This gets too complicated to explain here, but IF you
use a 30 Ampere extension cord, it will be smaller in
copper diameter than a 50 Ampere cord, will have one
less wire, ...... and will cause
MORE voltage drop that does the 50 Ampere cord.
A 50 Ampere cord contains TWO "HOTS"
and a neutral.......whatever loads are on DIFFERENT
"HOTs" have their currents 'meet' and cancel, so they
cause no neutral current...therefore cause no neutral voltage
drop.....(the imbalance/difference between the current
on the two 'hot' legs does return through the neutral.)
The point here is, you will have MORE voltage drop
through a 30 Ampere cord than through a 4 wire 50 Ampere
cord.