In outside-plant installations, conduit is generally installed underground to shield cables from damage and to facilitate cable placement for fast and future needs. You may also install Conduit Fittings inside buildings to facilitate pulling cable between two points like in the telecommunications closet (TC) to operate-area outlets, or from an equipment room into a TC. To protect, isolate, and identify the cables, innerduct–often known as subduct–may be installed inside existing larger-diameter conduit.
Conduit is defined as a rigid or flexible metal or nonmetallic raceway in which cables can 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. Various kinds of 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 just not recommended as a consequence of potential abrasion injury to the cable jacketing.
Metal conduit, which typically is available in 10-foot lengths, is fairly rigid and needs special tooling and accessories to sign up with it. Nonmetallic conduit is available on reels in longer, continuous lengths which do not really need to be joined as much.
“The only issue with installing EMT conduit is that it demands a special skill set and training, as well as lots of practice–or you find yourself making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit is available in 10-foot lengths so you have to do any nonstandard bends by hand, and that`s where the technician`s special skill is essential.”
Arnco Corp. (Elyria, OH) sells innerduct for the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “Inside a building, several types of duct are used–for example, riser- and plenum-rated–but all of our products are produced from thermoplastic materials, for example polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
You can find three various sorts (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 definitely not 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. And the third kind of duct is UL plenum-rated, generally a pvdf product, that is fire-retardant and smoke-resistant,” says Jensen.
According to Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most products which conduit and innerduct manufacturers make is made for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “often incorporating some type of silicon,” he says. “For premises cabling, Pyramid delivers a plenum raceway (tested to UL-910) as well as a riser raceway (UL-1666) for installation in vertical shafts.” Furthermore, the riser item is halogen-free and it is often useful for military, shipboard, or tunnel applications, depending upon the specifications.
Obviously contractors install conduit where building codes require it, but also in which the cabling system needs physical protection or protection from 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). “And we also do the installation for horizontal cabling, specifically in university campuses. From the living quarters, we install cable in conduit because it affords the cable extra protection, and hopefully, keeps it out of students` reach,” he says.
Some cabling contractors want to have other trades install conduit; for example, electricians who have more experience in performing this. “Generally, the only time we use Plastic Flexible Conduit takes place when we`re building a riser or penetrating a fire wall,” says Smith. “Typically, we will not install conduit from the wiring closet on the workstation outlet. For brief distances, up to 100 feet, we will install conduit between buildings depending on the existing infrastructure.
Along with the traditional smooth-bore type, innerduct is offered having a ribbed inner wall to minimize friction involving the cable sheath along with the innerduct wall. “A wave-rib within the duct reduces surface contact in between the cable along with the wall of the duct, thus reducing 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 states that, because of its cost, his company does not 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 sort 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. “While you pull the ducts off the reel (two to each and every reel), they go deep into a collector, which Dura-line supplies free of cost,” says Ray McLeary, vice president of sales. “Each duct carries a men and women part, that are snapped together, creating a multiple duct system. This saves time, space, and money, but the main savings is space.” He explains: “Normally, you can put three 1-inch innerducts into a 4-inch conduit. Using this system, you can fit four 11/4-inch or six 1-inch innerducts into the conduit.”
When choosing innerduct, you also have to be concerned with its tensile strength and crush resistance. “The thicker the wall material, the better the tensile rating,” says Stewart. “If you`re likely to pull it over a great distance, decide on a wall thickness that lets you pull the duct over that distance. The crush-resistance feature helps to ensure that the innerduct won`t be damaged during the placing process–or maybe you can`t pull in the cable,” he explains.
Due to limited amount of tensile pull that you can exert in the cable, people look for strategies to reduce the coefficient of friction inside the conduit. “You will find products available on the market like prelubricated conduit,” says Stewart. “And there`s also a different technology being used for placing cable, called air-blown fiber (or ABF), where fiber-optic cable is blown 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 available in the United States from Sumitomo Electric Lightwave Corp. (Research Triangle Park, NC).]
Conduit and innerduct have a very important factor in common: They facilitate pulling or replacing a cable for capacity within a premises cabling system. However, every contractor knows that for an installation grows, the number of cables grows to fill each of the space from the conduit. Therefore, picking out the correct trade dimensions are important, as you must leave sufficient clearance between the walls in the conduit and also other cables (start to see the eia/tia-569 standard). Typically, conduit trade sizes cover anything from 1/2 to 6 inches in diameter. Minimum conduit size suggested 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 total amount (being a percentage) of various kinds of cable you should use in a conduit. “The NEC typically covers power cables,” says Stewart. “With good-voltage cables, you must consider temperature and impedance, which really don`t apply in the case of data cables in conduit. The actual question for data cable is: Could you pull it into the dimensions of duct that you`ve selected?”
“The main decision when installing conduit is the dimensions of the conduit and clearance from 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 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 harm existing cables in the conduit. A good way to offer future changes is always to subdivide larger conduits with innerducts, which are smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“In an existing structure, many installers tend not to would like to pull new cable across the cable already from the conduit,” says Stewart, “because they risk damaging the existing cable. To optimize a greater conduit, they`ll install several smaller innerducts within it. They`ll pull a smaller fiber cable into among the 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 often used within buildings; however, 11/4-, 11/2-, and 2-inch innerducts are for sale to larger fiber cables. Although innerducts take up space inside a conduit, they provide additional protection and adaptability in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll wind up setting up three 1-inch innerducts: one for fiber, one for data, then one spare. What you want to do is pull all the dexlpky51 you are able to 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 from the inner wall in the innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct is used in plenum and riser products,” says D`Errico. “And, when manufactured from high-density polyethylene, it is actually typically used 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 is the cable jacket is “lifted” clear of and has a smaller section 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.
As outlined by Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s quicker to handle. If we`re pulling through a directional boring machine and it`s a multiple pull, then we use smooth innerduct. It can be easier to pull smooth innerduct in addition to an easy surface, plus it doesn`t kink as easily as ribbed innerduct.”
When utilizing innerduct, it is essential to verify whether it be a plenum or non-plenum area and to install the innerduct with all the appropriate support. In case the innerduct is secured with tie wraps in the plenum area, always employ plenum-rated products.
Innerduct is usually offered in just one color–orange for that fiber-optic communications industry. Color can often be installation-specific; as an 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 types of applications,” says Stewart. “Orange is usually communications, red can be for electrical power, and yellow for gas.”