The form of rigid metal conduit now used almost exclusively, consists of plain iron gaspipe the interior surface of which has been prepared by removing the scale and by removing the Irregularities, and which is then coated with flexible enamel. The outside of the pipe is given a thin coat of enamel in some cases, and, in other cases, is galvanized. Fig. 1 shows one make of enameled (unlined conduit.

Another form of rigid conduit is that known as the armored conduit, which consists of iron pipe with an interior lining of paper impregnated with asphaltum or similar compound. This latter form of conduit is now rapidly going out of use, owing to the unlined pipe being cheaper and easier to install, and owing also to improved methods of protecting the iron pipe from corrosion, and to the introduction of additional braid on the conductors, which partly compensates for the pipe being unlined. The introduction of improved devices - such as outlet insulators, for protecting the conductors from the sharp edges of the pipe, at outlets, cut-out cabinets, etc. - also decreases the necessity of the additional protection afforded by the interior paper lining.

Fig. 1. Rigid Enameled Conduit, Unlined.

Fig. 1. Rigid Enameled Conduit, Unlined.

Courtesy of American Conduit Mfg. Co., Pittsburgh, Pa.

Rigid conduits are made in gaspipe sizes, from one-half inch to three inches in diameter. The following table gives the various data relating to rigid, enameled (unlined) conduit:

Table I. Rigid, Enameled Conduit - Sizes, Dimensions, Etc

Standard Pipe Size

Thickness

Nominal Weight per 100 Feet

Number of

Threads per Inch of Screw

Actual

Outside

Diameter.

Inches

Nominal

Inside

Diameter.

Inches

1/2

.109

84

14

.84

.62

3/4

.113

112

14

1.05

.82

1

.134

167

11 1/2

1.31

1.04

1 1/4

.140

224

11 1/2

1.66

1.38

1 1/2

.145

268

11 1/2

1.90

1.61

2

.154

361

11 1/2

2.37

2.06

2 1/2

.204

574

8

2.87

2.46

3

.217

754

8

3.50

3.06

Tables II, III, and IV give the various sizes of conductors that may be installed in these conduits. Caution must be exercised in

using these tables, for the reason that the sizes of conductors which may be safely installed in any run of conduit depend, of course, upon the length of and the number of bends in the run. The tables are based on average conditions where the run does not exceed 90 to 100 feet, without more than three or four bends, in the case of the smaller sizes of wires for a given size of conduit; and where the run does not exceed 40 to 50 feet, with not more than one or two bends, in the case of the larger sizes of wires, for the same sizes of conduit.

Table II. Single Wire In Conduit

Size Wire, B. & S. G.

Loricated Conduit, Unlined; D. B. Wire

No. 14-4

1/2 inch

" 2

3/4 "

" 1

3/4 "

" 0

3/4 inch or 1 "

" 00

1 "

" 000

1 "

" 0000

1 "

250,000 C. M.

1 1/4 "

300,000 C. M.

1 1/4 "

350,000 C. M.

1 1/4 "

400,000 C. M.

1/14 " or 1 1/2 "

450.000 C. M.

1 1/2 "

500,000 C. M.

1 1/2 "

600,000 C. M.

1 1/2 " or 2

700,000 C. M.

2

800,000 C. M.

2

900,000 C. M.

2 "

1,000,000 C. M.

2 " or 2 1/2 "

1,500,000 C. M.

2 1/2 "

1,700,000 C. M.

3

2,000,000 C. M.

3

Table III. Two Wires In One Conduit

Size Wire,

B. & X.G.

Loricated

Conduit, Unlined, D. B. Wire

No. 14

1/2 inch

" 12

1/2 inch or 1/4 "

" 10

3/4 "

" 8

1 "

" 6

1 "

" 5

1" or 1 1/4 "

" 4

1 1/4 "

" 3

1 1/4 "

" 2

1 1/4 " or 1 1/2 "

" 1

1 1/2 "

" o

1 1/2 "

" 00

1 1/2 " or 2 "

" 000

2 "

" 0000

2 "

250,000 C. M.

2 " or 2 1/2 "

300 000 C. M.

2 1/2 "

350,000 C. M.

2 1/2 "

400,000 C. M.

2 1/2 " or 3 "

450,000 C. M.

3 "

500,000 C. M.

3 "

600,000 C. M.

3 "

700,000 C. M.

3 "

Table IV. Three Wires in One Conduit

Size Wire,

B. & S. G.

Loricated Tube, Unlined;D. B. Wire

Outside

Center

No. 14

No. 12

3/4 inch

" 12

" 10

3/4 "

" 10

" 8

1 "

" 8

" 6

1 "

" 6

" 4

1 1/4 "

" 5

" 2

1 1/4 "

" 4

" 1

1 1/4 inch or 1 1/2 "

" 3

" 0

1 1/2 "

" 2

" 2/0

1 1/4 inch or 2 "

" 1

" 3/0

2 "

" 0

" 4/0

2 "

" 2/0

250 M.

2 inch or 2 1/2 "

" 3/0

300 M.

2 1/2 "

" 4/0

400 M.

2 1/2 "

250 M.

450 M.

2 1/2 or 3 "

250 M.

500 M.

3 "

300 M.

600 M.

3 "

350 M.

700 M.

3 "

400 M.

800 M.

3 "

450 M.

900 M.

3 "

Unlined conduit can be bent without injury to the conduit, if the conduit is properly made and if proper means are used in making the bends. Care should be exercised to avoid flattening the tube as a result of making the bend over a sharp curve or angle.

In installing iron conduits, the conduits should cross sleepers or beams at right angles, so as to reduce the amount of cutting of the beams or sleepers to a minimum.

Where a number of conduits originate at a center of distribution, they should be run at right angles for a distance of two or three feet from the cut-out box, in order to obtain a symmetrical and workmanlike arrangement of the conduits, and so as to have them enter the cabinet in a neat manner. While it is usual to use red or white lead at the joints of conduits in order to make them water-tight, this is frequently unnecessary in the case of enameled conduit, as there is often sufficient enamel on the thread to make a water-tight joint.

When iron conduits are installed in ash concrete, in Keene cement, or, in general, where they are subject in any way to corrosive action, they should be coated with asphaltum or other similar protective paint to prevent such action.

While the cost of circuit work run in iron conduits is usually greater than any other method of wiring, it is the most permanent and durable, and is strongly recommended where the first cost is not the sole consideration. This method of wiring should always be used in fireproof buildings, and also in the better class of frame buildings. It is also to be recommended for exposed work where the work is liable to disturbance or mechanical damage.

Wires Run in Flexible Metal Conduit. This form of conduit, shown in Fig. 2, is described by the manufacturers as a conduit composed of "concave and convex metal strips wound spirally upon each other in such a manner as to interlock several concave surfaces and present their convex surfaces, both exterior and interior, thereby securing a smooth and comparatively frictionless surface inside and out."

The field for the use of this form of conduit is rapidly increasing. Owing to its flexibility, conduit of this type can be used in numerous cases where the rigid conduit could not possib1y be employed. Its use is to be recommended above all the other forms of wiring, except that installed in rigid conduits.

For new fireproof buildings, it is not so durable at the rigid conduit, because not so water-tight; and it is very difficult, if not impossible, to obtain as workmanlike a conduit system with the flexible as with the rigid type of conduit. For completed or old frame buildings, however, the use of the flexible conduit is superior to all other forms of wiring. Table V gives the inside diameter of various sizes of flexible conduit, and the lengths of standard coils. inside diameter of this conduit is the same as that of the rigid conduit; and the table given for the maximum size of conductors which be installed in the various sizes of conduits, may be used also for flexible steel conduits, except that a little more margin should be allowed for flexible steel conduits than for the rigid conduits, as the stiffness of the latter ma it possibleto pull in slightly larger sized conductors.

Fig. 2. Flexible Steel Conduit.