Circlotron,
Please understand that the replies you get are a mixed lot of valuable suggestions by some and utter crap by others. Just ignore the latter.
You asked a simple straightforward question. It is a 60Hz copper and iron power transformer, with a primary winding 'tapped'* for a lower current output from the secondary. Current, even into a short circuit, is limited by deliberately enhanced leakage between the two windings. It is the most common way of making cheap and reliable welding power source.
* the 'tapping' gives the *higher* current; the primary is overwound to reduce the output.
The only significant cause of wasting reactive current draw is the leakage reactance. The arc voltage is essentially in phase with its voltage, all be it distorted from true sine. The open circuit voltage would be at least 50V , to be dropped to 20 or 24 at the arc. This needs a reactive drop of --- not 30 to 26, but 45 to 40, because of the quadrature between them. So, it can be said that the current is largely at low power factor, not much over 0.4. At the lower 'tap', it is even lower.
There is NO reason to discredit the time honored pf corrector, i.e., capacitor. The current draw is bound to reduce if this is fitted.
The exact value is not critical, but it is common practice to use rather less microfarads than are needed to raise the overall pf to unity. An overall pf of 0.9 to 0.95 would be optimum. PF correction caps are rated directly by their VAr, not microfarads. Self healing MPP caps are needed, with built in discharge resistors. They can be left on, connected to the mains through a fuse, and not switched repeatedly. The leading current is generally not harmful to the mains. Very large caps will need in rush limiting switching, but your case of under 5kVAr should get through with just a switch and fuse/s. Please check if special wiring codes apply.
You can expect to reduce the mains current draw by about 30% while welding at full power. The caps will continue to draw almost the same current even when the welder is not feeding an arc, but this is a leading reactive current. Your purpose of not blowing mains fuse/s should be well met.
Good luck!