Jim Stewart wrote:

The business had a little extra cash before tax
time so my wife let me buy a Thermal Arc 185 TIG
welder.

I sorted out the plug/socket and installed an
outlet for the unit. I also want to fabricate
a 20' extension cord for the unit, and that's
where I got confused.

The welder specs call out 38 amps/240 volts as
the max current draw. The welder comes with a
line cord with 12 gauge conductors with a 50
ampere plug on the end. Anyone see the apparent
contradiction here?

I installed a 50 ampere breaker, the called-out
50 ampere socket, and 50 ampere-rated conductors
between the two. Should be no issue there.

The problem is the 20 foot extension cord. 8
or even 10 gauge portable cordage would seem
to be a waste given the 12 gauge cord on the
unit. OTOH, there's just something wrong about
putting a 25 amp cable on a 50 amp breaker running
to a 38 amp load.

As has been noted, the NEC allows the use of undersized conductors on
circuits feeding welders. This is ok in a commercial environment, but
not so good in a residential environment. If there is a 50A outlet on
the wall, someone will eventually try to connect a 50A load to it in a
residential environment, so everything up to it really ought to be
appropriately rated even if code doesn't require it. On the same idea, I
would make my extension properly rated as well to allow for other uses
in the future. Perhaps you'll get a kiln or heat treat furnace or
something.

Pete C.