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Figure 2.3 - Orange conduit for fiber optic cables

Ampacity for cables can be required to be reduced to 35% of
the original value. This section has several exceptions that should
be examined for each application to reduce oversizing cable.
As mentioned above, there can be challenges when applying the
NEC to larger underground medium voltage circuits. In order to
meet the fill requirements of the NEC, PVC conduit sizes larger
than 6” (6 inches) are often required for the cable used.
Many manufacturers produce 8” PVC electrical conduit, but the
NEC specifically limits the size of conduit used for each type.
The maximum size PVC conduit allowed by NEC is 6”. While
manufacturers do produce 8” conduit, it is not acceptable to use it
in conjunction with the NEC, and it will not usually be UL listed.
Underwriters Laboratories (UL) is a safety consulting and
certification company. It is up to the engineer to work with the
authority having jurisdiction to determine if the use of 8” conduit
is acceptable.
The National Electrical Safety Code (NESC) also contains a

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