In outside-plant installations, conduit is normally installed underground to guard cables from damage and to facilitate cable placement for fast and future needs. Also you can install Conduit Fittings inside buildings to facilitate pulling cable between two points like from your telecommunications closet (TC) to operate-area outlets, or from an equipment room to a TC. To guard, isolate, and identify the cables, innerduct–also known as subduct–might be installed inside existing larger-diameter conduit.
Conduit is described as a rigid or flexible metal or nonmetallic raceway whereby cables may be pulled. Furthermore, although conduit could be used to house various types of cable, the National Electrical Code (NEC) uses the expression “optical fiber raceway” in Article 770 to describe conduit, or raceways, for optical-fiber cable. Various kinds of conduit can be found, for example electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit is not recommended as a result of potential abrasion harm to the cable jacketing.
Metal conduit, which typically can be purchased in 10-foot lengths, is rather rigid and requires special tooling and accessories to sign up for it. Nonmetallic conduit is offered on reels in longer, continuous lengths which do not must be joined as frequently.
“A possible problem with installing EMT conduit is it takes a special skill set and training, in addition to lots 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 should do any nonstandard bends by hand, and that`s the location 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, various kinds of duct are employed–as an example, riser- and plenum-rated–but all of our products are manufactured from thermoplastic materials, including polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are simpler to install than metal.”
There are 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 often polyethylene and it`s definitely not rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which can be 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 happens to be fire-retardant and smoke-resistant,” says Jensen.
According to 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, “fairly often incorporating some form of silicon,” he says. “For premises cabling, Pyramid offers 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 utilized for military, shipboard, or tunnel applications, depending on the specifications.
Needless to say contractors install conduit where building codes require it, and also the location where the cabling system needs physical protection or protection from unauthorized access.
“We use conduit in riser and backbone systems through the building entrance towards the main distribution frame,” says Karl Clawson, senior vice president and partner, Clawson Communications (Greenwood, IN). “And we also install it for horizontal cabling, specifically in university campuses. Within the living quarters, we install cable in conduit because it provides the cable extra protection, and hopefully, keeps it all out of students` reach,” he says.
Some cabling contractors choose to have other trades install conduit; by way of example, electricians that have more expertise in 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 might not install conduit in the wiring closet on the workstation outlet. In short distances, just as much as 100 feet, we may install conduit between buildings dependant upon the existing infrastructure.
Besides the traditional smooth-bore type, innerduct can be obtained with a ribbed inner wall to minimize friction between the cable sheath as well as the innerduct wall. “A wave-rib on the inside of the duct reduces surface contact in between the cable along with the wall from the duct, thus decreasing the coefficient of friction and enabling you to pull cable over longer distances,” says Stewart.
Another variation will be the multicelled conduit system, that offers outerducts with pre-installed innerducts. Clawson states that, due to the cost, his company is not going to use conduit with pre- installed innerduct. “We keep leftover conduit available to work with on other jobs,” he says. “But pre-installed conduit is really a special application, so overages and underages are kind of costly to cope with.”
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 and every reel), they go deep into a collector, which Dura-line supplies totally free,” says Ray McLeary, vice president of sales. “Each duct features a male and female part, which can be snapped together, building a multiple duct system. This saves time, space, and funds, but the most significant savings is space.” He explains: “Normally, you can put three 1-inch innerducts into a 4-inch conduit. Using this system, you may fit four 11/4-inch or six 1-inch innerducts in to the conduit.”
When purchasing innerduct, you should also be worried about its tensile strength and crush resistance. “The thicker the wall material, the better the tensile rating,” says Stewart. “If you`re going to pull it spanning a great 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 in the placing process–or maybe you can`t pull within the cable,” he explains.
As a result of limited level of tensile pull that you could exert around the cable, people try to find approaches to decrease the coefficient of friction inside of the conduit. “There are actually products out there including prelubricated conduit,” says Stewart. “And there`s also a different technology being utilized for placing cable, called air-blown fiber (or ABF), where fiber-optic cable is blown into 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 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 in the premises cabling system. However, every contractor knows that for an installation grows, the amount of cables grows to fill all of the space in the conduit. Therefore, choosing the correct trade size is important, since you must leave sufficient clearance between your walls of the conduit and also other cables (see the eia/tia-569 standard). Typically, conduit trade sizes range between 1/2 to 6 inches in diameter. Minimum conduit size appropriate for backbone cables is 4 inches. Sufficient clearance should be accessible to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the amount (being a percentage) of different kinds of cable you should use inside 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 real 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 size of the conduit and clearance from your wall,” says Clawson. For external use, we use 4-inch PVC conduit, and that we try to install as much conduit in the trenches since we can for future use.”
Cables are continually added to 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 great way to provide for future changes would be to subdivide larger conduits with innerducts, which are smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“Inside an existing structure, many installers will not desire to pull new cable over the cable already from the conduit,” says Stewart, “mainly because they risk damaging existing cable. To optimize a greater conduit, they`ll install several smaller innerducts inside it. They`ll pull a reduced fiber cable into one of the innerducts, and then 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 for sale to larger fiber cables. Although innerducts use up space in a conduit, they provide additional protection and suppleness in constantly changing cabling installations.
“Generally, if you`re installing a 4-inch conduit,” says Smith, “you`ll find yourself putting in three 1-inch innerducts: one for fiber, one for data, then one spare. What for you to do is pull just as much dexlpky51 it is possible to at installation time.”
Typically produced from thermoplastic materials, innerduct includes 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 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 constructed from high-density polyethylene, it is typically utilized for short–1000 feet or less–installations.” Smooth wall is commonly used for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Flexible Metal Conduit Pipe is that the cable jacket is “lifted” away from and possesses a lesser section of experience of the pipe, lowering the coefficient of friction. Although the guideline is: the greater the hole, the simpler it`s going to be to pull the cable,” he says.
In accordance with Clawson, “We use ribbed innerduct if we`re pulling one innerduct, because it`s quicker to handle. If we`re pulling by way of a directional boring machine and it`s a multiple pull, then we use smooth innerduct. It is actually much easier to pull smooth innerduct in addition to an easy surface, and yes it doesn`t kink as easily as ribbed innerduct.”
When utilizing innerduct, you should verify whether it is a plenum or non-plenum area as well as install the innerduct together with the appropriate support. If the innerduct is secured with tie wraps in the plenum area, only 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 the like. “You will find a movement afoot to try to use color designations for various applications,” says Stewart. “Orange is generally communications, red can be for electrical power, and yellow for gas.”