Some facts:

From copper wire tables #14 is 2.58 ohm / 1000', #12 is 1.62

That's .00258 / .00162 ohm per foot respectively.

For a 30' run (60' of wire) that's .155 / .097 total ohms respv.

I'm assuming a 120 V circuit with a 1/2 HP pump that draws 10 amps. Rough WAG here but erring on the conservative high side.

That's a voltage drop of 1.55 / .97 V respv.

Power loss is E**2 / R or I**2 * R or 15.5W / 9.7W respv.

That power loss works out to be .52 / .32 Watts per foot of heating.

Conclusions:
With #14 you're wasting 5.8 watts of power.
I don't feel the additional heating is significant.
Cost delta of #12 vs #14 is small.
Go with #12. Even on the partial run.
If this is underground use UFB for underground direct burial, or a conduit.

Hope I did all the math right.
Al



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