How much current will flow if I deliberately touch a copper wrench across the live terminals?

You probably forgot a parallel path or misapplied a cable impedance.

Do the calculation right, and your equipment hums for 30 years. Do it wrong, and you earn a one-way ticket to the "Lessons Learned" presentation at the next IEEE conference.

Let’s pull back the curtain on this critical skill—without drowning in differential equations. Imagine plopping a 1000-kVA transformer into a factory. You think, “The load is only 400 amps. I’ll use a 600-amp breaker.”