Underground wire


Old 11-04-05, 06:35 PM
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Underground wire

I need to run apox 175 feet to a new house 200 amp breker box and then to a 200 amp breaker for a barn. How do i figuire out what size wire to use? How do i connect the two together? The meter will be on the house then i want to run 165 feet to a barn.
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Old 11-05-05, 05:00 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
You are asking something that cannot be answered quickly.

At a minimum you will need to learn about the following:
demand load calculations
conductor sizing for ampacity (NEC tables 310.15(B)(6) and 310.16)
conductor sizing for voltage drop
grounding and bonding

I suggest that you buy the book "Wiring Simplified", and read it through. Then get just about anything by Rex Cauldwell and read the appropriate sections. Then ask questions about the details that you do not understand.

The problem is not that we don't want to help. The problem is that there are_numerous_ details that must all be correct. To tell you everything that you need to know, the answer would fill a book. Thus you will save your own time and ours if you buy the books that are already written, and then ask for help on the details that you don't understand.

Whatever you do, don't start buying any materials until you have read the books, written out a complete plan, and then get some of us to review the plan.

You did ask a specific question: how do you size conductors for a given length.

1) Decide on your design criteria. This is the hard one, because there are no explicit code rules for voltage drop. There is the code suggestion of no more than 5% _total_ for the combination of branch circuits and feeders, but this is only a _suggestion_. Voltage drop is also a dynamic quantity, and it depends upon the current flowing in the wire; the greater the current flow, the greater the voltage drop. The suggestion in the code provides no guidance for what current to use as the basis for the voltage drop.

Many people simply use the trip rating of the breaker involved, and size presuming this full current. This works well for an individual branch circuit. However for a whole house load, you probably want to consider both the trip rating of the breaker (200A in your case) and the 'inrush' current of the dynamic load (probably associated with an air conditioning system, though with a barn this might be something like a large compressor or welder).

You also get to decide just how much % drop you want to permit. The larger the % drop, the more lights will flicker every time a large load is switched. The smaller the % drop, then the larger and more expensive the conductors.

2) Once you have your allowed % drop and your design current, then you run the voltage drop equations. Take % volt drop and multiply it by the supply voltage (240V in your case), and then divide by the design current. This gives you the maximum allowed wire resistance. Next take this maximum allowed wire resistance, divide by 2, and then divide by the length of the run. This gives you the maximum allowed resistance per foot. Then you go to a wire table and find wire with lower resistance per foot. It sounds somewhat complex, but each individual step is easy...there are even voltage drop calculators on the web that will do the grunt work for you. Step 2 has the math, but step 1 is the hard part because it involves the decisions.

3) You should note that voltage drop adds up; if you design for 5% drop from meter to house, and then 5% from house to barn, you will have a 10% drop from meter to barn. You can also do things like decide that you can tolerate more drop from house to barn, and apportion wire sizes suitably.

Example: presume that you have a large compressor in your barn. Rated current is 100A at 240V (30 hp or so, like I said, large) but the inrush current at startup is 500A. You decide that you want no more than an 10% voltage drop at the barn when the compressor is starting; but want no more than 3% at the house when this happens so that the lights don't flicker too much.

You start with the run to the house. 240V * 0.03 = 7.2V...7.2V/500A = 0.0144Ohm. 0.0144Ohm/2 = 0.0072Ohm 0.0072Ohm/175 feet = 0.000041ohms/foot. I look in a table and see that I need large 300 kcmil copper conductors to meet this voltage drop requirement.

Then the run to the barn. We have 7% drop 'left over' for the run to the barn. 240V * 0.07 = 16.8V 16.8V/500A = 0.0336 Ohm /2 / 165 feet = 0.0001 ohms/foot I look in the table and see that size 2/0 copper conductors would meet this voltage drop requirement.

4) Once you have determined the wire size based on voltage drop, you must check this against the wire size based on _ampacity_ (Nec 310.16 or 310.15(B)(6) depending on circumstances). You must use the larger of the two. For a 200A service to a home, NEC 310.15(B)(6) requires size 2/0 copper conductors, and for a 200A feeder to a barn NEC 310.16 requires size 3/0 copper conductors.

So for the feed to the home (in this example) you would use fat 300 kcmil conductors sized for voltage drop, and for the feed from the home to the barn, you would use size 3/0 copper conductors sized for ampacity.

Your situation will likely be very different, and you probably don't have a huge compressor using up half your service My guess is that you would be fine with conductors sized for ampacity, but you need to do the design calculations.

Good luck

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