Fiberglass Conduit

[lyla1711]

Has anyone ever used fiberglass conduit? I'm working on a site with heavily loaded below grade conduit runs with high load factor, and the fiberglass is supposed to help the cable ampacity derating (it does a better job of radiating heat than hdpe or PVC).

I've used the 90s, and they were great to work with, but most of the heavily loaded stuff I see is aerial, so I was hoping to get some feedback on how common this is, whether or not it actually works, or any other relevant experience with this stuff.

 

[7anoter4]

According to Neher&McGrath Table VII Constants for Use in Equations 41 and 41(A) [for cables temperature rise in a pipe-duct or conduit- calculations] it is not a big difference between metallic and fiber pipes. However, the thermal resistance [see Table VI Thermal Resistivity of Various Materials] for fiber duct is 480 but for a metallic duct thermal resistance is negligible. So, the temperature rise of a cable in a fiber duct it is more than in a metallic one.

 

[FacEngrPE]

Putting the cables in metal pipe, fiberglass duct, or plastic does nothing to the thermal resistance of the dirt. Duty cycle may be controlling as the dirt has a bunch of thermal inertia. Does Neher&McGrath address duty cycle?

 

[7anoter4]

It is very interesting how each part of the heat evacuation way reacts on variable heat load, indeed. All the known cable ampacity calculation standards consider steady state load-as far as I know.
However, in order to provide a good protection of power cables there are standards which present a way to appreciate the maximum permissible overload for different cable installation way and different overload durations as IEEE 242 /2001-chapter 9.5.2 Overload capacity-for instance. So, nor IEEE standards neither IEC standards, as IEEE 835 [according to Neher McGrath theory] or IEC 60287[IEC 60364-5-52 for low voltage and IEC 60502-2 for medium voltage] consider variable load ampacity calculation.

 

[7anoter4]

The IEC Standard 60287-1-1 chapter 1.4.2 Buried cables where partial drying-out of the soil occurs presents the calculation in the
case when the soil is drying around a power cable run underground directly buried in the soil. There is different study to state the critical
temperature in different soil type. See -for instance
On-line monitoring device for dry zone formation in the soil surrounding underground power cables.