Gauge Size for distance

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  #1  
Old 05-09-04, 04:26 PM
Mason Family
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Gauge Size for distance

We are trying to get 50 amps from our main power box down 450 feet to a barn. We have calculated that we need 50 amps for lights, refrigerator, microwave oven and outlets. We are novices in this and we would like to use the underground wire to get down the 450 feet. We are getting varied opinions on the gauge size. Any suggestions?
 
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Old 05-09-04, 06:15 PM
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Since these are 120V loads you will be using, I'll assume 25A on each of (2) 120V hots [2 hots, a neutral and ground]
You will need 3#2 copper plus ground for your circuit. If you wish to use aluminum, go one size larger. This will keep the voltage drop below 3%.
 
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Old 05-09-04, 08:19 PM
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To calculate voltage drop, you first figure out the current draw. I am going to assume as Ron did, that when you say '50 amps', you mean 50 amps at 120V. Because the feeder to the panel is supplying 240V, you only have a 25A, 240V load. This sounds pretty high to me for running a fridge, a microwave, and some lights. You may want to double check your capacity calculations.

Next you figure the 'allowed voltage drop'. A good rule of thumb is to limit the voltage drop to 3% in the feeders. 3% * 240V is 7.2V, so you want to have no more than a 7.2V drop with your maximum current flowing.

Next you figure conductor resistance. This is simply the allowed voltage drop divided by current, or 7.2/25=0.288 ohms.

Finally you select conductor size. There are tables that give wire resistance per unit length for different sizes. What you want to do is select a wire size that gives a lower resistance that 0.288 ohms. Your length is 900 feet (out and back again), so you want wire with a resistance of 0.288*1000/900 = 0.32 ohms per 1000 feet. #4 copper wire has a resistance of 0.321 ohms per 1000 feet, which is about 0.003% too high. I would recommend just using the #4, unless you expect power consumption to go way up in the future.

Normally for a 30A circuit you would use #10 phase conductors and a #10 ground. Since you are increasing the size of the phase conductors, you are required to increase the size of the ground wire in the same fashion. So you will need a #4 ground as well as the #4 phase and neutral conductors.

-Jon
 
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Old 05-10-04, 06:43 AM
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I agree. The 50-amp calculation seems suspect. At 450 feet, overengineering this solution would be expensive.
 
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Old 05-10-04, 11:01 AM
Mason Family
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Update to barn Situation

Thank you for all of the advice so far. We are confused about the amps we need. One book says one thing and another says we need far less. This is what we will have down at the barn as in electrical use.
1- incinerating toilet at 15 amps
1-old microwave at 12 amps
1- medium refrigerator at 5 amps
2- hunter Douglas fans with 4 60watt bulbs each.
2- 60 watt ea outside lights
3- inside lights with 100 watt bulbs ea
7-outlets
2- electrical surges for an electrical fence for horses at 1 amp
Can some one tells us what the calculations should be?
Also, what about underground wire without conduit. We have been told we can use and can't use it. We have a ditch 2 feet deep and it will be clearly marked. We would like to run the water in the same ditch. (Above or below the electrical cable?)
We have gotten varied answers on all aspects. We would greatly appreciate the advice. Thank you for all of your help.
Jerri
 
  #6  
Old 05-10-04, 11:41 AM
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If I just add those up, I get about 42 amps, not counting the receptacles. Of course, a receptacle doesn't use any power until you plug something in, so it depends on what you might plug in. There's a big difference between a radio and an air conditioner. Let's say I add another 10 amps to cover the receptacles.

The key thing to remember is that everything will not be running full bore simultaneously. That toilet will not be running 24x7. In fact, its duty cycle is probably only about 5%. And it probably doesn't use the full 15 amps when it is running.

Another thing to remember is that these things are all 120-volt devices (is this true?). So 52 amps at 120 volts is equivalent to 26 amps at 240 volts. For the rest of this post, when I say "amps", I'm talking about 240-volt amps.

You then must balance these mitigating factors against whatever you want for future growth. But remember, planning for future growth will be expensive so make sure it's a realistic future.

Let's pick 30 amps (same as winnie picked, and I think he's right). This will actually provide for everything you listed to be running simultaneously. But since that will never happen, the 30 amps leaves a reasonable, but not excessive, amount for growth.

To get 30 amps over 450 feet with around a 5% voltage drop requires at least 6-gauge copper. That can be done with either three insulated strands plus a bare of #6 THHN/THWN, or a 6/3 UF-B cable.

If flexibility is more important than money, consider heavier wire. I used 5% voltage drop, but as winnie suggested, 3% is more conservative. You have to use #4 or heavier to approximately achieve that.

If money is more important than flexibility, #6 will provide good service for what you have listed (especially since your actual usage will very rarely go above 15 or 20 amps).

You will of course need a subpanel and grounding rods at your barn. And pay attention to conduit size and burial depth requirements.
 
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Old 05-10-04, 02:19 PM
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Depending upon local codes, you may or may not be _permitted_ to use a cable without a conduit. The NEC permits direct burial of suitably listed cable.

There are cables which are listed for 'direct burial'; a google search for {"direct burial" cable } is a reasonable place to start. There are _many_ different kinds, including _cables_ with multiple individual conductors, and _individual conductors_ rated to simply go into the ground. Picking _which_ cable to use is more a question of what your local suppliers have the best price on. If we tell you to get '4ga 4conductor type USE' cable, and your supplier happens to have type TC cable at a better price (unlikely, but possible), then you lose out. In addition, the cost of direct burial cable _may_ be more than the cost of individual conductors plus conduit, so it may be cheaper to use conduit. Conduit also offers the advantage that if you upgrade later, you can pull new, larger wire into the conduit.

After you decide upon using #6 or #4 wire (with respect to John's post), you will need to get _4_ conductors, 2 'hot' conductors, 1 'neutral' conductor, and 1 'equipment ground' conductor. All of these conductors will need to be of the same size. Note that this is _not_ typical; usually when you need #4 wire, you only need a #8 or #10 ground, but see my note above about having to increase the size of the ground wire when you increase the size of your circuit conductors. When your supplier quotes you a price for cable, make sure it is for cable with the full sized ground.

The _type_ of cable that you need is any cable (or conductors) which are listed as 'suitable for direct burial'. Type UF wire is; so are many other types. Ask your supplier what they have.

-Jon
 
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