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 even install Conduit Fittings inside buildings to facilitate pulling cable between two points such as from your telecommunications closet (TC) to operate-area outlets, or from an equipment room to some TC. To shield, isolate, and identify the cables, innerduct–also referred to as subduct–may be installed inside existing larger-diameter conduit.
Conduit is defined as a rigid or flexible metal or nonmetallic raceway by 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 clarify conduit, or raceways, for optical-fiber cable. Various kinds conduit are offered, like electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit is just not recommended as a consequence of potential abrasion problems for the cable jacketing.
Metal conduit, which typically is available in 10-foot lengths, is pretty rigid and needs special tooling and accessories to sign up for it. Nonmetallic conduit is offered on reels in longer, continuous lengths that do not must be joined as frequently.
“A possible problem with installing EMT conduit is it demands a special skill set and training, in addition to 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 need to do any nonstandard bends by hand, and that`s where 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. “Inside a building, several types of duct are utilized–for example, riser- and plenum-rated–but all of 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 kinds (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 certainly not rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which is generally a thermoplastic material such as polyethylene or PVC with fire-retardant chemicals added to it. Along with the third kind of duct is UL plenum-rated, generally a pvdf product, which can be fire-retardant and smoke-resistant,” says Jensen.
Based on Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most items that conduit and innerduct manufacturers make is perfect for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “very often incorporating some type of silicon,” he says. “For premises cabling, Pyramid delivers a plenum raceway (tested to UL-910) plus a riser raceway (UL-1666) for installation in vertical shafts.” Moreover, the riser product is halogen-free which is often employed for military, shipboard, or tunnel applications, based on the specifications.
Needless to say contractors install conduit where building codes require it, and also where 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). “And that we also set it up for horizontal cabling, specifically in university campuses. From the living quarters, we install cable in conduit because it gives 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; for example, electricians who definitely have more expertise in performing this. “Generally, the only real time we use Plastic Flexible Conduit takes place when we`re creating a riser or penetrating a fire wall,” says Smith. “Typically, we might not install conduit from your wiring closet on the workstation outlet. For short distances, just as much as 100 feet, we would install conduit between buildings depending on the existing infrastructure.
In addition to the traditional smooth-bore type, innerduct is accessible using a ribbed inner wall to reduce friction involving the cable sheath and also the innerduct wall. “A wave-rib on the inside of the duct reduces surface contact between your 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, that offers outerducts with pre-installed innerducts. Clawson states that, due to its cost, his company is not going to use conduit with pre- installed innerduct. “We keep leftover conduit in stock to make use of on other jobs,” he says. “But pre-installed conduit is actually 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. “As you pull the ducts off the reel (two to every reel), they get into a collector, which Dura-line supplies totally free,” says Ray McLeary, vice president of sales. “Each duct includes a men and women part, which are snapped together, creating a multiple duct system. This saves time, space, and cash, but the most crucial savings is space.” He explains: “Normally, you may put three 1-inch innerducts in a 4-inch conduit. Using this system, you may fit four 11/4-inch or six 1-inch innerducts in to the conduit.”
When selecting innerduct, you should also be concerned with its tensile strength and crush resistance. “The thicker the wall material, the greater the tensile rating,” says Stewart. “If you`re going to pull it more than a long-distance, select a wall thickness that permits you to pull the duct over that distance. The crush-resistance feature helps to make certain that the innerduct won`t be damaged during the placing process–or else you can`t pull within the cable,” he explains.
Due to the limited level of tensile pull that you could exert about the cable, people look for strategies to reduce the coefficient of friction in the conduit. “You can find products on the market for example prelubricated conduit,” says Stewart. “And there`s even a different technology being utilized for placing cable, generally known as air-blown fiber (or ABF), the location where the fiber-optic cable is blown in the conduit. We manufacture whatever we call the `air-trak` system–a conduit system with chambers–to be used 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 something in common: They facilitate pulling or replacing a cable for added capacity in a premises cabling system. However, every contractor understands that being an installation grows, the amount of cables grows to fill every one of the space within the conduit. Therefore, selecting the correct trade size is important, simply because you must leave sufficient clearance between the walls of your conduit and other cables (start to see the eia/tia-569 standard). Typically, conduit trade sizes range from 1/2 to 6 inches in diameter. Minimum conduit size recommended for backbone cables is 4 inches. Sufficient clearance must 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 several types of cable you should use in the conduit. “The NEC typically covers power cables,” says Stewart. “With higher-voltage cables, you must consider temperature and impedance, which really don`t apply with regards to data cables in conduit. The genuine question for data cable is: Is it possible to 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 through the wall,” says Clawson. For external use, we use 4-inch PVC conduit, and we try to install as much conduit inside the trenches when we can for future use.”
Cables are continually put into 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 inside the conduit. A good 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.
“Within an existing structure, many installers usually do not want to pull new cable over the cable already in the conduit,” says Stewart, “simply because they risk damaging the present cable. To optimize a greater 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 to 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 around for larger fiber cables. Although innerducts consume space in just a conduit, they give 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 one spare. What you wish to do is pull all the dexlpky51 you are able to at installation time.”
Typically produced from thermoplastic materials, innerduct comes with a pull string already installed. It can be purchased 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 the innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct is utilized in plenum and riser products,” says D`Errico. “And, when produced from high-density polyethylene, it is typically utilized for short–1000 feet or less–installations.” Smooth wall can be used for direct-buried, trenching, plowing, aerial, and directional boring applications. “The Metal Flexible Conduit is the fact that cable jacket is “lifted” far from and possesses a reduced region of experience of the pipe, lowering the coefficient of friction. Nevertheless the rule of thumb is: the larger 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 easier to handle. If we`re pulling using a directional boring machine and it`s a multiple pull, we use smooth innerduct. It is actually much easier to pull smooth innerduct on the top of a smooth surface, and it also doesn`t kink as easily as ribbed innerduct.”
When using innerduct, it is very important verify be it a plenum or non-plenum area and to install the innerduct with the appropriate support. If the innerduct is secured with tie wraps inside a plenum area, always employ plenum-rated products.
Innerduct is often offered in one color–orange to the 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 types of applications,” says Stewart. “Orange is generally communications, red will be for electricity, and yellow for gas.”