Underground cable special condition

[marcjr]

Hi. I have a number of circuits running in a bank of underground ducts. This bank runs 38" below the surface of the ground through its length, except for a small part (35 ft) that runs 45" below the surface in order to avoid interference with water pipes. The question is: can I disregard this deeper section of the duct bank and calculate the cables as if they were running 38" below ground throughout the duct bank? That is, can I consider longitudinal heat flow will absorb the additional heating generated in these 35 ft of cable?

 

[Julius Right]

It is possible if the lower ampacity portion length is not more than 10 ft. or 10% of the circuit. See:
NEC 310.15 Ampacities for Conductors Rated 0?2000 Volts.
(A) General.
(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be
used. Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance
equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.

 

[GoldDigger]

Hi. I have a number of circuits running in a bank of underground ducts. This bank runs 38" below the surface of the ground through its length, except for a small part (35 ft) that runs 45" below the surface in order to avoid interference with water pipes. The question is: can I disregard this deeper section of the duct bank and calculate the cables as if they were running 38" below ground throughout the duct bank? That is, can I consider longitudinal heat flow will absorb the additional heating generated in these 35 ft of cable?

Depending on the ambient earth temperature compared to the air temperature, the deeper section may well be the cooler section, with a higher ampacity for a given wire size.

Also, in your case the 35 foot section is too long to take advantage of the exception that Julius cited. The longitudinal heat flow will not be adequate per the Code.

 

[marcjr]

Depending on the ambient earth temperature compared to the air temperature, the deeper section may well be the cooler section, with a higher ampacity for a given wire size.

Also, in your case the 35 foot section is too long to take advantage of the exception that Julius cited. The longitudinal heat flow will not be adequate per the Code.

That's exactly what I wanted to know. I'm using AmpCalc software to calculate the temperatures. I'll try to keep the deeper section within the 10 ft length then. Thanks a lot!

 

[don_resqcapt19]

While the ambient may be lower the deeper you go, the earth acts as an insulator a the deeper part of the run most often has a lower ampacity due to the heat build up caused by the additional insulation (earth).