Calculating voltage drop, wire gauge ampacity over distance

I'm confused by a couple of online calculators I've found.
I'm trying to determine the proper gauge wire to carry 20A, 120VAC over a distance of 300 ft. with an approx allowable 2% (per NEC) voltage drop. This UF cable will be buried for most of it's length.
This calculator: http://www.stealth316.com/2-wire-resistance.htm advises a 6 ga. wire would be sufficient (voltage drop 2.414V)
However, this calculator seems much more conservative: http://www.vihon.com/Vihon_Calculators/Vihon_Wire_Gage_Calculator/body_vihon_wire_gage_calculator.html It says a 0 (zero) gauge wire is required !
Also would like some real world advice on the voltage drop recommendations of the NEC. At what point does one see excessive/dangerous wire heating? 2% ? 5% ? 10% ? 20% ?
-- Paul
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http://www.vihon.com/Vihon_Calculators/Vihon_Wire_Gage_Calculator/body_vihon_wire_gage_calculator.html
Heating is not a concern as long as the wire size is adequate for the current. 12 gauge wire will generally cary 20 amperes over any distance with acceptable heating. Long runs require heavier wire only because of voltage drop under load, not because of heating. The extra heat generated is distributed over the extra wire length. Twice the heat with twice the wire length gives the same temperature along the wire. Two seperate, though related, concerns.
Don Young
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On Sun, 26 Aug 2007 18:40:06 -0400, "Paul"

A lot really depends on the expected load. If you are really just running a few lights out there you don't really care that much about voltage drop. If it is voltage sensitive things like motors you need the real FLA before you do the calc. The NEC suggestion (it is not a rule) is 5% total drop from service to load. #8 with a 16a load (reasonable max on a 20a circuit) at 300' comes out to a tad bit over 6%. That is still overkill if the load doesn't care about a little voltage drop. #6 should bring you in no matter what.
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drop it calls for #0

http://www.vihon.com/Vihon_Calculators/Vihon_Wire_Gage_Calculator/body_vihon_wire_gage_calculator.html
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