What size wire do I need?

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Old 08-22-07, 03:01 PM
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What size wire do I need?

Hello, What size wire do I need for a 20 amp service in a shed 100 feet from breaker box? there will be an a/c window unit 120V 6amp, a computer and printer, and one 4 foot flourescent light fixture.
Thanks for any assistance.
 
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Old 08-22-07, 03:24 PM
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The minimum cost option is #12/2 UF-B direct-burial cable. This doesn't give you much room for expansion, and you may experience unacceptable voltage drop above about 10A of load. If you want to use the full 20A, upsize to #10/2 UF-B cable.

A more flexible option is #10/3 UF-B cable for a 20A multi-wire circuit (or 30A subpanel) which also provides limited support for 240V tools and appliances. Do you anticipate the need for any more power than you listed?
 
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Old 08-22-07, 04:36 PM
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believe it or not, to maintain a voltage drop of less than 3%, at 20 amps (actual load) and 100 feet, you would need to use a #8 cu wire. If the load is no more than 9 amps, #12 cu is acceptable and #10 up through 14 amps load.

(at 120 volts)
 
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Old 08-23-07, 09:09 AM
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Originally Posted by ibpooks View Post
The minimum cost option is #12/2 UF-B direct-burial cable. This doesn't give you much room for expansion, and you may experience unacceptable voltage drop above about 10A of load. If you want to use the full 20A, upsize to #10/2 UF-B cable.

A more flexible option is #10/3 UF-B cable for a 20A multi-wire circuit (or 30A subpanel) which also provides limited support for 240V tools and appliances. Do you anticipate the need for any more power than you listed?
I really don't anticipate any more power need than what I mentioned.
Thank You
 
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Old 08-23-07, 09:13 AM
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Originally Posted by nap View Post
believe it or not, to maintain a voltage drop of less than 3%, at 20 amps (actual load) and 100 feet, you would need to use a #8 cu wire. If the load is no more than 9 amps, #12 cu is acceptable and #10 up through 14 amps load.

(at 120 volts)
Thank You for your help
 
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Old 08-23-07, 09:21 AM
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Somewhere in between the 12/2 and the 10/3 would be 12/3. You could use up to 20 amps on each side (40 amps total) and only have 3.2% voltage drop. Your expected load is far less than this, so the typical voltage drop would be much less. For best results, try to balance the load on the two legs as much as possible. No subpanel and no grounding rods required with this approach, although it will probably increase the burial depth requirements over a simple 12/2.
 
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