What is the general effect of a short run (2 ft) of 18 gauge wire in the middle of a long run (3ft per side) of 20 ga wire?

Hey Folks,

Because of previous wiring under my table using 18 ga wire, I may end up with a short run of 18 ga in the middle of a long run of 20 ga.   The string of wire will be hooked to a DC transformer, pumping out up to 18 V,  5 amps.   (It has a throttle, so I can increase or reduce it.)

Does the thicker wire in the middle "dampen" the voltage or amps going into it from the transformer, so that it is reduced by the time it reached the end of the last piece of 20 ga?  Or boost it?  Or nothing?     Or create a heat-up situation?

Thanks for all comments.

Mannyrock