In outside-plant installations, conduit is normally installed underground to protect cables from damage as well as to facilitate cable placement for fast and future needs. Also you can install Conduit Fittings inside buildings to facilitate pulling cable between two points for example through the telecommunications closet (TC) to work-area outlets, or from an equipment room to your TC. To guard, isolate, and identify the cables, innerduct–also known as subduct–may be installed inside existing larger-diameter conduit.
Conduit is defined as a rigid or flexible metal or nonmetallic raceway whereby cables may be pulled. Additionally, although conduit enables you to house various types of cable, the National Electrical Code (NEC) uses the term “optical fiber raceway” in Article 770 to illustrate conduit, or raceways, for optical-fiber cable. Several kinds of conduit are offered, including electrical metallic tubing (EMT), rigid metal conduit, PVC, fiberglass, and versatile conduit. For premises installations, how-ever, metal flexible conduit is not recommended because of potential abrasion injury to the cable jacketing.
Metal conduit, which typically comes in 10-foot lengths, is pretty rigid and needs special tooling and accessories to sign up with it. Nonmetallic conduit can be obtained on reels in longer, continuous lengths that do not must be joined as often.
“A possible problem with installing EMT conduit is that it takes a special skill set and training, together with a great deal of practice–or you wind up making swing sets,” explains Kevin Smith, project manager at MTS Services (Bedford, NH). “Metal conduit comes in 10-foot lengths so you must do any nonstandard bends yourself, and that`s the location where the technician`s special skill is important.”
Arnco Corp. (Elyria, OH) sells innerduct on the cable-TV, telecommunications, and electric utility markets, says Tom Stewart, electrical products sales manager. “In a building, several kinds 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 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 generally polyethylene and it`s not really rated. Then there`s a riser product, rated by Underwriters Laboratories [UL], which happens to be generally a thermoplastic material like polyethylene or PVC with fire-retardant chemicals included with it. And the third kind of duct is UL plenum-rated, generally a pvdf product, that is fire-retardant and smoke-resistant,” says Jensen.
Based on Mike D`Errico, regional director of sales at Pyramid Industries (Erie, PA), most products that conduit and innerduct manufacturers make is made for outside plant. Some manufacturers offer prelubricated innerduct and conduit, “frequently incorporating some form 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.” Additionally, the riser item is halogen-free and it is often used for military, shipboard, or tunnel applications, depending upon the specifications.
Needless to say contractors install conduit where building codes require it, but in addition where cabling system needs physical protection or defense against unauthorized access.
“We use conduit in riser and backbone systems in the building entrance for the main distribution frame,” says Karl Clawson, senior v . p . and partner, Clawson Communications (Greenwood, IN). “And that we also install it for horizontal cabling, specifically in university campuses. Inside 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 prefer to have other trades install conduit; as an example, electricians who may have more experience with performing this. “Generally, the only time we use Plastic Flexible Conduit occurs when we`re creating a riser or penetrating a fire wall,” says Smith. “Typically, we may not install conduit through the wiring closet on the workstation outlet. For brief distances, up to 100 feet, we might install conduit between buildings depending on the existing infrastructure.
In addition to the traditional smooth-bore type, innerduct is accessible by using a ribbed inner wall to lower friction involving the cable sheath as well as the innerduct wall. “A wave-rib within the duct reduces surface contact between your cable and the wall from the duct, thus reducing the coefficient of friction and letting you pull cable over longer distances,” says Stewart.
Another variation is the multicelled conduit system, which provides outerducts with pre-installed innerducts. Clawson says that, due to the cost, his company is not going to use conduit with pre- installed innerduct. “We keep leftover conduit in store to make use of 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, known as Hex-line, for multiple-duct applications between buildings. “As you pull the ducts off of the reel (two to each reel), they enter into a collector, which Dura-line supplies free of charge,” says Ray McLeary, v . p . 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 most significant savings is space.” He explains: “Normally, you can put three 1-inch innerducts in to a 4-inch conduit. With this particular 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 larger the tensile rating,” says Stewart. “If you`re going to pull it more than a long-distance, choose 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 through the placing process–or perhaps you can`t pull within the cable,” he explains.
As a result of limited quantity of tensile pull that one could exert about the cable, people look for approaches to lessen the coefficient of friction inside of the conduit. “There are actually products out there like prelubricated conduit,” says Stewart. “And there`s even a different technology being utilized for placing cable, referred to as 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–to use in ABF installations.” [Air-blown fiber is offered in america 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 additional capacity inside a premises cabling system. However, every contractor recognizes that for an installation grows, the number of cables grows to fill each of the space within the conduit. Therefore, selecting the correct trade size is important, simply because you must leave sufficient clearance involving the walls in the conduit and other cables (view the eia/tia-569 standard). Typically, conduit trade sizes range between 1/2 to 6 inches in diameter. Minimum conduit size suggested for backbone cables is 4 inches. Sufficient clearance needs to be accessible to allow pulling the cable without excessive friction or bending.
The NEC conduit-fill tables define the quantity (like a percentage) of different kinds of cable you should use within a conduit. “The NEC typically covers power cables,” says Stewart. “With higher-voltage cables, you must consider temperature and impedance, which really don`t apply when it comes to data cables in conduit. The genuine question for data cable is: Could you pull it into the size of duct that you`ve selected?”
“The most important decision when installing conduit is the size of the conduit and clearance through the wall,” says Clawson. For external use, we use 4-inch PVC conduit, and that we make an effort to install just as much conduit inside the trenches while we can for future use.”
Cables are continually included with conduit systems which 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 great way to look after future changes is usually 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 want to pull new cable across the cable already inside the conduit,” says Stewart, “mainly because they risk damaging existing cable. To optimize a more substantial conduit, they`ll install several smaller innerducts inside it. They`ll pull a smaller fiber cable into one of the innerducts, then have additional ducts to be used 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 two-inch innerducts are accessible for larger fiber cables. Although innerducts use up space inside 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 turn out investing in three 1-inch innerducts: one for fiber, one for data, then one spare. What you should do is pull all the dexlpky51 you can at installation time.”
Typically produced from thermoplastic materials, innerduct has 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 and the physical properties of the inner wall from the innerduct ensure less friction and tension when pulling cable.
“Corrugated innerduct is commonly used in plenum and riser products,” says D`Errico. “And, when manufactured from high-density polyethylene, it really is typically utilized for short–1000 feet or less–installations.” Smooth wall is 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 it has a reduced area of contact with the pipe, lowering the coefficient of friction. Nevertheless the general guideline is: the greater the hole, the easier it`s likely to be to tug 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 through a directional boring machine and it`s a multiple pull, then we use smooth innerduct. It can be quicker to pull smooth innerduct on top of a smooth surface, and it also doesn`t kink as easily as ribbed innerduct.”
When using innerduct, it is essential to verify be it a plenum or non-plenum area as well as install the innerduct with the appropriate support. In case the innerduct is secured with tie wraps in a plenum area, always employ plenum-rated products.
Innerduct is often offered in a single 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, etc. “There exists a movement afoot to try to use color designations for various types of applications,” says Stewart. “Orange is typically communications, red can be for electricity, and yellow for gas.”