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  #1  
Old 03/17/08, 11:59 PM
 
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Wire size for 200 amp service?

I am needing to run wire from a meter pole 260' to the service panel. It is a 200 amp service. This is going to be burried most likely in conduit and none of it will be indoors and will probably be THHN aluminum. I am not sure what size wire to use. I was told that according to electrical code, a 200 amp service can't be loaded beyond 80%. My question is, when calculating wire size do I use 160 amps (80%) or 200 amps? Or what wire size should I use?

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Last edited by SouthernThunder; 03/18/08 at 12:04 AM.
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  #2  
Old 03/18/08, 12:36 AM
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A sheer guess would be around double aught and the stuff ain't cheap!

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  #3  
Old 03/18/08, 12:58 AM
 
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I'm just a simple dirt farmer.

I use this voltage drop calculator to play 'what if' stuff. I'm told it is a little bit harsh, you can get by with less. I donno, electric motors will be happy if you don't get by with too much less. I think they use 3% drop, assuming you will get more drop in the wires in the building, making a total of 5% allowable drop. Some folks want to do the whole 5% drop in the feed wires......

http://www.elec-toolbox.com/calculators/voltdrop.htm

Maybe you can find different one that gives you a better answer.

Says 350 mcm which is about the size of your thumb. That is what the pros used to make 200 amp runs like that on my place a few years ago.

Looks like double aught would only carry 100 amp or less at this distance? Need more than that for such a long distance, for sure.

I think you need to use the full capacity when calculating wires, but at the end box you should only assume 80% is there.

But again, I'm just a simple farmer, get better advice than mine.

--->Paul

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Old 03/18/08, 01:35 AM
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Can you say BIG.

Use the 200 amps. Nothing stops you from using all 200 amps from the service.

80% is for single branch circuit loads. Some NEC calculations use a 125% margin and these, if worked backwards, will yield 80% (125 times .8= 100)

But for this you should worry about voltage drop at 200 amps.

600 kcmil would be about a 3% drop worst case and is oversized (all current on one side of the line) With 200 amps on each side of the line a 300 kcmil would have about a 3% drop.

4/0 would have a 5.2v drop (on a 125v side) but still be within the 5% total allowed for feeder and branch. How long is the 240 line on the other side of your meter? If it's long you should be more conservative on the wire size.

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Old 03/18/08, 09:18 AM
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I agree with idahodave. While I'm not on the engineering side of things, I'm a lineman at an electrical coop and usually see at least 350kcmil and have seen it parralleled for a similiar service (depending on the distance on the load side of the service panel).

While code is prescribed for "worst case" scenario, it's prescribed for a reason. I've shown up at a lot of folks houses and heard complaints of burned up appliances only to figure out that their service wire is too small on load side of the panel. I know that the wire is pricy, but I'd reccomend going with whatever the code book specifies.

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  #6  
Old 03/18/08, 09:29 AM
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I also agree with idahodave. My power company determined the minimum wire size for my 250' run. Even then my lights dimmed when the central air conditioner (5 ton unit) came on and I had a power surge limiter installed. Now everything works fine.

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  #7  
Old 03/18/08, 03:29 PM
 
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Ok thanks for the replies folks. Basically I need to go with 350 aluminum or equivalent. Next question... I have a 1000' of 3 conductor #2 copper. Could I take all 3 conductors and run them in parallel for each hot leg and get by that way? That is to say 3 runs of #2 copper to one hot, 3 runs to the other hot, and 3 to the neutral?

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  #8  
Old 03/18/08, 05:50 PM
 
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I think your electrical inspector would have a heart attack.......

Aside from that, it looks like, on the voltage drop calculator, a #2 wire will carry 40 amps 260 feet.

So...... doing it that way, you would end up with 120 amp max, not the 200 amps you want? If my math is right?

--->Paul

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  #9  
Old 03/18/08, 05:58 PM
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The National Electrical Code does not allow paralleling of conductors smaller than 1/0.
Jugum

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  #10  
Old 03/18/08, 06:00 PM
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NEC says parallel wires can be used for 1/0 and bigger, so not according to NEC....also three #3 AWG will have a resistance slightly greater than 4/0 aluminum and would drop 4.3v on the 125v side at 200 amps.

Edit...oops read #3 too many times. 3 runs of #2AWG would be about 3.2 volts drop on the 125 v side

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  #11  
Old 03/18/08, 11:28 PM
 
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Thanks again guys, Im gonna pick yer brains one more time here...

There is no code to speak of in my area but I have always built everything to code or better. I understand that there is a good reason for every rule in the NEC(usually something to do with keeping you from getting shocked or burned to death) That being said, in my non-electrician mind I can't realy understand why this would be a hazard. Even considering the slight increase in ohms I still come up with about a 3% voltage drop which I think is within code.

What I came up with was, for a copper wire to carry 240V @ 200A over 270' I would need a diameter of 193500 mil. #2 has a diameter of 6636mil and 3 of 'em is 199080mil which gives me the right diameter and a total voltage drop of 3%....

Guess my last question is just one of curiousity. Why are parallel conductors less than 1/0 not allowed by the NEC. What exactly is the safety/reliabilty issue here? Thanks!

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  #12  
Old 03/19/08, 12:09 AM
 
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Just what will this service cable be running to?



To calculate service drop you will have to know the load amps of your house or what ever it is your running the 200 amp service too. Just because you have a 100, 150,175,200, 400 amp service doesn't mean that's how much your load is. So this is kind of tricky.

When a electrical contractor wires your house they are suppose to do a load calculation of your home to determine the size of service for your home. If the load calculation comes up to 179 amps and you have a 200 amp service, then the 179 amps is what you would use in you voltage drop calculation.

Voltage drop calculation would be [(2 X Resistance of one circular mil foot of conductor) X Lenth of Wire X amps(load)] divided by Circular mil of conductor used.
2 X Resistance X Lenth X load amps divided by cm of wire.


Assuming you want to run this 200 amp service to a new house your building is there anyway you could get the electrical company to place the meter pole closer. Most generally a 4/0 URD cable is run from the meter pole to the 200 amp panel box. But if your house has a small calculating load you may not need to worry about voltage drop even at 260 feet. A lot of houses have a overkill on the service amps. They may be hooked up to a 200 amp service and their demand load may only be 90 amps. However this gives them room to build work shops and such and have enough service amps to carry the extra load.

Do you know what the load amps will be of whatever your are needing the 200 service to?

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  #13  
Old 03/19/08, 12:14 AM
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I can only guess...

Helps with standardized construction for electricians and others that do maintenance and modifications.

Lessens the chance of an overload should one conductor fail, break or have a poor connection.

It's cheaper to run one wire rather than two.

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  #14  
Old 03/19/08, 12:22 AM
 
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I don't understand why a person would want to run multiple wires - one fat one has much less problems.

In a really big wire, can be difficult to bend & run so multiple smaller is more manuverable.

Otherwise, on small wires, it is so easy to get one cut or broken & not realize it - other 2 are carrying full load & can't.

One fat wire should be cheaper than running 3 thin ones.

You seem to want to make this harder than it will be?

--->Paul

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  #15  
Old 03/19/08, 02:36 AM
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Quote:
Originally Posted by SouthernThunder View Post
What I came up with was, for a copper wire to carry 240V @ 200A over 270' I would need a diameter of 193500 mil. #2 has a diameter of 6636mil and 3 of 'em is 199080mil which gives me the right diameter and a total voltage drop of 3%....
I had the same problems with a 400' run at 200A and found out that it would cost me about $16,000 to install under ground. I am changing my mind on the building site since I was hoping to build my 40'x60' shop for less than $25,000.

Your math above seems good but unless I am totaly off base, I think it is flawed in that wire is provided to you in measurements of diameter but it is the total wire volume that is able to carry the load. If my failing memory serves me correctly, the area of a circle is Pi times the radius squared. So to compare, you would have to calculate the area of the three wires and add them together and compare this to the area of a single larger wire.

Based on your numbers you provided, three of your smaller wires would have an area of 3.1416 times half the diameter squared times three wires would be 103758791 square mils and the larger single wire would be 29407143150 square mils which is 283 times more surface area of the single larger wire compared to three smaller ones. I am certainly no electrician but the math seems correct.
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  #16  
Old 03/19/08, 10:13 PM
 
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Quote:
Originally Posted by chuckhole View Post
I had the same problems with a 400' run at 200A and found out that it would cost me about $16,000 to install under ground.
Really? I had my farm site rewired 2.5 years ago, 200 amp service, all underground, 11 buildings to hook up with 60 amps to each building; 200 amp runs to all but the last building in the line, 3 runs of 80 feet, 300 feet, & 360 feet. New panel in the house. All for $11,000. Was underground aluminumn wire. I beleive a lot was the 350 size, or at best one size less.

Don't think aluminum went up that much?

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  #17  
Old 03/19/08, 10:48 PM
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isn't most current conducted by the outer portion of a conductor and resistence related to the inner portion...or core? so perhaps one should calculate the surface area of 3 vs. 1 conductor and see what that reveals. i am just curious.

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  #18  
Old 03/20/08, 11:22 AM
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Skin current is a concern at radio frequencies (RF) not 60Hz power line. If the load had a high power factor, the impedance (Z) of the feeder would be considered (caused by the inductance of the wire) but it is usually so low as not to be a factor.

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Old 03/20/08, 01:42 PM
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thanks!

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  #20  
Old 03/20/08, 04:25 PM
 
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I'm no electician by any means but I have pulled enough wires to see that pulling nine wires through your conduit could be a real p.i.t.a. Lets say you have one of the conductors short to gound, your going to be redioing it anyway, so why not just use the correct size wire and use smaller conduit? my2 cents

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