500ft underground service run

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  #1  
Old 01-12-08, 10:55 AM
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500ft underground service run

I have to go 500 ft. underground from my main serivice to a remote 100amp service. Direct burial aluminum was installed 10+ years ago and I have had corrosion problems in recent years. Aluminum turns to white powder and service is lost.
I am thinking to go back with copper in PVC, cause my back is killing me and I'm tired of digging and splicing.
-OR-
I know its cheaper to go back with Aluminum in conduit but will I have the same problem in another ten years?

What gage copper should I use for the (2) lines and the neutral?
What gage of aluminum if I go that way?

Building has A/C window unit, refrigerator, water heater, heater, lights etc...

Thanks for your help friend.
 
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Old 01-12-08, 10:58 AM
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Expect a bill of almost 1k for the copper if you choose that route.
 
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Old 01-13-08, 09:14 PM
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You have several issues, the minimum code requirement and 'design issues'.

The minimum code requirement is that you use 4ga copper or 2ga aluminium for this feeder. However over the length of this run there will be _significant_ voltage drop; on the order of 10% if you ever use the full 100A. Such a high voltage drop is probably unacceptable in terms of your comfort; when the water heater thermostat cycles, the lights will flicker, motors will be hard starting, etc.

As you increase the size of the wire by 3ga values (eg to #1 Cu or #0 Al) you will halve the voltage drop and roughly double the cost.

The first thing that you need to figure out is what level of voltage drop is acceptable. The _best_ information is based upon how well your current system works. Ask yourself if you had problems with motors or lights flickering or other issues. Then evaluate the size of your current feeder; you should be able to figure out what wire gauge was used. If you had no problems, then your current feeder size is acceptable.

A copper conductor 2ga smaller will be approximately equivalent to your aluminium conductors. So if you have 0ga aluminium, you could replace it with 2ga copper.

An underground conduit will fill with water. The water is what is causing problems with your aluminium conductors. Water works its way into any insulation breaks, and a chemical reaction causes the wire to decay. This is much less of a problem with copper conductors. The benefit of conduit is that it lets you pull out the damaged conductors without digging, and protects the insulation from getting damaged by rocks and such. However for such a long run pulling the conductors in and out of the conduit will be quite a bit of work.

I'd recommend breaking the run up into a couple of smaller segments.

Good luck!

-Jon
 
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Old 01-14-08, 09:22 AM
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What type of building is the remote outbuilding? Do you actually use 100A out there?

Direct burial aluminum was installed 10+ years ago and I have had corrosion problems in recent years. Aluminum turns to white powder and service is lost.
This only happens when the insulation is penetrated, which means that the wrong cable was used, the cable was damaged at installation, or it was backfilled with rocky soil and no pea gravel barrier.

Assuming a good installation technique, you should have no problems with AL conductors in conduit.
 
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Old 01-14-08, 05:34 PM
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This is a long-shot, but would it be an option to have the power company extend the distribution lines and add a transformer to power this building?
 
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Old 01-14-08, 05:53 PM
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Originally Posted by Strategery View Post
This is a long-shot, but would it be an option to have the power company extend the distribution lines and add a transformer to power this building?
I'm sure they would do it for a price. It would be worth it to get a quote to compare to the cost of burying new secondary wire and conduit.
 
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Old 01-15-08, 09:35 AM
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Yes, the power company would install a second service to the outbuilding, but they will charge a monthly meter fee (forever), usually require the homeowner to pay some of the installation costs, and frequently charge a higher commercial rate for the outbuilding unless you are in an agricultural zone.
 
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