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# Wire gauge question (for underground supply wire)

## Wire gauge question (for underground supply wire)

#1
10-21-03, 10:20 PM
kmcdermed
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Wire gauge question (for underground supply wire)

Hello -

I need to run an electric feed to a barn from my service pole. The run is 400 feet and I want to go underground. The barn will have approximately 5 60w lights and a heater (or air con). I would like to have 240v service as I might use a light-medium duty welder there some day. I found a good deal on some zero-ot/zero-ot/2g aluminum URD cable and wondering if this would be sufficient. I know there will be some voltage drop, but, really don't understand what (if anything) can be done about that. The cable is made of three stranded seperate cables - two of which are zero ot and the third 2gauge for the ground.

Thanks for any advice!
Ken

#2
10-21-03, 10:38 PM
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Thats a long way, 4/0 4/0 2/0 alum in 3 conductor is common, and someone will look up the drop figures at that distance tomorrow as I am headed to bed now, but its not real good. I am guessing about 50 A at 5% if that.

#3
10-21-03, 11:48 PM
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The voltage drop will depend on the current you draw – the more you draw the larger the drop. Basically the wire acts as a resistor and ohm’s law applies (voltage drop = current drawn X resistance). Cannot remember the formula offhand to calculate the relationship between wire size and resistance but I’ve seen various online calculators that will tell you the voltage drop given the current drawn and the wire gauge. Once you work out your max expected current draw you can see if this is feasible or not.

Another possible solution (don’t know how practical this is) would be to step up the voltage and then step it down at the other end. By doing this you will require less current to be sent on the wires and thereby minimize the voltage drop. I don’t know if anyone sells a transformer set to allow this and also you will have losses in the transformers. Also a more expensive solution.

#4
10-22-03, 06:45 AM
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Most utilities won't allow you to run an underground feed from the meter without breaker protection, and if you want two separate feeds from the same "pole" you'll need two meters. Where does the feed start? 400' is a very long distance.

#5
10-22-03, 08:10 AM
kmcdermed
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More info

Yes, 400' is a long run. My utility company told me to expect about a 6v drop for that run w/ that wire. The things I'll have in the barn are a few lights (~5 60w), heater or air (for a 12x20 well insulated room w/ 8' ceiling). My 18yr old son wants to live in the room so I'm expecting there will be a tv and some smaller things as well.

In the future, I'd like to be able to run a 220v welder out there (probably 30-50 amp), but, wouldn't run it the same time as the air/heat.

I guess I don't understand if the concern with the 400-foot 1o/1o/2g alum wire is the amperage - or is it the voltage drop? What would be the harm ... heat build-up on the wire or the voltage drop and potential damage to components? If the voltage drop is ~6v, is that too much for everyday things (lights, tv, radio, etc)? Thanks a bunch for all the replys and info!!

Ken

#6
10-22-03, 08:26 AM
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Voltage drop will come into play as load increases. There will be virtually no drop with a small load but if you start adding 30A it will become a problem. Forget the 1/0 to start with. Go right to the 4/0 as the cost difference is minimal and the performance is so much better. I have a panel fed from service panel at 365 ft and I use the 4/0 and it will deliver 60 or 70A and stay at 2% I believe (from memory) The cost at a wholesale place should be 20 cents a foot difference over the smaller wire. I used a 200A panel with a main lug feed thru and a 200 at the other end so connections were not a problem with the large wire. There are adapters though for connecting to smaller breakers. You will need to come from a breaker panel, you will not be able to come directly from a meter.

#7
10-22-03, 10:25 PM
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Yes, you are correct in asking whether it is the amperage or the voltage drop that is the issue. Actually both are issues and they are related. As far as electricity is concerned the conductor is actually a resistor. When current passes through a resistor there is a voltage drop which results in power being dissipated in the resistor. In the case of a conductor, too large a gauge will provide a higher resistance and so will dissipate more energy in the wire. This will cause it to heat. At the same time higher resistance will cause a bigger voltage drop and the voltage at the load end (i.e. the equipment you are trying to power) may not work correctly.

I’ve done the calculations for you below based on the data you have supplied. You may need to adjust the values if my guesses are wrong.

5 lights @ 60W = 300W = 300/120 = 2.5 amps
Heater 1KW = 1000/120 = 8.3 amps
TV + other things = 1KW = 8.3 amps

Arc welder – not too clear if this figure you have quoted is the primary or secondary current. Most Arc welders are rated by the current at the electrodes since this is what is used for welding. Common secondary voltages are 35V to 45V. If we assume 40V at the secondary then at 50 amps (using the principle of VI = constant – excluding transformer losses) then the primary current – what you will actually draw form the mains supply will be (40*50)/220 = 9 amps.

So you total current will be 28.1 amps of which 9 amps will be from the welder and 19 amps will be for the rest.

Calculating voltage drop – Resistance of conductor is I believe RL/A where R is the coefficient of resistance L is length and A is area. R will be a fixed value for the type of metal (Al, Cu etc) and A will be pi*(radius)**2. I don’t’ know the relationship between gauge and area so I used an on-line calculator to come up with the result as follows:

Using 4 AWG the 120V circuit will suffer a 6.6 V drop (5.5% drop). The 220V circuit will suffer a 3.1V drop (1.3%). The online calculator can be found at electrician.com. This is marginal for the 120V supply so you should probably use 2 guage wire.

If you follow these step and adjust the numbers based on what you have in your installation you can calculate exactly if this will work or not. You can also do a power calculation to see if the wire is going to heat excessively.

#8
10-23-03, 08:21 AM
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Rav, I want one of those 9 amp input welding machines,,, ha, just teasing you. Wire feeders anwhere from 20-50A input depending on size and most small stick electrode machines are 50A input. That is the factor that changes all this along with AC units or any other power tools at this distance. You want to run any kind of air comp or similar tools you need the large wire.

#9
10-23-03, 12:40 PM
kmcdermed
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Well I was kinda wondering about the amperage on the estimate. The wire I'm considering is larger than 4 or 2 gauge. It's a 1o/1o/2 (three independent stranded wires that are braided together. Two of the wires are 1o and the other (smaller) ground is 2-gauge. It _is_ aluminum underground wire.

Thanks for all the help, research and information!!

Ken

#10
10-23-03, 02:42 PM
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that wire is going to give aprox 40-45A at 2% drop. Price the larger before making up your mind. To me its not worth it to go to all that trouble to get less than 60A of service, especially if there is a chance of running motors or welding on it. I bought 4/0 4/0 2/0 for right abound 1.20 a foot as far as I remember. Its going to make a big boost to the performance. That is a long ways. I think they make a connector to reduce the terminations so you can use smaller breakers, some of the other guys will probably know.

#11
10-23-03, 07:41 PM
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Hey sberry – yeah some of the figures I just made up as I went along purely for illustration. You are right of course – I’ve never seen a 220V welder @ 50A output. The range you quoted sounds right although I think I’ve seen a light duty one rated at 15A input.

Regarding the wire Ken – I would suggest you follow the calculations I’ve shown with the correct figures. If your welder is really 50A at the input you are looking at a load of around 70A. Although you said that you would not use everything at the same time I think it is wise to assume that you will and size accordingly. To be safe I would add an extra 10A on your calculations. You can certainly use wire that is of a lower gauge than the one the calcs recommend. The fact is it aluminum is also important – the online calculator I used does allow you to specify this.

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