In outside-plant installations, conduit is generally installed underground to safeguard cables from damage and also to facilitate cable placement for fast and future needs. You can also install Conduit Fittings inside buildings to facilitate pulling cable between two points such as in the telecommunications closet (TC) to work-area outlets, or from an equipment room into a TC. To protect, isolate, and identify the cables, innerduct–often known 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 may be used to house various kinds of cable, the National Electrical Code (NEC) uses the phrase “optical fiber raceway” in Article 770 to illustrate conduit, or raceways, for optical-fiber cable. Several types of conduit can be found, such as 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 injury to the cable jacketing.
Metal conduit, which typically can be purchased in 10-foot lengths, is rather rigid and needs special tooling and accessories to participate it. Nonmetallic conduit can be obtained on reels in longer, continuous lengths that do not have to be joined as often.
“The only problem with installing EMT conduit is that it requires a special skill set and training, in addition to lots of practice–or you end up making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit can be purchased in 10-foot lengths so you need to do any nonstandard bends yourself, and that`s where the technician`s special skill is important.”
Arnco Corp. (Elyria, OH) sells innerduct for the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “In a building, several types of duct are used–for example, riser- and plenum-rated–but all our products are produced from thermoplastic materials, like polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
You can find three different kinds (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 happens to be generally a thermoplastic material for example polyethylene or PVC with fire-retardant chemicals put into it. As well as the third sort of duct is UL plenum-rated, generally a pvdf product, which can be 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 designed for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “often incorporating some type 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 which is often employed for military, shipboard, or tunnel applications, depending upon the specifications.
Needless to say contractors install conduit where building codes require it, but also where the cabling system needs physical protection or defense against unauthorized access.
“We use conduit in riser and backbone systems from the building entrance towards the main distribution frame,” says Karl Clawson, senior vice president and partner, Clawson Communications (Greenwood, IN). “So we also set it up for horizontal cabling, specially in university campuses. From the living quarters, we install cable in conduit as it affords the cable extra protection, and hopefully, keeps it of students` reach,” he says.
Some cabling contractors choose to have other trades install conduit; for example, electricians who definitely have more experience with performing this. “Generally, the only real 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 the wiring closet for the workstation outlet. For short distances, just as much as 100 feet, we would install conduit between buildings depending on the existing infrastructure.
Besides the traditional smooth-bore type, innerduct is available using a ribbed inner wall to lower friction involving the cable sheath as well as the innerduct wall. “A wave-rib on the inside of the duct reduces surface contact between your cable and also the wall in the duct, thus lowering the coefficient of friction and letting you pull cable over longer distances,” says Stewart.
Another variation is definitely the multicelled conduit system, which offers outerducts with pre-installed innerducts. Clawson says that, because of its cost, his company fails to use conduit with pre- installed innerduct. “We keep leftover conduit in stock to use on other jobs,” he says. “But pre-installed conduit is a special application, so overages and underages are form of costly to handle.”
For premises applications, Dura-line (Knoxville, TN) has created a conduit, referred to as Hex-line, for multiple-duct applications between buildings. “When you pull the ducts away from the reel (two to each reel), they go deep into a collector, which Dura-line supplies free of charge,” says Ray McLeary, vice president of sales. “Each duct carries a male and female part, that are snapped together, building a multiple duct system. This saves time, space, and money, but the main savings is space.” He explains: “Normally, you may put three 1-inch innerducts right into a 4-inch conduit. With this particular system, it is possible to fit four 11/4-inch or six 1-inch innerducts into the conduit.”
When buying innerduct, you should also be concerned with its tensile strength and crush resistance. “The thicker the wall material, the larger the tensile rating,” says Stewart. “If you`re likely to pull it more than a long distance, decide on a wall thickness that lets you 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 you can`t pull in the cable,” he explains.
Due to limited volume of tensile pull that you can exert on the cable, people try to find methods to decrease the coefficient of friction in the conduit. “You will find products in the marketplace such as prelubricated conduit,” says Stewart. “And there`s also a different technology being utilized for placing cable, known as air-blown fiber (or ABF), the location where the fiber-optic cable is blown in to the conduit. We manufacture whatever we call the `air-trak` system–a conduit system with chambers–for usage in ABF installations.” [Air-blown fiber is accessible in the states from Sumitomo Electric Lightwave Corp. (Research Triangle Park, NC).]
Conduit and innerduct have something in common: They facilitate pulling or replacing a cable for added capacity inside a premises cabling system. However, every contractor understands that being an installation grows, the number of cables grows to fill each of the space inside the conduit. Therefore, selecting the correct trade dimension is important, as you must leave sufficient clearance in between the walls in the conduit and also other cables (begin 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 needs to be open to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the exact amount (as being a percentage) of different kinds of cable you can use inside a conduit. “The NEC typically covers power cables,” says Stewart. “With high-voltage cables, you have to consider temperature and impedance, which really don`t apply with regards to data cables in conduit. The actual question for data cable is: Is it possible to pull it into the size of duct that you`ve selected?”
“The most significant decision when installing conduit is the dimensions of the conduit and clearance from your wall,” says Clawson. For external use, we use 4-inch PVC conduit, and we attempt to install the maximum amount of conduit in the trenches as we can for future use.”
Cables are continually put into 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. A good way to offer future changes would be to subdivide larger conduits with innerducts, that happen to be smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“In a existing structure, many installers tend not to desire to pull new cable across the cable already in the conduit,” says Stewart, “simply because they risk damaging existing cable. To optimize a bigger conduit, they`ll install several smaller innerducts inside it. They`ll pull a lesser fiber cable into one of many innerducts, after which have additional ducts for use for future cable placement.”
Innerducts are classified by outside diameter (OD) whereas trade-size conduits use inside diameter (ID). One-inch innerduct is usually used within buildings; however, 11/4-, 11/2-, and 2-inch innerducts are accessible for larger fiber cables. Although innerducts consume space inside a conduit, they offer additional protection and flexibility in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll end up investing in three 1-inch innerducts: one for fiber, one for data, and something spare. What you wish to do is pull just as much dexlpky51 you can at installation time.”
Typically created from thermoplastic materials, innerduct features 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 along with the physical properties from the inner wall in the innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct can be used in plenum and riser products,” says D`Errico. “And, when manufactured from high-density polyethylene, it really is typically useful for short–1000 feet or less–installations.” Smooth wall is utilized for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Metal Flexible Conduit would be that the cable jacket is “lifted” away from and possesses a smaller part of experience of the pipe, decreasing the coefficient of friction. But the rule of thumb is: the larger the hole, the easier it`s likely to be to tug the cable,” he says.
Based on Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s simpler to handle. If we`re pulling through a directional boring machine and it`s a multiple pull, we use smooth innerduct. It can be much easier to pull smooth innerduct in addition to an effortless surface, and it doesn`t kink as easily as ribbed innerduct.”
When utilizing innerduct, you should verify be it a plenum or non-plenum area and to install the innerduct with the appropriate support. In case the innerduct is secured with tie wraps inside a plenum area, only take plenum-rated products.
Innerduct is generally offered in just one color–orange for the fiber-optic communications industry. Color can often be installation-specific; by way of example, one color for data cable, one for telephone, and the like. “There is a movement afoot to try and use color designations for various applications,” says Stewart. “Orange is usually communications, red could be for electric power, and yellow for gas.”