Electrical wire "requirements" for central air unit...

Last time I put in a central air, 2 tons needs 12 gage, and 2 1/2 tons needed 10 gage.
Of course, that was with a rotary compressor.
If memory serves 10 gage will handle 30 amp load, 12 gage will handle 20 amp load, and 14 gage will handle 15 amps.
Shorter is good, less line loss. You can always have an electrician shorten that wire. Also have a HVAC pro come out to clean the outdoor unit, and check the freon level. The HVAC guy may have the skills to shorten the wire. Keep the 10 gage, and shorten the 12.
Christopher A. Young Learn more about Jesus www.lds.org .
Was poking around my basement looking for paths to run new electrical wire/circuits when I traced the wire for my central air.
2 things struck me as odd:
First is there appears to be about 40 feet extra wire in the run to my outside a/c unit. My guess is that the compressor unit was moved from one side of the house to another but whomever installed the unit on the move didn't bother to shorten the run. So the wire runs along a joist then suddenly enters a junction box and reverses course 20 feet back.
Secondly the wire that comes out of the breaker panel to the "halfway" junction box is 12/2 (originally from 1967 builder) but the wire coming out of the box that finishes the run to the a/c is 10/2 and is much newer.
A/c unit is 15 years old, 1.5 ton. Claims to be 19 amps start, 13 amps nominal.
So my questions are this: is 12/2 actually acceptable, and is 12/2 acceptable for todays central air units as well (2 ton lets presume)? Also is there some voltage drop from the extra 40 feet of wire that could be costing me some money? Remember, in that 40 feet, 20 feet is 12 gauge and 20 feet is 10 gauge.
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