S9407-AB-HBK-010 Datasheet, PDF (39/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
a. It should be noted that conduit supplied in long lengths is made up of several short
lengths joined together by welding or brazing, and the inside diameter (ID) at each joint
(or splice) is reduced by a maximum of 0.028 inches. This reduction is caused by a
coupling ring (maximum wall thickness of ring = 0.014 inch) used as part of the
manufacturer's method of joining conduit. If the 0.028 inch reduction in diameter at
each joint will cause a clearance problem between a cable outside diameter (OD) and
the intended conduit size, then the intended conduit is, obviously, too small.
Table 3-1 contains the 11 sizes of conduit specified in appendix D, the conduit ID
(minimum), and the minimum ID at a conduit joint.
Table 3-1. Conduit Dimensions (Inches)
Conduit
Size
(nom ID)
1/4
3/8
1/2
5/8
3/4
1
1-1/4
1-1/2
2
2-1/2
3
Conduit
ID
(min)
0.245
0.370
0.495
0.620
0.745
0.995
1.245
1.495
1.995
2.495
2.995
ID at
Joint (min)
(conduit ID
minus 0.028)
0.217
0.342
0.467
0.592
0.717
0.967
1.217
1.467
1.967
2.467
2.967
b. One approach to determining a conduit size is the general rule of thumb used by one of
the shipyards, which states that if a cable OD approaches 90 percent of a conduit ID,
the next larger size conduit should be selected to facilitate installation. Consider, for
example, a cable OD which is 90 percent of a 5/8-inch conduit ID. The same cable in
the next larger size conduit (3/4-inch) is 75 percent of the conduit ID. Another example
is that of a cable OD which is 90 percent of a 3/4-inch conduit ID. This cable in the next
larger size conduit (1-inch) is 67 percent of the conduit ID.
c. In addition to the rule of thumb, the various installation methods of shielding cable, as
well as conditions at the installation site, such as length of cable run and number of
bends along the run, should be considered in determining the amount of clearance
required between cable OD and conduit ID.
For example, one method used where cable has been previously inserted into the
conduit elsewhere, is to install the cable and conduit as a unit. Another method
involves installing the conduit first, and then pulling the cable through the conduit. This
method might be used where the weight and length of a cable/conduit unit would cause
handling difficulties. The bend radius of the cable and the number of bends should be
considered in determining the required clearance. This method would normally require
more clearance than the previous method. A cable shielded in this manner can also be
readily replaced, if necessary.
Another method of shielding a long cable run, at the installation site, is to use several
short lengths of conduit (such as 20 to 30 feet each). The first length of conduit is
slipped over the long cable, then a reusable-type conduit-coupling is used to join the
CHANGE 1 of Revision 2
3-7