In outside-plant installations, conduit is normally installed underground to protect cables from damage and to facilitate cable placement for immediate and future needs. You may also install Conduit Fittings inside buildings to facilitate pulling cable between two points for example from the telecommunications closet (TC) to operate-area outlets, or from an equipment room into a TC. To safeguard, isolate, and identify the cables, innerduct–also known as subduct–can be installed inside existing larger-diameter conduit.
Conduit is identified as a rigid or flexible metal or nonmetallic raceway by which cables may be pulled. Furthermore, although conduit may be used to house various types of cable, the National Electrical Code (NEC) uses the phrase “optical fiber raceway” in Article 770 to clarify conduit, or raceways, for optical-fiber cable. Various kinds conduit can be purchased, for example electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and flexible conduit. For premises installations, how-ever, metal flexible conduit is not recommended as a consequence of potential abrasion harm to the cable jacketing.
Metal conduit, which typically comes in 10-foot lengths, is fairly rigid and requires special tooling and accessories to join it. Nonmetallic conduit is accessible on reels in longer, continuous lengths that do not need to be joined as much.
“One problem with installing EMT conduit is that it requires 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 comes in 10-foot lengths so you must do any nonstandard bends by hand, and that`s in which the technician`s special skill is essential.”
Arnco Corp. (Elyria, OH) sells innerduct on the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “In the building, several kinds of duct are employed–as an example, riser- and plenum-rated–but all our products are manufactured from thermoplastic materials, like polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
You will 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 normally polyethylene and it`s not necessarily rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which is generally a thermoplastic material including polyethylene or PVC with fire-retardant chemicals included in it. Along with the third sort of duct is UL plenum-rated, generally a pvdf product, which happens to be fire-retardant and smoke-resistant,” says Jensen.
As outlined by Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most items that conduit and innerduct manufacturers make is for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “fairly often incorporating some type of silicon,” he says. “For premises cabling, Pyramid offers a plenum raceway (tested to UL-910) along with a riser raceway (UL-1666) for installation in vertical shafts.” Additionally, the riser item is halogen-free and it is often useful for military, shipboard, or tunnel applications, based upon the specifications.
Needless to say contractors install conduit where building codes require it, but in addition the location where the cabling system needs physical protection or protection from unauthorized access.
“We use conduit in riser and backbone systems from the building entrance on the main distribution frame,” says Karl Clawson, senior vice president and partner, Clawson Communications (Greenwood, IN). “And that we also install it for horizontal cabling, specifically in university campuses. From the living quarters, we install cable in conduit as it allows the cable extra protection, and hopefully, keeps it out of students` reach,” he says.
Some cabling contractors choose to 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 happens when we`re constructing a riser or penetrating a fire wall,” says Smith. “Typically, we will not install conduit in the wiring closet for the workstation outlet. For short distances, just as much as 100 feet, we may install conduit between buildings according to the existing infrastructure.
In addition to the traditional smooth-bore type, innerduct is offered using a ribbed inner wall to lower friction in between the cable sheath and also the innerduct wall. “A wave-rib on the inside of the duct reduces surface contact in between the cable along with the wall in the duct, thus lowering the coefficient of friction and letting you pull cable over longer distances,” says Stewart.
Another variation will be the multicelled conduit system, which offers outerducts with pre-installed innerducts. Clawson states that, due to the cost, his company will not use conduit with pre- installed innerduct. “We keep leftover conduit in stock to utilize on other jobs,” he says. “But pre-installed conduit is actually a special application, so overages and underages are sort of costly to cope with.”
For premises applications, Dura-line (Knoxville, TN) has designed a conduit, known as Hex-line, for multiple-duct applications between buildings. “While you pull the ducts off the reel (two to every single reel), they get into a collector, which Dura-line supplies cost-free,” says Ray McLeary, vice president of sales. “Each duct features a female and male part, which are snapped together, setting up a multiple duct system. This saves time, space, and funds, but the main savings is space.” He explains: “Normally, it is possible to put three 1-inch innerducts in a 4-inch conduit. Using this system, it is possible to fit four 11/4-inch or six 1-inch innerducts to the conduit.”
When selecting innerduct, you also need to be concerned with its tensile strength and crush resistance. “The thicker the wall material, the higher the tensile rating,” says Stewart. “If you`re planning to pull it more than a cross country, choose a wall thickness that allows you to pull the duct over that distance. The crush-resistance feature helps to make sure that the innerduct won`t be damaged throughout the placing process–or maybe you can`t pull inside the cable,” he explains.
Due to the limited amount of tensile pull you could exert on the cable, people try to find methods to decrease the coefficient of friction inside of the conduit. “You will find products in the marketplace for example prelubricated conduit,” says Stewart. “And there`s even a different technology used for placing cable, called air-blown fiber (or ABF), the location where the fiber-optic cable is blown in the conduit. We manufacture what we call the `air-trak` system–a conduit system with chambers–to be used in ABF installations.” [Air-blown fiber is accessible 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 the premises cabling system. However, every contractor knows that as being an installation grows, the volume of cables grows to fill all of the space from the conduit. Therefore, choosing the correct trade size is important, since you must leave sufficient clearance in between the walls from the conduit and other cables (begin to see the eia/tia-569 standard). Typically, conduit trade sizes cover anything from 1/2 to 6 inches in diameter. Minimum conduit size suitable for backbone cables is 4 inches. Sufficient clearance must be open to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the quantity (like a percentage) of several types of cable you should use within a conduit. “The NEC typically covers power cables,” says Stewart. “With higher-voltage cables, you must consider temperature and impedance, which really don`t apply in the case of data cables in conduit. The true question for data cable is: Is it possible to pull it into the actual size of duct that you`ve selected?”
“The most significant decision when installing conduit is the dimensions of the conduit and clearance through the wall,” says Clawson. For external use, we use 4-inch PVC conduit, and that we attempt to install as much conduit inside the trenches when we can for future use.”
Cables are continually included with conduit systems which can be often filled to capacity with generations of older cable. When new cables are added, friction and pulling tension may damage existing cables within the conduit. One way to provide for future changes would be to subdivide larger conduits with innerducts, that happen to be smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“In an existing structure, many installers will not want to pull new cable across the cable already inside the conduit,” says Stewart, “mainly because they risk damaging the present cable. To optimize a bigger conduit, they`ll install several smaller innerducts inside it. They`ll pull a lesser 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 2-inch innerducts are available for larger fiber cables. Although innerducts occupy space in a conduit, they give additional protection and suppleness in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll turn out putting in three 1-inch innerducts: one for fiber, one for data, then one spare. What for you to do is pull all the dexlpky51 you are able to at installation time.”
Typically manufactured from thermoplastic materials, innerduct has 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 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 actually typically used for short–1000 feet or less–installations.” Smooth wall can be used for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Flexible Metal Conduit Pipe would be that the cable jacket is “lifted” from and contains a smaller region of experience of the pipe, lowering the coefficient of friction. Although the rule of thumb is: the greater the hole, the easier it`s will be to pull the cable,” he says.
As outlined by Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s much easier to handle. If we`re pulling via a directional boring machine and it`s a multiple pull, then we use smooth innerduct. It is actually simpler to pull smooth innerduct in addition to an easy surface, plus it doesn`t kink as easily as ribbed innerduct.”
When you use innerduct, you should verify whether it is a plenum or non-plenum area and also to install the innerduct with the appropriate support. In case the innerduct is secured with tie wraps in a plenum area, only use plenum-rated products.
Innerduct is generally offered in just one color–orange for the fiber-optic communications industry. Color can sometimes be installation-specific; by way of example, one color for data cable, one for telephone, and the like. “There is a movement afoot to attempt to use color designations for various applications,” says Stewart. “Orange is typically communications, red can be for power, and yellow for gas.”