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This article originally appeared in Cabling Business Magazine --

Working with Metal Raceway


By David Herres


How to install metal raceway in various environments--

Low-voltage cabling, like all electrical work, can be done in a variety of ways. Manufacturers offer numerous cabling products that can legitimately be used to create the installed circuitry. These often have widely differing appearances and other attributes even though the finished installation may function the same.

A structured cabling design can take many forms. When running Category 5e throughout a new building or adding to pre-existing wiring in an older structure, many choices present themselves. The entire system can be concealed behind walls or above suspended ceilings, if a high-grade finish appearance is desired, as in an upscale restaurant or hotel lobby. The other extreme, perhaps, would be a Class I Division 1 classified (hazardous) area in an industrial setting where Rigid Metallic Conduit (RMC) strapped to the wall would be the appropriate solution. In some areas of medium sensitivity where a degree of protection is desired for retrofit work, Wiremold raceway (from Wiremold/Legrand) is the way to go.

In this article, we will take a look at Electrical Metallic Tubing (EMT), a thinwall steel (or less frequently aluminum) raceway that cannot be threaded in the field and so is put together with setscrew couplings and connectors. EMT offers a good degree of protection although it is not as strong as the less commonly used RMC and Intermediate Metallic Conduit (IMC) which, with their threaded joints and thicker sidewalls, are needed where explosive gases or liquids are handled. They are much more expensive and labor intensive and so are not seen in ordinary environments.

EMT, like all wiring methods addressed in Chapter 3 of the National Electrical Code, has an article devoted to its use. It is Article 358, where requirements, installation specifications and permitted and prohibited uses are found.

Inexperienced persons sometimes assume that NEC is not concerned with wiring under a certain voltage, but that is not the case. Even though low-voltage cabling presents reduced risk of fire initiation and shock, nevertheless important safety issues exist. Examples are fire propogation and smoke generation. Also, it is possible to pick up higher voltages if low-voltage cabling comes in contact with power wiring that has faulty insulation.

NEC Chapter 7, Special Occupancies, and Chapter 8, Communications Circuits, deal with low-voltage cabling. Some of the major topics are "Propogation of Fire or Products of Combustion," "Removal of Abandoned Cables," "Separation from Other Conductors," and "Grounding and Restricted Usage" (plenum, riser, general purpose and dwellings).

Many of the mandates contained in these chapters can be met by using EMT or other raceway. Conductors within are not considered accessible and so do not have to be removed, thus eliminating a very big problem for retrofit projects. And this exemption makes sense because conductors inside a raceway are much less a source of smoke propogation. Moreover, old cable inside a raceway can be used as a pull rope when installing new media such as fiber optic.

It isw noteworthy that some articles in Chapters 7 and 8 do not include references to earlier chapters, which means power and light rules do not have to be followed in all cases. Certain requirements such as cable fill are not in force. Tn the cases of power wiring, tables in Annex C have to be consulted if all conductors are the same size and calculations based on cross-sectional area must be performed if the conductors are not all the same size. But for low-voltage cabling, you are on your own. In other words, there is no specific number of cables allowed in various sizes of raceway, but you do need to make sure the pipe is not overfilled, causing damage to the insulation. Usually heat from current flow will not be a problem.

But even though the low-voltage cabling within the raceway is not always covered by rules in earlier NEC chapters, nevertheless the raceway installation itself must comply.

EMT, as mentioned, is treated in Article 358. For a start, you should look at 358.10, "Uses Permitted," and 358.12, "Uses Not Permitted."

EMT is permitted for exposed and concealed work. It can be installed in concrete or underground (although RNC is better suited for these applications) or in a severely corrosive environment with suitable protection.

EMT can be used outside or in any wet location, but different fittings are needed. The connectors and couplings must be compression fittings that are made up wrench-tight, not the indoor set-screw fittings that are sometimes mistakenly used outdoors.

Uses Not Permitted (358.12) includes where raceway is subject to severe physical damage, most classified (hazardous) locations, and for support of equipment other than conduit bodies having trade sizes no larger than the pipe itself. Article 358 contains a number of directives, which must be scrupulously observed but they are quite easy to access because they are all right together in less than two pages. EMT must be supported adequately -- every ten feet and within three feet of terminations (with exceptions). All cut ends of EMT must be reamed to remove any sharp edges that could damage cable when being pulled into place. There are allowed no more than the equivalent of four quarter bends between pull points. Total distance is unlimited although certain cabling usages experience signal degradation if maximum distances are exceeded -- not a Code issue.

Very important: bends must be made so that the tubing is not damaged and the internal diameter is not effectively reduced. Hand and power benders are designed to make smooth bends not less than the correct radius so that kinking does not occur.

These are some of the principle Code mandates. Before starting an installation article 358 should be carefully reviewed to make sure that nothing is missed.

Doing an EMT installaation is easy and straightforward, except for calculating the bends, which is part science and part art. EMT in the smaller sizes bends easily with a hand bender. First measure the desired length, mark with a fine felt tip pen and cut with a hacksaw, chopsaw or portable bandsaw. Ream the cut off end to remove burrs. Some reaming tools smooth the outside as well, which makes the pipe slide into couplings and connectors more easily. Then measure and mark where you desire the bend, insert into the bender aligning with the appropriate mark on the bender,u place the pipe on a flat level floor and with firm foot pressure on the bender so that the tubing stays in the channel, make the bend to the correct angle.

Making a bend is quite easy but, as we shall see, many complications arise, especially when there are several bends facing different directions as is the case when it is necessary to install the tubing in tight quarters going around obstacles. Beginners frequently create what is known as a "dog's leg" which is a piece having the bends facing the wrong way relative to one another or not laying in the same plane if that is what is required. Sometimes, cutting between two bends and inserting a coupling so that they face the right way can rescue the piece. Purists disallow any extra couplings and insist on discarding the piece and starting over.

The pipe should conform to the building structure, rarely cutting through open space. If two box terminations are three feet apart, the pipe should have a pair of offset bends where it comes out of one box, run along the wall and be clamped in place, then have another pair of offset bends where it approaches the other box. An offset bender is a great labor saver for this type of work.

For intricate obstacle-filled pathways, it is sometimes best to transition to Flexible Metallic conduit (FMC). Liquidtight Metallic or Non-Metallic Conduit are also good choices.

Here are the principle bends and the methods for creating them:

STUB UP (90 DEGREE) CONDUIT BEND:

1.Insert the pipe into the bender.

2.Lay the stationary leg on a flat, level floor.

3.Bend the moving leg until it is vertical, nice and plumb.

4.You will have to go a few degrees beyond vertical because when you release pressure, it will spring back. Some benders have a bubble vial to aid in getting a perfect bend and some benders have a second bubble vial for making a 45-degree bend. This all assumes a level floor.

5.When making the bend, step down on the footrest and press down while bending, otherwise the pipe will come out of the channel and you will have an inaccurate radius.

6.This will give a perfect 90-degree bend provided the starting point and/or the location of the bend has not been predetermined, as when you place the vertical segment anywhere along a wall or you are going to couple the horizontal leg to another length and go from there, or if you are going to terminate at a box and its exact location is not critical.

STUB-UP DEDUCTIONS:

If you are picking up from a predetermined location and going to a right angle bend at an adjacent wall, either vertically or horizontally, an additional calculation is involved, known as a stub-up deduction. This is because the bend will add to the length of the run, based on the horizontal distance required for the radius. The amount has to be deducted from the distance between the termination or coupling and the "brick wall." The amount to be deducted varies with the size of the pipe. The amount to deduct is usually stamped on the bender. The common deductions are --

  • For 1/2" EMT, deduct five inches.
  • For 3/4" EMT, deduct six inches.
  • For one-inch EMT, deduct eight inches.

1.Measure from the shoulder of the coupling or connector where the pipe will actually begin to the adjacent "brick wall" where the bend will lay.

2.Deduct the required amount from this measurement.

3.Using a fine felt tip pen, mark the pipe at that distance.

4.Place the pipe in the bender with the mark at the arrow on the bender.

5.Make the bend and you will get a perfect fit.

The stub-up bend is the simplest. Other bends are a little more difficult and it is beyond the scope of this article to treat them in full detail, but here are some basic concepts:

OFFSET BEND:

Another very common bend is the offset. This is actually two bends, both the same angle. The object is to bring the pipe to a different plane, but continue in the same direction. The second bend, accordingly, is in the opposite direction from the first so that finally the pipe segments are parallel. The center-to-center dimension between the two bends is equal to the depth you want to make the offset times the cosecant of the bend angles. A cosecant is a trigonometric function and can be looked up in trigonometric tables for any angle. These are the frequently used angles and their cosecants:

  • 10-degree cosecant is 5.76.
  • 22.5-degree cosecant is 2.6.
  • 30-degree cosecant is 2.0.
  • 45-degree cosecant is 1.414.
  • 60-degree cosecant is 1.15.

Most electricians use a pair of 30-degree bends to make an offset because it is easier to figure, although using a hand calculator, they are all easy. Thirty-degree bends are too big an angle for offsets that are less than four inches.

Using 30-degree bends, determine the desired depth of the offset; multiply it by two and that is the distance center-to-center between bends. For any other pair of angles, the cosecant of the angle is the multiplier.

THREE-POINT SADDLE BEND:

The purpose of a saddle bend is to run conduit past an obstacle, frequently another raceway or water pipe that is in the way. The three-point saddle is used when there is only one pipe to traverse. The first and third bends are equal and the angle of the middle bend is equal to the sum of them. The opening and closing bends are 22.5 degrees and the center bend is 45 degrees.

FOUR-POINT SADDLE BEND:

This is similar to the three-point saddle bend, but there are important differences. It is used when more than once obstacle has to be avoided. All angles are the same. It is actually made up of two offsets any distance apart -- whatever is needed to avoid the offending pipes.

SEGMENTED OR MULTI-SHOT BEND:

Often used when a number of parallel conduits have to make a 90-degree or some other turn. It is possible to accomplish the same objective by bending all pipes at the same angle, but in a setting where appearance is of the essence, the segmented approach is preferred. The innermost pipe can have a 90-degree bend, for example, composed of nine ten-degree bends. The number of bends remains constant but they are set ever farther apart for each succeeding run so that parallel spacing is maintained.

Bending conduit is part science and part art. It should be in every electrician's body of knowledge and is a great potential field of interest for low-voltage cable installers where metal raceway is desired. Internet and print resources abound. Among the best on the Web:

Mike Holt

In Print:

"Benfield Conduit Bending Manual," by Jack Benfield
--END--

Books for electricians --

Here is a selection of the most significant electricians' books available online today, at the best prices around. Clicking on any logo provides access to reviews and ratings by electricians. A good place to start is with the 2008 NEC Handbook, which contains the complete text of the current code plus extensive commentary, diagrams and illustrations. Other books of interest for the electrician are available as well.

Low Voltage, Telecom, Fire Alarm Books --


This site is created and conducted By David Herres, NH Master Electrician License #11335M

E-mail: electriciansparadise@hughes.net


HOME | Best Web Host | Question of the Week | Archived Questions | More Archived NEC Questions | Still More Archived Questions | Still More Archived Questions-2 | Still More Archived Questions-3 | Articles | Electrical Deficiencies | More Electrical Deficiencies | Electricians Tools | Online computers | Cybercorner | Electrician's License | Electronics Tutorials | Electricians' worksaving ideas | Electronic Theorems | Satellite Dish | Digital Cameras and Equipment | HTML Color Chart | Electronic Acronyms | Electronic Definitions | Electrician's Soldering Tutorial | Photovoltaic Power | Wind Power | Fire Alarm Basics | More Fire Alarm Info | Working with MC and EMT | Electricians' Color Code | Wiring Commercial Garages | Managing Your Emergency Lights | Lighting Design | Industrial Wiring | Wiring Ethernet | Residential Wiring | Low Voltage Wiring | PLC Overview | Electrical Troubleshooting Techniques | Using Loop Impedance Meter | Ten Common Grounding Errors |NEC and Low-Voltage Wiring | Raceway Protection and NEC | Working with Metal Raceway | Inductance and Characteristic Impedance | Understanding Capacitance | History of the Ethernet | Twisting Data Conductors | NEC Article 800, Communications Circuits | NEC Article 810, Radio and Television Equipment | NEC Article 820, Community Antenna and Radio Distribution Equipment | NEC Article 830, Network-Powered Broadband | Troubleshooting Submersible Well Pumps | Wiring Healthcare Facilities | First Edition National Electrical Code 1897 | Books for Electricians | Links