Any Derating of Ampacity for Wiring In Sealed U/G Conduit

In oil and gas installations, hazardous area code requirements essentially require the conduits to be sealed or plugged usually done by pouring sealing compound.

Once proper sizing of cable is done by taking into account derating factor in accordance with any of the codes there is no need for any breather nor there is such a practice.

yes you have to consider derating.as current derating depends on where you laid cable ,what the temp of atm,whether it is touching or not .

The derating factor mainly depends on the location of installation of the cable in ground or in air, the manufacturer's catalogue gives a clear detail about these factors if they are installed in air or ground with some grouping factor also should be taken care.

grouping factor is taken if more than two cables are laid horizontally next to each other in ground within in a certain distance. This data can be found in the manufacturer's catalogue. More over you have not specified the type of cable with its armouring and insulation to clearly give you some data.

The following data extracted from a catalogue for your help, it is a LSF type cables :

The NEC assumes no air movement in the conduit. Cooling of the wire is via contact between the wire and the pipe, convection (through the still air in the pipe) and radiation.

De-rating is based upon the environment outside the conduit.

• The NEC assumes a certain thermal conductivity of the earth. If your installation has unusual soil conditions, that must be considered by a qualified engineer.
• Further de-rating is required if multiple conduits & conductors are installed adjacent to one another. This is because the area between the conduits will be subjected to more heat (from several conductors) than the area around the perimeter.

As a point of reference, nearly all commercial installations have no breathing capability for conductors. The conduit terminates into a closed box (transformer, switchgear, pull box, etc.) on both ends, which prevents free circulation of air.