I don't come by here much anymore but I saw this post and can throw in my two cents based on theory and my real life experience of adapting GM HEI's to at least 5 Chevy and GMC 6's
Being an electrican I can understand all this stuff at least in theory.An HEI coil is usually around .5 Ohms at the vehicle running voltage of a nominal 14 volts.Ohm's law says amps is volts divided by resistence,.5 Ohms in this case.So there's a potential in theory for the HEI coil to draw 28 amps.In reality it's less than 28 amps due to short time period the coil is allowed to "fill" with current between sparks.And I do believe the HEI module limts the current to the coil to prevent overloading.
A short run of #14 wire can conduct a steady flow of 20 amps with acceptable voltage loss,The National Electrical Code amperage flow for wire sizes doesn't apply to automotive use.
But the HEI current flow isn't steady,there's an inrush of current 100's of times a second depending on engine RPM's.Beside the current inrush,electrical flow isn't instantanous,there's a slight lag when more current is called for.A larger wire will help to overcome this.Kinda like a drag car taking off,it takes a brief second for all the forces to react.
So the 10 gauge wire is a safety factor so to speak,covers all bases.The large wire keeps voltage losses acceptable in a worst case senario.
In normal situations,like my low rpm truck engines not needing all there is to fire the plugs,a #14 or 12 wire is adequate from my personal experience .
