electrical metallic tubing
(EMT)

 

Metal tubing meant to contain electrical conductors. Often called thin-wall conduit, to contrast it with rigid metal conduit (RMC), which has a similar sequence of sizes but much heavier walls. Compared with RMC, EMT is about a third the price, and is also lighter and easier to install. The disadvantage is that it is more easily crushed and, unlike RMC, a nail driven through a wall can penetrate EMT.

All grounded metal conduit helps to suppress electromagnetic interference (EMI), by preventing the conductors inside the conduit from acting as an antenna, either broadcasting interference or picking it up.

Most EMT is steel, galvanized on the outside and painted on the inside with an organic coating. It is also available in aluminum and some copper alloys, generally for use in special environments, and with the exterior coated with PVC. It is sold in nominal 10- and 20-foot lengths, usually ten.

All dimensions in the table are in inches. The minimum radius is to the center line, not the nearest wall of the tubing. “Min. straight” gives the lengths of straight sections required on both sides of the bend.

Trade
size,
inches
Metric
desig-
nator
Weight,
100 feet,
pounds
Diameter Wall
thickness
Bending
inside outside Minimum
radius
Min.
straight
½ 16 30 0.622 0.706 0.042 4
¾ 21 46 0.824 0.922 0.049
1 27 67 1.049 1.163 0.057 1 7/8
35 101 1.380 1.510 0.065 2
41 116 1.610 1.740 0.065 2
2 53 148 2.067 2.197 0.065 2
63 216 2.731 2.875* 0.072 10½ 3
3 78 263 3.356 3.500* 0.072 13 3 1/8
91 349 3.834 4.000* 0.083 15
4 103 393 4.334 4.500* 0.083 16 3 3/8

* Same outside diameter as rigid metal conduit of the same nominal size, so inside diameter is greater then RMC.

 

Installation

Installation of EMT, like anything to do with electrical wiring, is governed in great detail by the National Electrical Code® (NEC) and state and local building codes. For example, EMT must be supported at no more than 10-foot intervals, and within 3 feet of fixtures like outlet boxes. No more than four 90° bends or the equivalent (360°) are permitted between points at which wires can be inserted or withdrawn (if more changes in direction are needed, consider substituting a pull box for a bend).

Because its wall is so thin, EMT cannot be threaded. Connectors use set-screws, clamps or specialized forms of fastening like indentation. Special water-resistant connectors are required in moist environments.

Metallic conduit almost always carries the ground. For this reason, it is essential that connectors be properly installed and that the run is not broken by a non-conductor, such as PVC tubing. Because the conduit is an element of the circuit, code limits how long a run of conduit can be from system ground to the outlet.

EMT should be cut with a hacksaw, not the roll-type tubing cutters used on RMC. Those cutters create a burr on the inside. The reamer that removes the burr will flare EMT’s thin walls (it does not flare the heavier RMC). The flare makes it difficult to install connectors properly. Reamers designed specifically for EMT are available.

How many wires?

The nature and number of the conductors that can be placed in the tube is also governed by code. According to the NEC regulations, if there is only one wire it can occupy 52% of the cross sectional area of the conduit. If there are 2 wires, their combined cross-sectional area can total no more than 31% of the conduit’s area. Three or more wires are limited to 40%. The percentages are slightly different for lead-covered conductors. Calculating the cross sectional area of wires is unnecessary, because the NEC provides tables showing the number of conductors allowed for each conduit size, wire type, and wire gauge.

Running telecom cables

EMT is often used as raceway for ethernet and other communications conductors. Additional standards apply to this use, specifically the recommendations of EIA/TIA 569. A good place to begin is Tony Casazza’s article at www.lanshack.com/DesigningConduitRuns.aspx

take warning!

This information is intended to inform a layperson of some of the considerations involved in the use of EMT, and is in no way a substitute for the latest edition of

NFPA 70: National Electrical Code®.
National Fire Protection Association.
Quincy, MA 02269. www.nfpa.org/catalog/product.asp?pid=7011SB&order_src=A291

and state and local codes. Installation of electric wiring ought to be done only by licensed electricians, who will be familiar with all the often complex requirements.

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