Conduit Wall Determination

Conduit and duct products come in a wide range of sizes, spanning 1/4 -inch (5mm)
to 24-inch (610mm) bore casings. The standard dimension ratio, SDR, of a conduit is
defined as the ratio of the average conduit diameter divided by the minimum wall
thickness. Wall thickness typically ranges between SDR 9 to SDR 17. (Larger SDR
numbers indicate a thinner wall thickness.)
Conventions exist that work off of either the average outside diameter (SDR) or the
average inside diameter (SIDR). Internally sized (SIDR) are usually chosen when the
inside diameter clearance must be very carefully controlled. This usually does not
apply to most duct installations because, as noted above, the free clearance between
the cable and the inner wall of the conduit is not usually that close. Bore casings,
on the other hand, offer situations that can benefit from close ID control because
many times several innerducts are tightly fit into a casing. In this latter case, the
conduit wall can be increased or decreased relative to service conditions without
jeopardizing the inside clearance fit. Internally sized dimension tables tend to
preserve the minimum ID above the nominal conduit size, whereas, externally sized
conduits often fall below the nominal ID as the wall thickness increases.
For most conduit installations, SDR sizing is utilized because the OD control lends
itself to better joint formation using external couplers. This becomes very important
when air-assisted placement methods are used for placing the cable. On the other
hand, large diameter conduits (4 and above) typically undergo butt fusion as a
means of joining.
Determination of the wall thickness becomes a function of either the method by
which the conduit is placed, or the nature of environmental stresses that it will be
exposed to over the service life. ASTM F 2160, ”Standards Specification for Solid Wall
Chapter 14
Duct and Conduit
481
High Density Polyethylene (PE) Conduit Based on Controlled Outside Diameter (OD)”,
explains the conduit sizing systems fully.