IIRC a 5% voltage drop over length is allowed- check here:

This is the bible from which you will work
these guys have done a great job over the years:

http://www.faqs.org/faqs/electrical-wiring/part1/preamble.html

Dave

At 09:31 PM 8/17/04, you wrote:
>Wire gauges are rated according to resistance drop.  While it's
>important to keep the heat of the wire down, 12GA requires much more
>than 20A to heat up appreciably.
>
>From
>http://www.epanorama.net/documents/wiring/wire_resistance.html
>----------
>Most current ratings for wires (except magnet wires) are based on
>permissible voltage drop, not temperature rise.
>...
>You will find tables of permitted maximum current in national electrical
>codes, but these are based on voltage drop (not the heating which is no
>problem in the current rating those codes give).
>...
>Here's a quick table for normal situations. Go up a size for more than
>100 foot runs, when the cable is in conduit, or ganged with other wires
>in a place where they can't dissipate heat easily:
>
>                Gauge           Amps
>                14              15
>                12              20
>                10              30
>                8               40
>                6               65
>----------
>I hope this helps!
>
>-Adam
>
>Matt Redmond wrote:
>
>>To those smarter & more resourceful than I,
>>
>>I'm in the middle of wiring & sheet-rocking my detached garage.  It is a
>>two-car, about 22'^2.  I've run 12 gauge cable from my fuse box (in the
>>front corner of the garage) to each of various collections of outlets spaced
>>therein.
>>
>>My question is:  How long a run of 12 gauge solid copper cable will handle
>>at 20A @ 120V?  Will my 12 gauge cable carry 20A to any spot in the garage -
>>or do I need to put 15A breakers in?
>>
>>Thanks!
>>
>>-matt redmond
>>
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