higher voltage calculations

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copper123

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I know little to nothing about the higher voltage distribution systems. I have always been an inside wireman. I just had a friend call and ask me about a calculation for medium voltage cable being buried for three houses about two miles from the utilities overhead power line. The utility cost for overhead lines, single phase is 90,000 per mile! Anyhow they are going to hire a lineman contracting company to lay the cable and he asked if the figures were right. I did some calcs but it is not my specialty at all, and it shows how ignorant I am on distribution.

It is a 13.6 V power line. I guess the utility is going to do primary metering and will let the private folks tap directly off the line. I am not sure how any of this would work either. The company suggested 100-amp alum cable. What would this cable look like? Is it a #2 or something with a ground and shield wrapped around it? How many solid conductors get carried out? I just sat down and did some very elementary Ohms law calcs and was also wondering if these would work as well.
13,600 E x 100 I = 1,360,00 P
1,360,000 watts potential that can be carried on that line.

1,360,000 P / 240 E (secondary) = 5667 amps

I am sure you have some other factors that have to be applied, but is that somewhat correct? Also, I would imagine that the 100-amp cable is a standard underground gauge that would be hardy enough to bury underground.
Thanks
 
Re: higher voltage calculations

First the voltage is probably 13.2 or 13.6 kv 3 phase. 7600 volts phase to ground(about)
Use ohms law to caculate the VD. You can assume 3 houses might be 75 kva. 75000 va/7600 volts = 10 amps. You won't have a VD problem. Use the resistance of the wire size for an estimate.
R for 2 miles of #2 AL = 3.3 ohms.
10 amps x 3.3 ohms = 33 volts. This is primary VD. This is just an estimate to show VD is no problem.
I assume this is a rural area. If so you can use
a 400 ft span on 40 ft poles with 2# 2 al and get about 14 spans. I don't have an up to date cost of the poles and wire. $90K sounds absurd.
What is the utility in business for if not to serve the customer.

[ January 19, 2006, 10:52 PM: Message edited by: bob ]
 
Re: higher voltage calculations

The utility wanted $30,000 to bury about 300 feet on some property I was developing. It was above ground and I wanted it buryed to eliminate a pole in the front yard of one lot.
 
Re: higher voltage calculations

Thanks Bob,
To answer the question, no they are not going to have the utility put the overhead powerline in. It is just way to much. They are going to install a private line out to the three properties and have a line man contractor install the cable underground for them. But thanks Bob, i am going to look at your numbers and see if I can make sense of it all!
 
Re: higher voltage calculations

Cooper
How long a run is the UG cable from the OH line to each house? Is this one of the utilities that will bring the line to your property but will not extend it into the property? The problem with this setup is that when lightning hits the power line it sends a high voltage wave down the
line and when it comes to an UG dip it will follow the cable blowing out the insulation as it travels along the cable. If you live in a area prone to heavy lightning storms you may be replacing the cable often. Ask the utility to install lightning arresters on each side of the pole with the UG cable. What area are you in?
Where I live the utility will install a pole line right up to the house at no cost. I was with the "Mother of all Utilities", Southern Company an we would extend lines to trailers. Many of the trailers stayed on the lot til the rent came due and then leave. We wised up a little and made them install a trailer slab, sewer lines and a pump before the service was attached but there was no cost for the line extension. Not the same for a commercial account.
 
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