underground aluminum conductors

[cvirgil]

Does anyone know of some literature that compares installing aluminum and cooper in undergroud (duct bank) installations or care to share thier thoughts and experience. I would think that alumnum would be the least desirable due to it's thermal properties compared with copper.

 

[dpc]

Underground utility distribution cable is probably 99% aluminum. Industrial facilities tend to use copper.

The economic tradeoffs are pretty straightforward. With copper, you use a smaller conductor size, but the cable is still much heavier, and more costly, than the equivalent aluminum.

Aluminum requires larger conductor cross section but is much lighter, easier to install, and is cheaper. Since most 15 kv distribution circuits are not all that large anyway, aluminum always wins out.

For industrial facilities or power plants with heavy amperage feeders, and relatively shorter runs, copper makes more sense since it allows fewer conductors and smaller conduits to be used.

Both use the same insulation, so the thermal differences tend to wash out because when using aluminum, a larger size conductor is used.

 

[cvirgil]

The application is low voltage. I know when installing underground conductors you need to take into account the soil's thermal resistivity and also derate the cables 6% for each foot below 30".

I would think aluminum would be at a disadvantage in this case.

 

[dpc]

I don't see that it would make any difference in that regard. The same insulation systems are used and that establishes the maximum allowable surface temperature of the conductor. The cable will have a rated current carrying capability.

Per NEC, there isn't any difference.

Also, I believe the derating you refer to only applies over 2000 V, per NEC.

 

[cuky2000]

Here is some reference to compare AL & Cu.

http://www.tpub.com/neets/book4/11e.htm

 

[Electic]

Within their ratings, both aluminum and copper will perform fine.

From a practical point of view, aluminum cable is fussier for terminations, but if properly prepared will work okay. If the cable insulation gets nicked upon installation, aluminum will fail in a short time whereas copper may continue to perform. Where reliability is paramount (industrial installations) aluminum is usually considered substandard.

 

[rempman]

As you know in the underground utiltiy systems there are generally two classes of systems: URD - 200 Amps or less and the 600 Amp systems. These are classified by the ratings fo the accessories that are available such as terminations and splices. If there is a chance that moisture may get into the conductor the aluminum will deteriorate much quicker than the copper. I am avoiding the whole discussion on water treeing here.

The predominate cable size for URD Systems is the 1/0 Aluminum cable which is equivelent to the #2 Copper cable for ampacity (roughly 150 amps). In the vast majority of applications the limitation of load on the cable system is not the ampacity of the cable or accesories but fuse coordination of the system. As mentioned above if you are looking for the maximum use for the cable then you will want to direct bury it. (Bare in mind that almost all road crossings will be in conduit.) However if you want to be able to replace the cable in the future with minimal disturbance of landscaping, and or roadway you install the cable in conduit. Like every other choice we make it is a set of trade-offs you will need to make.

 

[alehman]

Some utilities use aluminum for URD and copper for feeders.

In spite of a big marketing campaign by one particular aluminum manufacturer, there seems to be very little aluminum used in non-utility applications.