerduct is often used within buildings; however, 11/4-, 11/2-, and two-inch innerducts are available for larger fiber cables. Although innerducts consume space in just a conduit, they supply additional protection and suppleness in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll end up putting in three 1-inch innerducts: one for fiber, one for data, and another spare. What you want to do is pull as much dexlpky51 you may at installation time.”
Typically constructed from thermoplastic materials, innerduct includes a pull string already installed. It comes in ribbed-, corrugated-, and smooth-wall styles. Some types have prelubricated inside walls. These special coatings along with the physical properties of the inner wall of your innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct can be used 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 that the cable jacket is “lifted” clear of and possesses a lesser region of contact with the pipe, decreasing the coefficient of friction. Although the principle is: the larger the hole, the better it`s gonna be to drag the cable,” he says.
Based on Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s much easier to handle. If we`re pulling using a directional boring machine and it`s a multiple pull, then we use smooth innerduct. It can be easier to pull smooth innerduct on top of an effortless surface, and it also doesn`t kink as easily as ribbed innerduct.”
When utilizing innerduct, you should verify be it a plenum or non-plenum area and also to install the innerduct with the appropriate support. When the innerduct is secured with tie wraps inside a plenum area, always use plenum-rated products.
Innerduct is generally offered in a single color–orange to the fiber-optic communications industry. Color can often be installation-specific; for example, one color for data cable, one for telephone, etc. “There exists a movement afoot to try to use color designations for various applications,” says Stewart. “Orange is generally communications, red will be for electrical power, and yellow for gas.”