[Boatanchors] OT: 230vac house circuit runs

[eugene at hertzmail.com]

Hello all, happy thanksgiving.

Sorry for the off topic post.  I am looking to run a 230vac circuit to
my shack for an amplifier (URT-23/Harrs RF-130) --yes it has tubes and
could easily anchor a boat, so its not too far ot).  Just running this
circuit is a major project. The approximate length of the run is 140
feet. This includes the ups and downs, acrosses and overs that this wire
will have to do.

The amplifier specifies the input power as 230vac (+/- 10%) 20 amps
(this thing consumes 4500 watts!).  

The thing is, I already have a 230vac wire run that gets me 75% of the
way there (from a no longer used air conditioning compressor). So I am
really hoping I can use this and just handle the remaining 25% of the
run. 

Here comes the question.  This existing run is 12 AWG.  Is 12 AWG
sufficient to handle this application at 140 feet in length? I don't
mind if the final voltage is lower (probably would only cause a little
less output) but my primary concern is safety and passing an electrical
inspection. Certainly, if I were to do the complete run from scratch, I
might use 10 AWG, but what do you all think about 12 AWG?

Thanks very much!
73 Eugene W2HX