> Well, the heat is caused by amperage. Amperage is caused by voltage and ohms. If the resistance is high the amperage is low and the heat is low. John T gave the ohms for 6v and 12v coils which will prevent too much amperage. The size of the wire is pretty much independent of the heat generated so long as the total resistance of the coil is in line.
You miss the point. The TOTAL heat generated by the resistance in the primary circuit of a 12 volt ignition system will be the same whether it uses a "12 volt coil" or a "6 volt coil" plus an external resistor. The difference is that the with the no-external-resistor 12 volt coil, ALL the heat is generated INSIDE the coil, while if an external resistor coil is used with a 6 volt coil, only HALF the heat is inside the coil. That's why 12 volt coils run hot.
The other issue is how the additional resistance is built into the "12 volt" coil. As John said, there's no discrete resistor inside the coil. (I'm not entirely certain that's true, since the heat seems to be concentrated near the top of the coil, but we'll assume there's no discrete resistor.) So where does the additional resistance come from? You can't just add primary windings, since that will change the coil inductance. So you have to use a primary conductor with higher resistance per foot than is used in the "6 volt" coil. Either they use a different material, such as aluminum instead of copper, or they use a smaller gauge conductor. Either way, the primary conductor of the "12 volt" coil is more prone to burning out than that of the "6 volt" coil. Aluminum has a lower melting point than copper. Smaller gauge conductors burn out before larger gauge conductors.
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