In outside-plant installations, conduit is typically installed underground to safeguard cables from damage as well as facilitate cable placement for immediate and future needs. Also you can 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 to a TC. To shield, isolate, and identify the cables, innerduct–also referred to as subduct–might be installed inside existing larger-diameter conduit.
Conduit is described as a rigid or flexible metal or nonmetallic raceway whereby cables might be pulled. Moreover, although conduit could 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. Several types of conduit are offered, such as electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and flexible conduit. For premises installations, how-ever, metal flexible conduit is not really recommended as a result of potential abrasion damage 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 join it. Nonmetallic conduit is accessible on reels in longer, continuous lengths which do not really need to be joined as frequently.
“The only issue with installing EMT conduit is it takes a special skill set and training, together with a great deal of practice–or you find yourself making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit comes in 10-foot lengths so you have to do any nonstandard bends by hand, and that`s where technician`s special skill is necessary.”
Arnco Corp. (Elyria, OH) sells innerduct to the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “Inside a building, various kinds duct are employed–by way of example, riser- and plenum-rated–but all of our products are produced from thermoplastic materials, such as polyvinylide fluoride [pvdf] and polyvinyl chloride [pvc]. The thermoplastic materials are easier to install than metal.”
You will 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 usually polyethylene and it`s not necessarily rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which is generally a thermoplastic material like polyethylene or PVC with fire-retardant chemicals included in it. And also the third sort 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 form of silicon,” he says. “For premises cabling, Pyramid offers a plenum raceway (tested to UL-910) and a riser raceway (UL-1666) for installation in vertical shafts.” In addition, the riser product is halogen-free and is often employed for military, shipboard, or tunnel applications, based upon the specifications.
Obviously 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 through the building entrance towards the main distribution frame,” says Karl Clawson, senior v . p . and partner, Clawson Communications (Greenwood, IN). “And that we also do the installation for horizontal cabling, especially in university campuses. Inside the living quarters, we install cable in conduit as it affords the cable extra protection, and hopefully, keeps it all out of students` reach,” he says.
Some cabling contractors want to have other trades install conduit; for instance, electricians that have more experience of performing this. “Generally, really the only time we use Flexible Plastic Conduit for Cables is when we`re building a riser or penetrating a fire wall,” says Smith. “Typically, we would not install conduit from the wiring closet towards the workstation outlet. For short distances, as much as 100 feet, we may install conduit between buildings depending on the existing infrastructure.
Besides the traditional smooth-bore type, innerduct is offered using a ribbed inner wall to minimize friction in between the cable sheath and also the innerduct wall. “A wave-rib on the inside of the duct reduces surface contact between the cable as well as the wall of your duct, thus decreasing the coefficient of friction and letting you pull cable over longer distances,” says Stewart.
Another variation is definitely the multicelled conduit system, that provides outerducts with pre-installed innerducts. Clawson says that, because of its cost, his company will not use conduit with pre- installed innerduct. “We keep leftover conduit available to utilize on other jobs,” he says. “But pre-installed conduit is a special application, so overages and underages are type of costly to cope with.”
For premises applications, Dura-line (Knoxville, TN) has designed a conduit, called Hex-line, for multiple-duct applications between buildings. “While you pull the ducts off of 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 includes a men and women part, that are snapped together, making a multiple duct system. This saves time, space, and funds, but the most important savings is space.” He explains: “Normally, you may put three 1-inch innerducts into a 4-inch conduit. With this system, you can fit four 11/4-inch or six 1-inch innerducts in the conduit.”
When selecting innerduct, you must 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 gonna pull it spanning a cross country, pick a wall thickness that allows you to pull the duct over that distance. The crush-resistance feature helps to ensure the innerduct won`t be damaged in the placing process–or else you can`t pull from the cable,” he explains.
Due to the limited volume of tensile pull that you could exert on the cable, people search for strategies to lessen the coefficient of friction inside the conduit. “There are products out there such as prelubricated conduit,” says Stewart. “And there`s even a different technology getting used for placing cable, known as air-blown fiber (or ABF), where the fiber-optic cable is blown into the conduit. We manufacture what we call the `air-trak` system–a conduit system with chambers–for use in ABF installations.” [Air-blown fiber is available 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 additional capacity in the premises cabling system. However, every contractor understands that for an installation grows, the amount of cables grows to fill every one of the space inside the conduit. Therefore, picking out the correct trade dimensions are important, since you must leave sufficient clearance in between the walls from the conduit and also other cables (start to see the eia/tia-569 standard). Typically, conduit trade sizes range between 1/2 to 6 inches in diameter. Minimum conduit size suitable for backbone cables is 4 inches. Sufficient clearance should be offered to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the quantity (as a percentage) of different kinds of cable you should use inside a conduit. “The NEC typically covers power cables,” says Stewart. “With good-voltage cables, you will 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: Can you pull it into the size of duct that you`ve selected?”
“The most important 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 try to install just as much conduit inside the trenches as we can for future use.”
Cables are continually included with conduit systems that happen to be often filled to capacity with generations of older cable. When new cables are added, friction and pulling tension can harm existing cables within the conduit. A good way to look after future changes is to subdivide larger conduits with innerducts, which are smaller in diameter than conduit, generally nonmetallic, and semiflexible.
“Inside an existing structure, many installers tend not to wish to pull new cable on the cable already within the conduit,” says Stewart, “since they risk damaging the current cable. To optimize a bigger 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 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 normally 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 supply 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, and one spare. What you should do is pull all the dexlpky51 it is possible to at installation time.”
Typically manufactured 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 the physical properties from the inner wall from 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 constructed 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 is the cable jacket is “lifted” far from and it has a smaller region of connection with the pipe, lowering the coefficient of friction. But the guideline is: the larger the hole, the better it`s going to be to drag 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 using a directional boring machine and it`s a multiple pull, we use smooth innerduct. It is actually quicker to pull smooth innerduct along with an effortless surface, and it doesn`t kink as easily as ribbed innerduct.”
When utilizing innerduct, you should verify whether it be a plenum or non-plenum area as well as install the innerduct using the appropriate support. When the innerduct is secured with tie wraps inside a plenum area, always employ plenum-rated products.
Innerduct is often offered in a single color–orange to the fiber-optic communications industry. Color can sometimes be installation-specific; as an example, one color for data cable, one for telephone, and the like. “There exists a movement afoot to try to use color designations for various applications,” says Stewart. “Orange is generally communications, red will be for power, and yellow for gas.”