How much current is circulating in the coil? The short answer is: a lot. This is why the thick copper conductor gets hot. The high current and frequency also means it is not trivial to measure. Tesla coil builders have ways to measure the current on their primaries, but it is tricky. Trying to calculate it with the V=IR approach is also tricky, because R is difficult to determine. Skin effect and proximity effect are significant in this situation. If you want to calculate it, I think using the capacitor reactance is a better approach. For this parallel resonant circuit the capacitor current is roughly: Ic = 14 * Vin * f * C So for example at 48v input, 100kHz, and 2uF, the RMS capacitor current is ~135A. The coil current would just be this plus losses/load. If the "no load" current draw of the circuit is say 10A, then you could estimate the coil current at 145A. This has some assumptions built into it of course, but it should get you close. Why do you want to know what the current is? How much current do you suppose is flowing in a 308 case neck?